STUDIES ON THE EMBRYOLOGY O f THE MINK

DISSERTATION

-Presented in Pa r t ia l Fu l f il l m e n t of tnc Requirements FOR TNE DEaREE DOCTOR OF PHILOSOPHY IN TNE Graduate School of The Oh io Sta te Un iv e r s it y

By

ABBOTT THEODORE KISSEN, B . A ., M. A .

••••

The Oh io State Un iv e r s it y 1956

Approved s y :

^ADVISER Department of Zoolooy ano E ntomslooy ACKNOWLEDGMENTS

The present stuoy was conoucted in the zoological

LABORATORIES OP THE OHIO STATE UNIVERSITY, COLUMBUS, OHIO.

I WISH TO ACKNOWLEDGE MY INDEBTEDNESS TO OR. JOHN W. PRICE,

Propessor op Zoology at the Oh io State Un iv e r s it y , por h is

ADVICE AND SUPERVISION, TO THE MEMBERS OP READING COMMITTEE,

Dr . J ames G. Haub and Dr . Wil b u r M. T ido por t h e ir c r it i­ c is m , AND TO THE ENTIRE STAPP OP THE ZOOLOGY DEPARTMENT POR

THEIR ENCOURAGEMENT.

I WISH TO EXPRESS MY APPRECIATION POR THE SPIRIT OP

INTERDEPARTMENTAL COOPERATION EXHIBITED BY DR. RALPH K nOUPP and Dr . Ru s s e l l Ha y e s , both op the Anatomy department , who

GRACIOUSLY OPPERED THEIR TIME AND FAC ILITIES TO THE DEVEL­

OPMENT OP THIS STUDY.

A SPECIAL EXPRESSION OP GRATITUDE IS EXTENDED TO

T he So c ie t y op The Sigma X i por f in a n c ia l a id in the form

op a Grant prom the Sigma X i - R E S A Research Fund. TABLE OF CONTENTS

Pape

I ntroduction ...*...... 1

Ma t e r ia l s and Metho ds ...... 5

De s c r ip t io n of Developmental Stag es ...... 10

Ho r izo n 1...... 10

Ho r izo n I I ...... 13

Ho r izo n I I I ...... 15

Ho r izo n IV ...... 20

Ho r izo n IX ...... 22

Ho r izo n X ...... 25

Ho r izo n X I ...... 28

Ho r izo n X I I ...... 32

Ho r izo n X I I I ...... 35

Ho r izo n X IV ...... 3&

Ho r izo n XV ...... ^2

Ho r izo n X V I ...... ^5

Ho r izo n X V II ......

Ho r izo n X V I I I ...... 5**

D is c u s s io n ...... 5®

Summary ...... 70

Abbreviations Used in Pl a t e s ...... 72

E xplanations of Pla t e s and Pl a t e s ...... 7^

B ibliography ...... 92

Autobiography ...... 9^

111 LIST OP PLATES

Pace

Plate I. (F ig s * I, 2, 3* 5, 6, ~ft 8 , 9 , 10} ...... 75

Plate II. (Figs. 11, 12, 13, 1**, 15, 16, 17, 18 , 19 , 20, 2 1 ) ...... 77

Plate III. (Figs. 22, 23, Zk, 25, 26, 27, 28 , 29, 30, 3 ', 32, 33)...... 79 Pla te " • e,

Pla te V. ( F ig s . kk, 1*5, 1*6, 1*7, 1*8, 1*9, 50, 51, 52, 53, 5*0 ...... 83

Pla te VI. (Figs. 55, 56, 57, 58 , 59 , 60, 61, 62, 63, 61*, 65) 7 ...... 85

Plate VII. (Figs. 66, 67, 68 , 69 , 70, 71, 72, 73, 7U, 75, 76 ) ...... 87

Plate VIII. (Figs. 7 7 . 78 . 79 . 80 , 81 , 82 , 8 3 , 8 4 , % ; &;%)...... : ...... 89

Plate IX. (Figs. 88 , 89 , 90 , 91 , 92 , 93 , 9 * , 9 5 , .....9 6 ) ...... 9 '

IV INTRODUCTION

Wit h re v e x c e p t io n s , stud re * o r the f a m il y Mu s t c lio a c have been

CONriNED TO THE PHYSIOLOGY Or REPRODUCTION, USUALLY WITH SPECIAL EM­

PHASIS ON THE PHENOMENON OP DELAYED IMPLANTATION* AN EXAMINATION Or

THE LITERATURE REVEALS NO ATTEMPT TO DESCRIBE THE EMBRYOLOGY Or THE

MINK HUSTELA VISON. PAPERS BY HANSON (1 9 ^7 ) AN0 EnDCRS (1952) PROVIDE

AN APPROACH TO THE PROBLEM WITH DESCRIPTIONS o r EARLY AND LATE BLASTO­

CYSTS, BUT THEY WERE PRIMARILY CONCERNED WITH THE PHYSIOLOGY o r RE­

PRODUCTION RATHER THAN THE EMBRYOLOGY OF MINK. INDEED, THE ONLY PAPERS

DISCOVERED RELATING TO EMBRYONIC DEVELOPMENT IN THIS PAMILY ARE THOSE or Mainland (1931 and 1932) and Hamilton (193^ AND 1937) on the fe r­ ret, Hamlett ( l932) on the badger, Enders and Pearson ( 19 *13) on th e fisher. There are several factors which render embryological investi­ gations on mink especially d iffic u lt and which might explain the

ABSENCE OF SUCH RESEARCH TO DATE. THE MINK IS A SAVAGE ANIMAL AND EX­

TREME CARE MUST BE TAKEN IN ROUTINE MAINTENANCE, TO AVOID SERIOUS PER­

SONAL in j u r y . Another consideration is the economics in v o lv e d in a pro­ je c t OF THIS SORT. THE MINK IS A VALUABLE FUR-BEARING ANIMAL. THE PELT

ATTAINS A PRIME CONDITION DURING THE MONTHS OF NOVEMBER AND EARLY DECEM­

BER. T he EMBRYOS ARC NOT RECOVERED UNTIL SOME TIME AFTER THE SPRING

MATING SEASON. THE INVESTIGATOR IS THEN FACCO WITH THE UNHAPPY DILEM­

MA OF MAINTAINING CASTRATED FEMALES UNTIL THE FOLLOWING WINTER OR AC­

CEPT INO A LOSS ON INFERIOR PELTS. THE PHENOMENON OF DELAYED IMPLANTA­

TION SERIOUSLY IMPAIRS THE ABILITY OF THE INVESTIGATOR TO RECOVER

DESIRED EMBRYONIC STAGES. THIS TROUBLESOME FACTOR IS INTENSIFIED BY

THE FACT THAT THE MINK HAS ONLY ONE MATING SEASON PER YEAR. In view op thc existing papers on the mink by Hanson (19**7)

ANO ENOERS (1 9 5 2 ), IT IS QUITE NEEDLESS TO DO MORE THAN BRIEFLY SUM­

MARIZE THE ANATOMY AND BREEOING HABITS OF THIS ANIMAL AT THE PRESENT

TIME*

ACCOROING TO ENOERS (1 9 3 2 )i OVULATION IN THE MINK OCCURS APPROX­

IMATELY FROM 36 TO 39 HOURS POST-CO ITUM AND THC FGG IS FERTILIZED IN

THE OVIDUCT. THE SEGMENTING EGG TRAVERSES THE TUSO-UTCRINC JUNCTION

INTO THC ANTERIOR PORTION OF THE UTERINE HORN. As LONG AS NINETEEN DAYS

FOLLOWING COPULATION, BLASTOCYSTS HAVE BEEN FOUND RESTING IN A STATE OF

ARRESTED DEVELOPMENT IN THC ANTERIOR PORTION OF THE UTERINE HORN, AND

THE GREATEST NUMBER OF CELLS IN ONE UN IMPLANTED BLASTOCYST HAS BEEN

COUNTED AS ^ .

Probably under control of the p it u it a r y gland , the m ig r a t io n ano

ACCELERATED GROWTH OF THE ALREADY SPACED BLASTOCYSTS TAKE PLACE WITHIN

THC FIRST TWO WEEKS OF APRIL. THE WELL DEFINED ZONA PELLUCIDA WHICH

SURROUNDS THE BLASTOCYST IS LOST AND A CENTRAL OR SUPERFICIAL TYPE OF

IMPLANTATION TAKES PLACE.

The uterus of the mink is a bicornuatc uterus. There is a re­ m arkable CHANGE IN BOTH THE SIZE AND COLOR OF THIS ORGAN AS THE MINK

PROGRESSES FROM A NON-PAROUS TO A PAROUS CONDITION. RESPECTIVELY, THE

CHANGE IS FROM A WHITE THREAD-LIKE TUBE TO ONE OF GREATER THICKHES8 AND

DEEP RED.

THE AUTHOR HAS CHOSEN TO FOLLOW THE INNOVATION OF STREETER

(1930) WHO SEGREGATED HUMAN EMBRYOS INTO "GROUPS OR PERIODS THAT

REPRESENT LEVELS IN THEIR STRUCTURAL ORGANIZATION AS A WHOLE*', RATHER

THAN ON THC BASIS OF AGE, SIZE, OR NUMBER OF SOMITES. THESE GROUPS OR PERI00S ARC RCPCRRCO TO A t HORIZONS. INSOFAR AS APPLICABLE, THC PRESENT

STUDY PRESENTS SUCCCSSIVC HORIZONS IN MINK EMBRYOLOGY SASCO UPON CRITCRIA

USCD SY STRCCTCR IN STAQING HUMAN CMSRYOS. STREETER'S HORIZONS ARC SCT

FORTH IN ASSRCVIATCD FASHION IN THC FOLLOWING LISTS

As s r c v ia tc o l is t in g of Strcctc r' s " h o r izo n s " OF HUMAN CMSRYOS*

1. Onc c c llc d cg g.

2 . SCOMCNTING CGG.

3* FRCC SLASTOCYST.

IMPLANTING OVUM.

5* Ovum im p la n t e d , sut s t il l a v il l o u s .

6 . Primitive v illi, distinct yolk sac.

7* B ranching v i l l i , a x is of gcrm d is c d c f in c d .

8 . Henson's node, prim itivc groovc.

9 . Neural fo ld s , elongated notochord.

10. E a r ly s o m it e s .

11. Form atio n ano closurc of a n te r io r ncuroporc, o t ic invagination ,

FRCSCNCC OF TWO FAIRS OF BRANCHIAL SARS (MANDISULAR AND HYOIO).

12. Closure of p o s te r io r neuroforc , thrcc f a ir s of b r a n c h ia l b a r s .

13. Arm bud ridge, leg bud, prominent heart.

1 ^ . I nvagination of o p t ic l e n s .

15. Lens vesicles closed, olfactory placodes submerging as oval

DEPRESSIONS, ARM BUD OIVISIBLE INTO DISTAL NANO SEGMENT AND

PROXIMAL ARM AND SHOULDER SEGMENT.

16. N ostrils with overhanging borders, eyes show dark tinge due to

EARLY RETINAL PIGMENT. HAND REGION DIFFERENT!ATCD INTOCARPUS

AND DIGITAL PLATE. Head r e l a t iv e l y lar g er , m a in a x is o r the trunk s t r a ig h t e n , o l ­ factory F IT MOVED MEDIALLY AND VENTRALLY, DISTINCT NASO-FRONTAL

GROOVES, LIMS SUDS EXHIBIT DIGITAL RAYS IN HANO FLATE*

E mbryo c u s o id a l in s h a fe , extremities longer and d ig it a l rays

Or THE HAND DEFINITELY NOTCHEO, EYELID FOLDS FRESENT IN OLDER

SPECIMENS, DISTINCT TIF OF NOSE SEEN IN PROFILE.

E x t r e m it ie s extend near ly s t r a ig h t forward , toe rays p ro m inent

BUT NO INTERDIGITAL NOTCHES, PRIMITIVE CORNEAL BODY, SMALL

SLENDER OPTIC NERVE, TIF OF COCHLEA TURNS UP, BEGINNING FORMA­

TION OF RENAL VESICLES.

Arms in cr eased in length and s l ig h t l y ben t at elbows , hands

WITH SHORT STUBBY FINGERS, APPEARANCE OF DELICATE, FRINGE-

LIKE VASCULAR PLEXUS IN SUPERFICIAL TISSUE Or HEAD, HYPOPHYSIS

POSSESSES LONG SLENDER STALK.

F ing ers longer w it h swollen t e r m in a l ph alan g es , hanos s l ig h t l y

FLEXED AT WRISTS, CORNEAL BODY IS A COMPACT MESODERMAL SHEET -

2 TO 5 CELLS THICK, RETURN DOWN CURVE OF COCHLEA, HYPOPHYSIS

HAS THREAD-LIKE STALK - BEGINNING ABSORPTION.

B e g in n in g or eyelid closure, tragus and antitragus assuming de­

f i n i t e FORM, DESCEMET'S ENDOTHELIUM, SHEATH LAYER BEGINNING TO

FORM ON OPTIC NERVE, REMNANT OF INCOMPLETE HYPOTHYSEAL STALK

AT EITHER END, A FEW LARGE GLOMERULI.

Th ic k , f ib r o u s corneal body , ear ly o p t ic nerve sheath , cochlear

TIP TURNS UP A SECOND TIME, LONG SECRETORY TUBULES IN KIDNEY. MATERIALS AND METHODS

Ad u lt m in k used in t h is investigation vcrc r c c c iv c o as o o n a t io n s

FROM NUMEROUS MINK RANCHERS IN THC STATE OF OHIO. I n VIEW OF THIS FACT

THERE HAS SEEN NO ATTEMPT TO L IM IT THE DEVELOPMENTAL DESCRIPTION CON­

TAINED IN THIS WORK TO ANY ONE OF THC NUMEROUS MUTANTS OF MUSTCLA

VISON.

A TOTAL OF TWENTY-FIVE FEMALE MINK WERE BRED DURING MARCH IN

CITHER 1954 OR OF THESE/ l 6 FEMALES WERE BRED TWICE/ THC RE­

MAINDER ONLY ONCE. IN THE FORMER GROUP THC SHORTEST INTERVAL BETWEEN

MATINGS WAS 8 DAYS/ THC LONGEST WAS 12 DAYS. THE EARLIEST MATING OC­

CURRED on M arch 8 ; the last, on March 26.

An im a l s from w h ic h u t e r in e horns were to be removed were

ANESTHETIZED BY INJECTIONS OF NEMBUTAL SODIUM SOLUTION (6 0 MG. PER

CC.) IN 0.35 CC. DOSAGES ADMINISTERED PER I TONEALLY. SURGICAL HAIR

CLIPPERS WERE USED TO REMOVE THC FUR FROM THC VENTRAL PELVIC REGION.

A 1 3/4 INCH INCISION/ PERFORMCO SLIGHTLY TO ONE SIDE OF MIO-LINC,

PROVIDED SUFFICIENT EXPOSURE OF THE VISCERA. NEXT/ IT WAS NECESSARY

TO SUTURE THC LARGER BLOOD VESSELS WHICH COURSE THROUGH THC BROAD

LIGAMCNTARY ATTACHMENT OF THC UTERINE HORN TO THC BODY WALL. THESE

VESSELS, WHICH ENLARGE CONSIDERABLY DURING PREGNANCY, ARC EASILY

VIS IB LE AND ACCESSIBLE EVEN BEFORE IMPLANTATION-SITC SWELLINGS IN THE

HORN ARE EVIDENT. A FINAL SUTURE WAS APPLIED TO THC MOST CAUDAL POR­

TION OF THC HORN TO BE REMOVED, CLOSE TO ITS CONNECTION WITH THE BOOY

OF THC UTERUS. THE HORN WAS THEN TRANSECTED CRANIAO TO THC LAST MEN­

TIONED SUTURE AND THE CUT WAS CONTINUED ANTERIORLY THROUGH THC BROAD LIGAMENT. WITH THE SEVERANCE OP THC SUSPENSORY LIGAMENT, THC

UTERINE HORN WAS REMOVED FROM THC SODY CAVITY, DIVIDED INTO SEGMENTS

OP DESIRED SIZE AND PLACED INTO BO U IN 'S F IX A T IV E . In THE PEW INSTANCES

OP PROFUSE BLEEDING, PERIODIC SWABBINGS WITH THROMBOZYMC-SOAKCD COTTON

EFFECT IVCLY COAGULATED THE BLOOO. DURING THE COURSE OF THC OPERATIONS

STRICT ASEPTIC PRACTICES WERE FOUND TO BE QUITE UNNECESSARY ANO WERE

CONFINED TO BATHING THC OPERATIVE INSTRUMENTS AND HANDS IN 8 ^ ^ ALCOHOL.

I t MIGHT BE WELL TO INSERT HERE THE OBSERVATION THAT THESE ANIMALS ARC

REMARKABLE IN THEIR STAMINA OURING SURGERY ANO ALSO IN THEIR RECUPER­

ATIVE POWERS.

Fo l l o w in g the rem o val of the u t e r in e horn the in c is e d a b o o m in a l

WALL WAS SEWN WITH SURGICAL THREAD AND THE CUT SURFACES OF THC SKIN

WERE DRAWN TOGETHER WITH METAL CLAMPS. IN THC PAST SOME DIFFICULTY

HAS BEEN EXPERIENCED IN MAINTAINING SURGICAL CLOSURE WHEN THREAD OR

CATGUT WAS USED FOR THAT PURPOSE (SCHACKELFORD 1 9 5 2 )* THE ANIMALS

E XH IBIT A TENDENCY TO CLAW AND BITE THE OPERATION SITE THEREBY RE­

OPENING THC WOUND. T here was no e v id e n c e of t h is t y p e of BEHAVIOR

WHEN CLAMPS WERE USED AS THE AGENTS OF CLOSURE.

Af t e r a p e r io d of t im e , th e e x t e n t of w h ic h was d e t e r m in e d b y

WHAT WAS FOUND IN THE FIRST HORN TO BE REMOVED, THE ANIMAL WAS SACRI­

FICED BY INJECTING AN OVERDOSE OF NEMBUTAL. THE CLAMPS WERE REMOVED,

THE BODY CAVITY RE-ENTERED AND THE REMAINING UTERINE HORN QUICKLY RE­

MOVED AND PLACED IN BOUIN'S FIXATIVE.

T he p r a c t ic e e m p lo y e d in t h is work of b r e e d in g each f e m a le

TWICE (EXCEPT IN THE INSTANCES ALREADY CITCO) HAS NO SIGNIFICANCE

OTHER THAN TO INCREASE THE POTENTIAL OF SUCCESSFUL BREEDINGS AND SUBSEQUENT IMPLANTATIONS. THE REAOER IS REPERREO TO PAPERS BY HANSSON

( 1947 ) ano Johansson (1951) in which the conclusion is orawn that

"SUPERFCTATION IS THC RULE RATHER THAN AN EXCEPTION IN THE MINK WHEN

A SECOND OVULATION IS INDUCED BY RCMATINO.” THE SHORTEST PERIOD OP

TIME BETWEEN A BREEDING DATE AND THC RECOVERY OP UTERINE TISSUE PROM

A GIVEN FEMALE WAS 13 OAYS WHILE THC LONGEST INTERVAL WAS 4^ DAYS.

I n THOSE CASES WHERE UTERINE HORN SWELLINGS WERE NOT PRESENT,

THC HORN WAS TRANSECTED INTO SEGMENTS, THC NUMBER OP WHICH WAS DETER­

MINED BY THC TOTAL LENGTH OP THC HORN. ON THC OTHER HAND, WHEN IM­

PLANTATION SITES WERE EVIDENT, THC HORN WAS TRANSECTED SO THAT ONE IM­

PLANTATION SITE WAS ENTIRELY CONTAINCD WITHIN ONE SEGMENT.

A l l TISSUES WERE REMOVED PROM th e B o u in 's SOLUTION AFTER 24

HOURS OF FIXATION AND STORED IN J O )t ALCOHOL. THE TISSUES WERE IM­

BEDDED IN PARAFFIN AFTER TREATMENT IN THE REGULAR ALCOHOL SERIES.

The m a t e r ia l was se c tio n e d 20 m icro ns in th ic k n e s s and whenever pos ­ s ib l e THESE SECTIONS WERE MOUNTED IN SERIAL ORDER. ALL TISSUES WERE

STAINED WITH EHRLICH' s GLYCERINC-ALUM HACMATOXYLIN AND COUNTER­

STAINED WITH EOS IN AFTER WHICH THEY WERE MOUNTED IN PlCCOLITC. ALL

SECTIONS WERE MICROSCOPICALLY EXAMINED FOR THE PRESENCE OP BLASTO­

CYSTS ANO/ OR EMBRYOS.

Table I l is t s the total number of embryos recovered , s e c t io n ­ ed , AND IN A CONDITION SUITABLE FOR INCLUSION IN THIS WORK. THE

CMSRYOS ARC ARRANGED IN A DEVELOPMENTAL SEQUENCE AND ADDITIONAL IN­

FORMATION IS INCLUDED SUCH ASj

A) THC UTERINE HORN (RIGHT OR LEFT) FROM WHICH

THE EMBRYOS WERE RECOVERED, s ) THC SEGMENT (ANTERIOR, MEOIAN, POSTERIOR) OP

THC UTERINE HORN PROM WHICH THE CMSRYOS WERE

RECOVERED,

C) THC BREEDINQ DATES,

O) RECOVERY DATES.

Macro- and micro-photography as w ell as camera cue i da draw ­ in g s WERE EMPLOYED TO ILLUSTRATE SIGNIFICANT ASPECTS AND STAGES OP

DEVELOPMENT. THESE TECHNIQUES, UNFORTUNATELY, WERE NOT PERFORMED

UPON PRESH MATERIAL SUT RATHER AFTER THE EMBRYOS HAD BEEN FIXED.

Although the author rec o g n izes the tendency of t is s u e s to undergo

MARKED ALTERATIONS IN SIZE AND TO A LESSER EXTENT IN STRUCTURAL

APPEARANCES AFTER FIXATION, IT IS ALSO THE CASE THAT ALL TISSUES

PHOTOGRAPHED OR DRAWN WERE TREATED PREVIOUSLY IN AN IDENTICAL MAN­

NER AND THEREFORE, THE IMPORTANCE OF THIS PROCEDURE AS AN UN­

DESIRABLE VARIABLE IN THE RESULTS IS MINIMIZED. 9

IABLCJ.

E moryos Recovered , L is t e d ay Ho r iz o n * w it h I ntrauterine Lo c a tio n and Aoe at Recovery .

Ho r izo n No.or N*nch IJt e r .H orn Horn Seoment Breed . .Reco v . Br eed in g Sp e c . No . R -R iq h t A-A n t e r io r Dates Oa tes Recovery L-L e ft M-Me d ia n I n te r v a l * P-P o s te r io r in Days

I None II 1 5? R A 3/16 4/7 22 III 2 6 R - 3/14-3/23 4/io 18-27 8 R - 3/ 15- 3/23 4/13 21-29 IV 1 59 RA 3/16 4/7 22 V-VIII None IX 1 20 L M 3/ 13- 3/22 4/21 30-39 X 5 20 L A 3/13-3/22 4/21 30-39 7 R A-M-P 3/ 15- 3/23 V i 3 21-29 13 R m 3/20 Vi 5 26 XI 2 12 R A-M 3/19 V i 9 31 XI1 2 84 L M-P 3/;i0 - 3/ i § 4/20 33-J*i XIII 4 84 L A 3/10-3/18 V20 33- 41 6 L A-M-P 3/14-3/23 4/ i 8 26-35 XIV 6 9 L A-M-P 3/16 4/ i 4 29 16 RM 3/23 4/16 24 7 L A-P 3/15-3/23 V I7 25-33 XV 3 15 R A-M 3/21 4/16 26 13 L P 3/20 4/20 31 XVI 2 11 L A-M 3/19 4/19 31 XVII 2 8 L A-P 3/15-3/23 4/22 30-38 XVIII 4 16 L A-M-P 3/23 4/22 30 87 L A 3/17 V19 33 DESCRIPTION OF DEVELOPMENTAL STAGES

(T he reader is referred to Table II for a summary of FEATURES NOT ONLY OF THIS HORIZON, BUT OF ALL THE HORIZONS OESCRI BED IN THIS WORK).

Ho r izo n I

The embryology of any form l o g ic a l l y b e g in s w it h the zygote

PHASE OF DEVELOPMENT. ALTHOUGH NO SPECIMENS OF THIS AGE GROUP WERE

OBTAINED FOR STUDY IN THIS WORK THERE IS , FORTUNATELY, THC WORK OF

ENOERS (1938) WHICH DEALS WITH THC OVUM OF MINK AND WHICH IN PART IS

QUOTED HERE. ENDCRS RECOVERED THREE TUBAL OVA WHICH WERE PHOTOGRAPHED

IN THE LIVING CONDITION. THE FOLLOWING IS ENDCRS' ACCOUNT!

"The ovum was free of fo llic le cells. The zona pcllucida was

SHARPLY DEFINED, RELATIVELY NARROW THE EGG PROPER, OR VITELLUS,

DID NOT FILL THE ZONA PCLLUCIDA. EACH EGG HAD A PER IV I TELL INC SPACE

FILLED WITH GRANULES. THE CELL MASS IS VERY DARK BECAUSE &LOBULCS OF

LIPOID MATERIAL ARE DISTRIBUTED THROUGHOUT THE CYTOPLASM THERE IS

NO TENDENCY TOWARD POLAR ACCUMULATION IN ALL PROBABILITY THC EGGS

HAD SEEN FERTILIZED, FOR SPERM HEADS WERE OBSERVED IN EACH." THE

AVERAGE TOTAL DIAMETER OF THC THREE TUBAL OVA MENTIONED ABOVE WAS

1^7 MICRONS, THE DIAMETER OF THE CYTOPLASMIC MASSj VITELLUS WAS 104

MICRONS, AND THC AVERAGE THICKNESS OF THC ZONA PCLLUCIDA WAS APPROX­

IMATELY 11 MICRONS.

Ma in l a n d ( 1 9 3 0 f in d s s im il a r features w it h respect to the ovum

OF THE FERRET. "THE CYTOPLASM CONSISTS CHIEFLY OF A MASS OF LIPO ID -

BEARING MATERIAL, THIS MASS BCG IN SURROUNDED BY A GRANULAR ZONE WITH

10 LITTLE LIPO10 . " HOWEVER* MAINLAND STATES THAT OCPINITC NOLAN ITT OP

THC CYTOPLASM WAS CSTASLISHCD IN 29 OUT OP 100 OVA. IN A SUSSCQUCNT work* Mainland (1932) examined pcnrct ova, bcqinhino with thc

SECOND POLAN SPINDLE STAGE TO THAT OP THC PIRST SEGMENTATION SPINDLE.

Thc disappcarancc op thc zona oranulosa could sc dctcctco as thc ova

ADVANCCD PROM TNC PIRST TO THC LAST OP THCSC STAGES. THC MAJORITY OP

THC OVA SHOWCD RADIAL STRIAE IN THC ZONA PCLLUCIDA* SUT THCRC WCRC

ONLY IN ITS OUTCRMOST PART CXCCPT IN THOSE OVA WHCRC CCLLS OP THC

COROHA RADIATA DEMONSTRABLY PENETRATED PAR INTO OR THROUGH THE ZONA p c l l u c io a . There appears to sc a remarkable dippcrcncc between m in k

AND PCRRET OVA WITH RCSPECT TO THC THICKENING OP THC ZONA PCLLUCIOA.

£nDCRS* MCASURCMCNT OP THC THICKNCSS OP THC ZONA PCLLUCIDA OP MINK

OVA WAS APPROXIMATELY 11 MICRONS* WHILC MAINLAND'S PIOURC POR THC

SAMC STRUCTURE IN THC PCRRET WAS "ROUGHLY **-6 MICRONS." MAINLAND

CONCLUDES THAT THC SPHERICAL OR ELLIPSOIDAL BODIES POUND IN THC ZONA

PCLLUCIDA WCRC PRODUCTS OP THC ZONA GRANULOSA* ARISING IN THC SAMC

WAY AS DID PART, AT LEAST OP THC ZONA PCLLUCIDA* AND THAT THC SODICS

WCRC ASSORSCD BY THC OVUM. BOTH MAINLAND AND ENOCRS AGREE THAT THC

PCRIVITCLLINE SPACES OP THC PCRRCT ANO MINK OVA* RCSPECTIVCLY* CON­

TAINED SOME GRANULAR MATCRIAL.

Ha m ilt o n (193*0* also r epo r tin g on the eggs op pcrrct* made

THE POLLOWING OBSERVATION ON AN UNPCRTILIZED OVUM! THE CGG

"...O STAIN C D PROM AN ANIMAL 4 l HOURS APTCR INSCMINAT ION* THC WHOLE

SPERM* HEAD* AND TAIL WAS SEEN IN PROFILE IN THC PCRIVITCLLINC

SPACE* SO PERTILIZATION HAD NOT TAKEN PLACE. THE PCMALC PRONUCLCUS

APPEARED AS A UNIFORMLY DARKLY STAINED KIDNEY-SHAPED MASS AT THC PERIPHERY OP THC OVUM. A PEW OP THE SPINDLE PISERB OP THC SCCONO

MATURATION OIVISION WCRC STILL VISIBLE, ANO A SMALL INTERMEDIATE BODY

WAS PRESENT ALSO.. • .TkCRC WAS NO POLARITY OP THC CYTOPLASM OP THC

OVUM." IN ANOTHER CASE, AN CGG REMOVED PROM THC ANIMAL HOURS APTCR

INSEMINATION, ALSO WAS STUDIED. IN THIS INSTANCE, WFCRTILIZATION HAD

OCCURRED AND THC ENTIRE SPERM WAS POUND IN THC CYTOPLASM OP THE £ 0 6 ."

The cytoplasm op t h is ego e x h ib it s p o l a r it y and the polar b o o ic s have

SEPARATED AT THC OEUTOPLA8 MIC POLE. A SUPERNUMERARY SPERM WAS OB­

SERVED IN THC PER IVITELL INC SPACE.

HAMLCTT (1 9 32 ) PRESENTS THC POLLOWING DATA CONCERNING THC DES­

CRIPTION OP A PULL-GROWN OVARIAN EGG OP THE BADGER, PRIOR TO THC PIRST

MATURATION DIVISIO N : "THE EGG IS ELLIPTICAL IN SECTION, MEASURING

114 BY 150 M ICRONS....A THIRD DIMENSION OP ABOUT 100 MICRONS.. . .

AROUND THC OVUM *... IS A THICK, DENSE, HOMOGENEOUS ZONA PCLLUCIDA...

RANGING BETWEEN 3 , 6 , AND 5 MICRONS* (lN THICKNESS). 13

Ho r izo n I I

Only a singlc example or a cleavage stage in Horizon II was

OSTAINEO IN THIS PRESENT STUDY ( r iG . 1 )*

The blasto cyst is s m a ll , e x h ib it in g a blasto co el but la c k in g

AN INNER CELL MASS. THE ZONA PELLUCIDA IS PRESENT ANO IS CLOSELY AP­

PLIED TO THC CELLS OF THC TROPHOBLAST BENEATH IT . ON THE BASIS OF

THIS SINGLC SPECIMEN IT IS IMPOSSIBLE TO ESTABLISH WITH CERTAINTY

THC NATURE OF THC BLASTOMCRCS AND PATTERN OF CLEAVAGE. IF ONE CON­

SIDERS ENDCRS* DESCRIPTION OF THE ZYGOTO TOGETHER WITH THC STRUCTURAL

CHARACTERISTICS OF THE BLASTOCYST CONTAINED IN THIS WORK, A PROBABLE

SUPPOSITION IS THAT THC MINK EMBRYO CLEAVES IN A TYPICALLY MAMMALIAN

FASHION.

Hamilton (193*0* reporting on the two-cell stage o r the f e r ­ r e t EMBRYO, STATES THAT, ALTHOUGH THE FIRST TWO BLASTOMCRCS APPEAR

TO BE OF DIFFERENT SIZES, SUCH DIFFERENCES ARC NOT SIGNIFICANT.

There was considerable v a r ia t io n in the t im e at w hich the two- c e ll

STAGE APPEARS. A THREE-CELL STAGE IS REPORTED IN THC FERRET. "WHEN

THC VOLUMES OF THE CELLS AT THE FOUR-CELL STAGE ARC EXAMINED, ONE

CELL IN EACH EGG IS FOUND TO BE SMALLER THAN THC OTHERS, A POINT

STATISTICALLY TO BE SIGNIFICANT. OWING TO THE FACT THAT THC D IV I­

SIONS ARE NOT SYNCHRONOUS AT THC FOUR-, EIGHT-, AND SIXTEEN-CELL

STAGES, INTERMEDIATE STAGES WITH ODD NUMBERS OF CELLS OCCUR. At

THE SIXTEEN-CELL STAGE ONE CELL IS CENTRALLY PLACED....AT THC STAGE

OF MORE THAN SIXTEEN CELLS A CAVITY APPEARS IN THE MORULA.. . .THE

CENTRAL CELL OF THC SIXTEEN-CELL STAGE MULTIPLIES TO FORM THC INNER « CELL MASS." 14

Thc observations or H a m ilto n ( 193*0 on the two-cell stage or

THE rERRET EMBRYO IN ONE SENSE SUPPORTS THC PINDINGS OP HlLL (1 9 24 )

CONCERNING THE SAMC STAGE o r THC CAT EMBRYO. HAMILTON ANO H lLL RE­

PORT OirrCRCNCES IN THE SIZES OF THC TWO BLASTOMCRCS. HOWEVER, WHILC

Ha m ilt o n co nsio ers the m a t e r ia l homogeneous , Hi l l , reports a h ig h e r

CONCENTRATION o r FAT GLOBULES IN THE LARGER BLASTOMCRC. THE TWO

DIVISIONS OF THC SECOND CLEAVAGE ARE NOT SYNCHRONOUS BUT SUCCESSIVE,

PRODUCING A THREC-CELL STAGE. THE BLASTOMCRCS OF THC FOUR-CELL

STAGE BCCOMC ARRANGED AS TO FOAM AN INTERLOCKING CROSS-SHAPED CROUP.

TWO CONSTITUENT PARTS OF THE BLASTOCYST, THE INNER CELL MASS AND

TROPHOBLAST ARE CLEARLY DISTINGUISHABLE IN THE SIXTEEN-CELL STAGE. 15

Ho r IZON I 11

Two EMBRYOS WERE SELECTED TO REPRESENT THE PRESENT AGE GROUP.

They were 6l and 8 r .

The free blasto cyst is found l y in g in the m a in uterus c a v it y .

E ven at t h is stage in development the blasto cyst is d e f in it e l y p o la r ­ iz e d WITH RESPECT TO THC RELATIVE POSITIONS OF THE INNER CELL MASS

AND THE MESOMETRIUM. I n ALL SPECIMENS EXAMINED, THC INNER CELL MASS

AND LATER THC EMBRYONIC DISC ARE ANT IMESOMETRIAL•

The sm allest inner c ell mass ( f i g . 2) exam ined was oval in

SHAPE. T he ENTODERM IS a DISTINCT LAYER of CELLS WHICH COMPLETELY

LINES THE INNER CELL MASS AND CONTINUES A SHORT DISTANCE BEYOND ITS

MARGIN BUT DOES NOT EXTEND ENTIRELY BENEATH THC TROPHOBLAST CELLS.

The entodcrmal c e lls arc s p ih o l e - shaped , w id e ly spaced , and each

POSSESSES A SINGLE OVAL NUCLEUS. THE NUCLEI OF THE SPHERICAL INNER

MASS CELLS, ON THE OTHER HANO, ARC LARGE, SPHERICAL, MULT I-NUCLCO-

LATED AND RESEMBLE THOSE OF THE TROPHOBLAST. THE TROPHOBLAST AP­

PEARS TO COVER THC INNER CELL MASS AS A CONTINUOUS LAYER. THE ZONA

PCLLUCIDA IS PRESENT ALTHOUGH DAMAGED IN THIS SPECIMEN* REMARKABLE

CHANGES HAVE TAKEN PLACE IN THE APPEARANCE OF THC UTERINE HORN FOLDS.

|N THE ANC8 TROUS UTERINE HORN THC ENDOMETRIUM IS HIGHLY FOLDCO, THC

LUMEN ALMOST OBLITERATED. JUST PRIOR TO IMPLANTATION, PROBABLY IN

RESPONSE TO A STIMULUS PRODUCED BY THC PRESENCE OF THC BLASTOCYST,

THC ENDOMETRIAL FOLDS, IN THC AREA OF THC BLASTOCYST, DIMINISHES

IN LENGTH ANO, IT WOULD SEEM, BECOME INDENTED AT THEIR APICES

FORMING A SCRIES OF SMOOTH, CURVED SURFACES SURROUNDING THE BLASTO­

CYST ( f i g . 3 ). These implantation crescents, as they might be 16

CALLED; DCCOME MORE AND MORE EXPANSIVE AS THC SLASTOCYST ENLARGES.

Another specimen demonstrates a pronounced thickening o r th e

CPIBLAST LAYER OP THC INNER CELL MASS. IT SHOULD BE NOTED HERE THAT

THC INNER CELL MASS O r THC MINK BLASTOCYST IS RELATIVELY SMALL; NEVER

BECOMING AS PRONOUNCED A FEATURE AS IN THC CASE OP THC P IG ; RABBIT;

MAN; AND MONKEY (NELSON; 1 9 5 3 )* THERE APPEARS TO BE AN ABSENCE OP

TROPHOBLAST CELLS (R a UBCR'S LAYER) OVERLYING THC INNER CELL MASS.

T here is a ls o e v id e n c e o r f u r t h e r proliferation op cnto dcrm al c e l l s

ON A WIDELY SPACED MANNER; UNDERLYING THE TROPHOBLAST. IT CANNOT

SC DETERMINED IN THIS SPECIMEN IP CNTODCRMAL CELL PROLIFERATION HAS

PROGRESSED TO THC POINT AT WHICH THE BLASTOCYST MIGHT BE DCSCRIBCO

AS TRULY BILAM INAR. IN THIS SPECIMEN THE ZONA PCLLUCIDA HAS SEEN

ALMOST COMPLETELY TORN PROM THC BLASTOCYST; PROBABLY DURING SECTION­

ING; BUT PORTIONS OP IT REMAIN SCATTERED W ITHIN TNE LUMEN OP THE

UTERINE HORN.

I n AN OLDER SPECIMEN ( p i g . 4 ) IT WAS NOTED THAT THC INNER

CELL MASS; OR EMBRYONIC DISC AS IT MAY NOW BE CALLED; HAS ENLARGED

PROPORTIONATELY TO A GREATER EXTENT IN LENGTH ANO WIOTN THAN IN

THICKNESS. THE RESULT IS THE DEVELOPMENT OP A BILAMINAR "FORMATIVE a r e a " OP THC BLASTOCYST. THE CPIBLAST IS ONE OR TWO-CELLS IN THICK­

NESS WHILE THE HYPOBLAST IB A MONOCELLULAR LAYER. THIS FIGURE

CLEARLY SHOWS THC THIN ZONA PCLLUCIDA WHICH APPEARS TO BE T R I-

LAMINATC COVERING THC CPIBLAST LAYER OP CELLS. THE EXTRA-

EMBRYONIC PORTION OP THC BLASTOCYST IS NOT COMPLETELY BI LAMINAR

AT THIS STAGE.

E ndcrs ano P ear so n (1 9 ^ 3 ) r e p o r t in g on the blastocysts op THE FISHER* STATE THAT THE BLASTOCYSTS ARE "SIMILAR IN GENERAL AP­

PEARANCE TO THOSE OP THE MARTEN* ALTHOUGH SLIGHTLY LARGER*. . .THE MOST

CONSPICUOUS FEATURE IS THE THICK SURROUNDING ZONA PELLUCIDA WHICH

AVERAGES l4 . 4 MICRA IN THICKNESS.* .CONSIDERABLY THICKER THAN IN THE

MARTEN* WEASEL OR SAOGER* * * .THE INNER CELL MASS IS LESS COMPACT THAN

IN THE MARTEN* AND IS COMPOSED OP A GREATER NUMBER OP CELLS. THC IN­

NER CELL MASS OP TWO OP THC BLASTOCYSTS PROM THE JANUARY SPECIMEN CON­

TAINED 798 ANO 807 n u c le i. T h is compares WITH ABOUT 350 IN th e mar­ te n * 150 IN THC WEASEL AND 50 IN THE BADGER." An UNIMPLANTED BLASTO­

CYST OP THC FISHER RECOVERED IN FEBRUARY WAS POUND TO CONTAIN AS MANY

AS 844 NUCLEI.

AS FOR THC BLASTOCYST OP THE PCRRCT (HAMILTON 193*0 »TAT£S THAT

"AFTER ITS FORMATION IT INCREASES IN SIZE VERY RAPIDLY* AND THC WELL-

KNOWN CONDITIONS POUND IN CERTAIN OTHER MAMMALS ARC REALIZED* V IZ . THC

TROPHOBLAST BECOMES ATTENUATED* THC INNER CELL MASS BECOMES LENTI­

CULAR AND LATER FORMS THC EMBRYONIC DISC. THE CENTRAL CELL MASS IS

DEFINITELY ATTACHED AT ONE POINT TO TROPHOBLAST* ANO THE DISAP­

PEARANCE OP THE COVERING TROPHOBLAST OR (TAUBER'S LAYER TAKES PLACE

RELATIVELY EARLY IN THC TERRCT.” THE LIVING BLASTOCYST PILLS THC

LUMEN OP THC UTERINE HORN AT THC POINT OF ITS POSITION AND IS SPHER­

ICAL. Fla tte n e d cntodcrmal c e lls a r is e prom the convex surface op

THE EMBRYONIC DISC AS A CONTINUOUS LAYER. AFTER THC ESTABLISHMENT

OP EMBRYONIC DISC POLARITY; THC PLAT CNTOOCRMAL CELLS AT THC AN­

TERIOR END OP THC DISC BECOME COLUMNAR FORMING THC SO-CALLED

"PRECHORDAL PLATE."

I n a b r ie f paper concerning the blasto cyst op thc dog* \8

VAN OCR STRICHT {1 9 ^3 ) DIVIDES THC SLASTOCYST PHASE OP EMBRYONIC DE­

VELOPMENT INTO POUR STAOCS. IN THC PIRST STASC THC TROPHOBLAST IS A

UN I LAMINAR LAYER, THC INNCR SURPACC OP WHICH IS ATTACHED TO THC IN­

NER CELL MASS. T h is INNER CELL MASS PROJECTS INTO THC PLUIO OP THC

BLASTODCRMIC CAVITY AND; ACCORDING TO VAN OCR STRICHT, GRADUALLY

PLATTCNS OUT AS A RESULT OP THC INCREASINGLY HIGH PRCSSURC OP THC

PLUID. T hc blasto cyst is u n il a m in a r in it s extra - em bryonic region

AND BI LAMINAR IN ITS PORMATIVE RCQION. In THC SCCOND STAGE, THC FOR-

MATIVC ARCA EVENTUALLY BCCOMC8 UNILAMINAR, THIS DCVELOPMCNT AS WCLL

AS THE DISAPPEARANCE OP RaUBCR'S LAYER OVERLYING THC PORMATIVE ARCA

BCING ATTRIBUTED TO THE INCREASING PRCSSURC OP THC BLASTOCOCL PLUID.

Thc t h ir d stage is CHARACTERIZED BY the appearance op hypoblast ce lls

WHICH APPEAR TO BE COMPRESSED TOWARD THC INNCR SURPACC OP THC FLAT­

TENING EMBRYONIC DISC* SOME OP THCSC CELLS EXTEND TO THE INNER SUR­

FACE OP THC NEIGHBORING TROPHOBLAST, BUT THEIR COMPLETE DCLAMI NAT ION

PROM THC EMBRYONIC DISC HAS NOT YET OCCURRED. IN THE FOURTH AND

PINAL STAGE OP THC DCVELOPMCNT OP THE BLASTOCYST, VAN DCR STRICHT

REPORTS A COMPLETE SEPARATION OP THC EPIBLAST AND HYPOBLAST LAYCRS

OF THC FORMATIVE ARCA. T hC HYPOBLAST CELLS ARC FLATTENED BUT TO BE­

COME COLUMNAR IN THC FUTURE CEPHALIC HALF OP THC FORMATIVE ARCA WHICH

MAY THEN BE CONSIDERED AS POSSESSING BI LATERAL SYMMETRY. THE HYPO­

BLAST CELLS CONTINUE TO EXTEND THC LIM IT OP THEIR GROWTH ALONG THC

INNER SURPACC OP THC TROPHOBLAST WHICH EVENTUALLY BECOMES ENTIRELY

SI LAM!NAR* H lLL (1924) REPORTS WITH ONE EXCEPTION A STRIKING SIM­

ILARITY BETWEEN THC CAT ANO THE DOG IN THC DEVELOPMENT OP THC BLASTO­

CYST. Th is v a r ia t io n p e r t a in s to the intercalation op the em bryonic OI»C WITH THE TROPHOBLAST ANO THE FORMAT ION Of THE HYPOBLAST. |T WILL

BE RECALLED THAT IN THE DOG, VAN OER STRICHT REPORTS THAT THE EM­

BRYONIC DISC FLATTENS ANO BECOMES INTERCALATED WITH THE TROPHOBLAST

L0N8 BEFORE THC APPEARANCE OF HYPOBLAST CLEMENTS. H lLL* ON THE OTHER

HANO* MAINTAINS THAT IN THC CAT THE EMBRYONIC DISC FIRST UNDERDOES

DIFFERENTIATION INTO EPIBLAST AND HYPOBLAST LAYERS) THIS BE INO FOL­

LOWED BY INTERCALATION WITH THC TROPHOBLAST WITHOUT PRELIMINARY FLAT­

TENING.

On THC BASIS OF MATERIAL CONTAINED IN THIS WORE) THC HYPOBLAST

CLEMENTS IN MINK APPEAR BELOW THC CPIBLAST LAYER OF THE INNCR CELL

MASS BEFORE THC FLATTENED EMBRYONIC DISC IS FORMED. IN THIS RESPECT

THC MINK MORE CLOSELY RESEMBLES THE CAT THAN THC DOG* 2 0

Ho r izo n IV

One embryo (5?RA) was found to represent the present aoe

GROUP.

The most conspicuous characteristic of t h is age group is the

DEVELOPMENT OF THE MESOOCRM. THE GASTRULATION PROCESS WHICH SEGAN

IN THE PRECEDING HORIZON WITH THE FORMATION OF THC HYPOSLAST IS CON­

TINUED IN THIS AGE GROUP. IN THE SPECIMEN EXAMINED THC HYPOSLAST

COMPLETELY LINES THE CMSRYONIC DISC AND THC TROPHOBLAST, PRODUCING

A BlLAMINAR BLASTOCYST. IN ADO IT ION TO THC CPIBLAST AND HYPOBLAST,

SECTIONS THROUGH EMBRYONIC DISCS DISCLOSE THC DISPERSAL OF MESODERMAL

CELLS BETWEEN THESE LAYERS WHICH MAY NOW BE REFERRED TO AS ECTODERM

AND ENTODERM (F IG . 5 ) . THE ARCA OF MESOOCRM CELL PROLIFERATION

MARKS THC SITE OF THC PRIMITIVE STREAK. THE MESODERM APPEARS TO

ARISE FROM THE ECTODERM. TRANSVERSE SECTIONS OF SPECIMENS IN THIS

GROUP DEMONSTRATE THE LIMITATION OF MESODERMAL GROWTH TO THC CON­

FINES OF THC EMBRYONIC DISC. THE LONGITUDINAL SECTION, HOWEVER,

CLEARLY SHOWS THE SPREAD OF MESODERM (cAUDALLY, WITH RESPECT TO THC

PRIMITIVE STREAK AXIS) EXTENDING WELL BEYOND THC LIM ITS OF THC EM­

BRYONIC DISC.

The ZONA PCLLUCIDA is s t il l present OVER THE em bryo nic d is c

IN ALL SPECIMENS, AND EXTENDS OVER THC TROPHOBLAST IN SOME. IN RE­

GIONS OF CONTACT BETWEEN THE TROPHOBLAST AND ENDOMETRIUM, HOWEVER,

THC ZONA PCLLUCIDA SEEMS TO HAVE DISAPPEARED.

The MESODERM ORIGIN IS QUITE SIMILAR IN THC FERRET (HAMILTON,

1937 ). In this case the firs t MESODERMAL cells appear to be

01SPERSCD BETWEEN THC ECTODERM ANO ENTODERM IN THE POSTERIOR QUARTER OF THE EMBRYONIC DISC. THESE MART MESODERMAL CELLS, "WHICH

ARE ROUNOEO, APPEAR TO ARISE FROM THE ECTODERM ANO MARK THE SITE OF

THC FUTURE PRIMITIVE STREAK."

Fo llow ing t h is ho rizo n there is a s iz a b l e gap of four h o r i­ zons DUE TO A DEARTH OF MATERIAL. THE READER IS REMINOCD OF THC WORK of Ha m ilt o n (1937) w hich roughly covers t h is pcr io o in the closely

RELATED FERRET. 2 2

H o riz o n IX

Only onc sp e c im e n (20LM) was founo to represent the present

AQE GROUP.

The body of the embryo is s t r a ig h t w it h no in d ic a t io n of f le x u r e . There is a w ell d e f in e d neural groove . Th ic k e n e d neural fo lds approximate one another at m id - body but div e r g e considerably posteriorly and to a le s s e r extent in the c e p h a lic r e g io n . Caudal

TO THE CEPHALIC REGION, LATERAL FOLDING IS IMMINENT BUT NOT DE­

FINITELY EXPRESSED. IN THE CEPHALIC REGION, LATERAL AND ANTERIOR

UNDERCUTTING TOGETHER WITH ANTERIOR GROWTH OF THE HEAD HAVE PROOUCED

THE HEAD FOLD WHICH IS SEPARATED FROM THE UNDERLYING EMBRYONIC DISC

BY THE SUB-CEPHALIC POCKET. THE MOST ANTERIOR SECTION OF THE SPECI­

MEN REPRESENTING THIS AGE GROUP AT THE LEVEL OF THE PRESUMPTIVE

FOREBRAIN ILLUSTRATES THE NEUROCOEL COMMUNICATING WITH THE OUTSIDE

IN THREE DIRECTIONS - DORSALLY, ANTERIORLY, AND VENTRALLY. THE SUB-

CEPHALIC POCKET IS LIMITED TO A BARELY DISCERNIBLE S LIT-LIK E CAVITY

BETWEEN THE HEAD FOLD AND THE BILAMINAR EMBRYONIC DISC (F IG . 6).

The fore - gut forms as a b l in d sac w hich is o ir e c t e d anter ­

io r l y . The sac is compressed dorso - vc n tr ally and assumes a cres ­

cent SHAPE WITH THE CONCAVITY DIRECTED DORSALLY* A FEW SECTIONS

CAUOAL TO ITS BLIND ENO, THE FLOOR OF THE FORE-GUT FUSES WITH THE

VENTRAL ECTODERM OF THE HEAD FOLD, THE COMBINED TISSUE BEING RE­

FERRED TO AS THE ORAL OR PHARYNGEAL MEMBRANE (F IG . 7 )* THIS FIGURE

ALSO ILLUSTRATES THE RATHER DENSE MESENCHYME IN THE HEAD REGION.

Caudal by several s e c t io n s to the lo c a t io n ju s t m en tio n ed is the

ANTERIOR INTESTINAL PORTAL WHICH IS SHOWN IN (F IG . 8 ). Unfortunately , some o r the s e c t io n s o r t h is s p e c im e n were

DESTROYED AND IT IS IMPOSSIBLE TO DETERMINE THE COMPLETE NUMBER OF

SOMITES. AN ESTIMATE WHICH MIOHT BE VENTURED ON THE BASIS OF DEGREE

OF DEVELOPMENT IS APPROXIMATELY FOUR. THERE ARE AT LEAST TWO PAIRS

OF WELL DEVELOPED SOMITES IN THE ANTERIOR REGION. A MYOCOEL WAS OB­

SERVED IN SOME OF THE SOMITES. THE INTERMEDIATE MESODERM IS PRESENT

BUT UNMODIFIED. THE LATERAL MESODERM IS SPLIT INTO WELL DEFINED

SOMATIC AND SPLANCHNIC LAYERS (F IG . 9 ) .

The heart is in the p a ir e d tubular c o n d it io n but f u s io n o r

THE ENDOCAROIAL TUBES IS IMMINENT AND ALTHOUGH NOT EVIDENT IN THIS

SPECIMEN IS TO BE CONSIDERED A CHARACTERISTIC OF THIS AGE GROUP.

Ag a in , in the s p e c im e n e x a m in e d , there is no pronounced t h ic k e n in g

OF THAT PORTION OF THE SPLANCHNIC MESODERM WHICH ENTERS INTO THE

FORMATION OF THE EPIMYOCARDIUM. ALTHOUGH THE HEART IS STILL IN THE

PAIRED TUBE STATE THE ABSENCE OF THE VENTRAL MESOCARDIUM IS NOTED.

Dorsal to the h ear t , the p a ir e d la y e r s o r s p la n c h n ic mesoderm a p *

PROACH ONE ANOTHER ONLY SLIGHTLY. THIS IS THE ONLY PRESENT IN D I­

CATION OF THE DORSAL MESOCARDIUM WHICH IS TO APPEAR IN A LATER AGE

GROUP.

The lac k of a complete s e r ie s of s e c t io n s p la c e s the exact

NATURE OF THE PRIMITIVE STREAK IN THE REALM OF UNCERTAINTY. A n EX­

AMINATION oV THE AVAILABLE MATERIAL, HOWEVER, WOULD LEAD ONE TO

DESCRIBE THE PRIMITIVE STREAK AS POSSESSING HENSON'S NODE AND BEr

ING TYPICAL IN OTHER RESPECTS BUT QUITE SHORT IN COMPARISON TO THE

OVERALL LENGTH OF THE EMBRYO*

One or the p o s te r io r s e c t io n s of the s p e c im e n e x h ib it s what 2k

>OSSIBLY MAY BC AN ALLANTOIC DIVERTICULUM, NUT AGAIN, TMC CONDITION

OF TMC MATERIAL PROHIBITS CERTAIN IDENTIFICATION. 2 5

Ho r izo n X

O r THE FOUR SPECIMENS REPRESENTING THIS HORIZON ANO AVAILABLE

TOR STUDY* 7RA AND J 9 P WERE SECTIONED TRANSVERSELY* 20L ANO 7 ^

SAG ITTALLY. THE FEMALE WHICH SORE THE 7R SERIES OF EMBRYOS WAS

BRED TWICE ANO THE UTERINE HORN WAS REMOVED 2 $ DAYS FOLLOWING THE

FIRST* OR 21 DAYS FOLLOWING THE SECOND MATING RESPECTIVELY. THE

FEMALE WHICH BORE EMBRYO 20L ALSO WAS BRED TWICE AND THE UTERINE HORN

WAS REMOVED 39 DAYS FOLLOWING THE FIRST MATING* OR 28 DAYS FOLLOWING

THE SECOND MATING RESPECTIVELY. SlNCE THE EMBRYOS ARE ALL OF AP­

PROXIMATELY THE SAME DEGREE OF DEVELOPMENT* IT WOULD SEEM THAT THEY

ARE FROM 2 6 -2 9 DAYS OF AGE. THE READER IS REMINDED OF THE PHENOM­

ENON OF DELAYED IMPLANTATION WHICH IS CHARACTERISTIC OF THIS ANIMAL*

AND THAT THE PRESENTATION OF THE ABOVE AGE FIGURES IS ONLY OF

ACADEMIC INTEREST. THE SOMITE NUMBER OF ALL SPECIMENS STUOIED IN

THIS AGE GROUP RANGES FROM 10 TO 13 .

T here is closure and f u s io n of the neural fo ld s throughout at

LEAST THE CRANIAL TWO-THIRDS OF THE EMBRYO. HOWEVER* THE NEUROCOEL

IS STILL CONTINUOUS WITH THE OUTSIDE BY WAY OF THE MINUTE ANTERIOR

AND WIDER POSTERIOR NEUROPORES. THE ANTERIOR NEUROPORE IS NOT E V I­

DENT IN SECTIONS MADE OF EMBRYO 20L* BUT THIS IS ATTRIBUTED TO THE

FACT THAT THE SAGITTAL SECTIONS WERE VIRTUALLY PARALLEL TO THE

LONGITUDINAL AXIS OF THE EMBRYO AND THE KNIFE PASSED THROUGH CITHER

SIDE BUT NOT THROUGH THE ANTERIOR NEUROPORE.

T he s t o m o d c a l d e p r e s s io n is n o t p r o n o u n c e d . T h e e c t o d e r m o r

THE HEAD IN THE STOMODCAL REGION AND THE LINING OF THE FORE-GUT IN

THE SAME REGION ARC TWO DISTINCT LAYERS. THE DORSO-VCNTRALLY AND ONLY ONC RATHER BROAD PHARYNGEAL POUCH I t EVIDENT. CAUDALLY,

THC FORE-GUT I t IN OPCN COMMUNICATION WITH THE PRIMITIVE OUT BY THE

WIDE B LIT-LIK E ANTERIOR INTEtTINAL PORTAL* FAR POSTERIORLY, ON THE

VENTRAL AtPECT OF THE EMBRYO, THERE I t A OORtALLY DIRECTED INVA-

BINATION - THE ANLAOE OP TNE HIND-OUT (PIO* 10)*

IN tPECIMEN ~IRA DIFFERENTIATION OF THC INTERMEDIATE MCBODCRM

INTO A RUOIMCNTARY PRONCPHRIC BYtTCM WAS OBtCRVCD* PAIRCO, BEOMENTAL

PRONCPHRIC TUBULCt AND CONTINUOUt PARALLEL PRONCPNRIC DUCTt APPEAR­

ED TO OR 101 NATE AT THC tlX T H BOMITC AND CONTINUED UNTIL THE 12TH ANO

FINAL tOMITE OF T H It tPECIMEN* THE PRONCPHRIC TUBULCB ARC EXTREMELY

RUDIMENTARY AND 0 IFF ICULT TO DCMONBTRATC WITH CERTAINTY. THE PRO­

NCPHRIC DUCT, ON THC OTHER HAND, I t CAtlCR TO D ltT IN O U ItH AND, IN

DOME tCCTIONt, I t CANALIZED. ONE tCCTION OF EMBRYO 7RA WAS tCLCCTCD

TO ILLUBTRATC A PRONCPRIC TUBULE ANO DUCT AT THE LEVEL OF THC tCVENTH BOMITC (F IG . 1 1 ).

PORTIONt OF THC CIRCULATORY BYtTCM ARC CtTABLIBHCO ALREADY.

T he d o r s a l a o r t a c were o b b c r v e d a b w e l l a b t h e f i r b t a o r t ic a r c h

IN ALL BPCCIMCNt OF TH It GROUP (FIG . 1 2 ). THE EMBRYONIC BLOOO

V C ttC L t WHICH HAVE DEVELOPED ARC PRCBCNT AS A PAIRED SYMMETRICAL

BYtTCM* T he VENOUS SYSTEM I t IN THE PROCESS OF FORMATION. THERE

ARC ISOLATED INSTANCES OF BLOOD VESSEL DEVELOPMENT IN THC MESEN­

CHYMAL TISSUE OF THC EMBRYO AND IN REGIONS LATER TO BE OCCUPIED

BY THC ANTERIOR AND POSTERIOR CARDINAL SYSTEMS. THE FUSION OF

THC TWO ENDOTHELIAL TUBES FORMING A SINGLE TUBE, THE ENDOCARDIUM,

WAS ACCOMPLISHED IN A PREVIOUS HORIZON. THE DEVELOPMENT OF TNE 2 7

HEART HA» PROGRESSED BCYONO THIS POINT IN ASSUMING A DECIDED DCXTRAL

FLEXURE AND SY VIRTUE OF LOCALIZED SWELL I NOS IT IS POSSIDLE TO IDEN­

TIFY REGIONAL DIFFERENCES (F IG . 1 2 )* THERE IS NO EVIDENCE OF FUSION

OF THE DORSAL AORTAC.

IN THE REGION OF THE HEART THE SOMITES NAVE BEGUN TO DIFFER­

ENTIATE. W it h in the r e l a t iv e l y large myococl a sm all clump of c e lls

WAS OBSERVED. NELSON (1953) REFERS TO THEM AS "A SMALL EPITHELIAL

CORE OF CELLS." THESE CELLS CONTRIBUTE TO THE SCLCROTOMIC MATERIAL.

The early differentiation of s o m it e s is therefore rec o g n ized as a

KEY CHARACTERISTIC OF THIS AGE GROUP. THE NOTOCHORD IS PARTIALLY

DEVELOPED, THAT IS , IT IS ONLY IN THE PORTION OF THC EMBRYO CAUDAL

TO THC 10TH AND llT H SOMITE THAT THC NOTOCHORD HAS ASSUMED ITS DE­

FIN ITIV E CYLINDRICAL SHAPE* CEPHALIC TO THIS POINT THC NOTOCHORD

GRAOCS INTO A FLATTENED NOTOCHORDAL PLATE WHICH TERMINATES IN THC

HINO-SRAIN REGION.

At it s a n te r io r e x t r e m ity the neural tube o is p l a y s ev id e n c e # OF THC FORE-BRAIN EXPANSION. IN SPECIMEN 7&A A SECTION WAS SELECT­

ED TO ILLUSTRATE THIS REGIONAL DIFFERENTIATION. COINCIDENT WITH THC

FORE-BRAIN EXPANSION IS THE LATERAL EVAGI NAT IONS (OPTIC VESICLES)

( f i g . 13). 2 6

Ho r izo n XI

Two SPECIMENS, 12RA AND 12RM, WERE POUND TO REPRESENT THIS

HORIZON.

D isappearance op the a n te r io r ncuropore , and persistence op

THE POSTERIOR NEUROPORE ARE CHARACTERISTICS WHICH INTRODUCE THIS AOE

OROUP. The two specim ens a v a ila b le POR s tu d y WERE OBTAINED PROM THE

SAME UTERINE HORN 31 DAYS POST-CO ITUM. ONE OP THE SPECIMENS HAS SEEN

8 CCTIONED 8 AGITTALLY AND IS DESIGNATED AS 12RA, THE OTHER HAS BEEN

SECTIONED TRANSVERSELY ANO DESIGNATED AS 12RB. THE SOMITE NUMBER OP

12RA IS 19 AND, WHILE THE NUMBER OP SOMITES OF 12RA COULD NOT BE DE­

TERMINED PRECISELY, IT IS APPROXIMATELY THC SAME. In BOTH SPECIMENS

THC ORAL MCMBRANC IS IN THC PROCESS OP RUPTURING ANO THC LUMEN OP

THC PHARYNX IS CONTINUOUS WITH THC INCOMPLETE AMNIOTIC CAVITY. THE

PHARYNX IS BROAD, FLATTENED DORSO-VCNTRALLY, AND TWO PHARYNGEAL

POUCHES ARC EVIDENT. THE MANDIBULAR ANO HYOIO ARCHES ARC VISIBLE

EXTRCNALLY. THERE WAS NO INDICATION OP RUPTURE OP THC VISCERAL OR

CLOSING PLATE WHICH SEPARATES THC PHARYNGEAL POUCH AND CORRESPOND­

ING GROOVE. With THE EXCEPTION op the pharyngeal pouches, the pore- g u t IS RELATIVELY UNMODIFIED. THERE IS NO THYROID ANLAGC DEPRES­

SION, LIVER BUD CVAQINATION, OR RESPIRATORY GROOVE. POSTERIORLY

THC PORE-GUT REMAINS IN OPEN COMMUNICATION WITH THC YOLK SAC BY

WAY OP THE NARROWING ANTERIOR INTESTINAL PORTAL. THE HIND-GUT

AND ALLANTOIS OCCUR AS A SINGLE CHAMBER CAUDAL TO THC POSTERIOR IN­

TESTINAL portal. This chamber is crescent-shaped with the wings

OP THC CRESCENT EXTENDING LATERALLY AND SLIGHTLY ANTERIORLY. A

SAGITTAL SECTION THROUGH THIS REGION IS SHOWN IN PIG. lU . Commencing at the level op the 6th somite theme was osscmvcp

A LINEAR MASS OP TISSUE CXTENOING CAUOALLY. ON THE SASIS OP ITS

PO SITIO N ANO POINT OP O RIG IN TH IS WAS JUDGED TO BE THE PRONEPHRIC o u s t . How ever i t w as n o t t r a c e a b l e so p a r a s th e c l o a c a .

The presence op blood cells in the divisions op the heart and

IN the venous system, plus the pact THAT THC COMMON cardinal veins

HAVE BRIOQCD THC COELOM IC TRACT INTO THC SINUS VCNOSUS INDICATES

THAT AT LEAST A PRIMITIVE TYPE OP BLOOD CIRCULATION IS UNDER WAY

IN THIS AGE GROUP. ADVANCEMENT IN THIS AGE GROUP IS ALSO EVIDENT

IN PURTHCR REGIONAL DIPPCRCNTI AT ION OP THE HEART. THERE IS A WELL

DCPINCD ATRIUM, VENTRICLE AND BULBUS (P IB . I j ) . A SINGLE SECTION OP

EMBRYO 12RM ILLUSTRATES PUSION OP THC DORSAL AORTAC AT THC LEVEL OP

THC 10TH SOMITE. I n ALL OTHER SECTIONS THC DORSAL AORTAC ARC PAIR­

ED INDICATING THC COMMENCEMENT OP THC PUSION PROCESS. IN BOTH

SPECIMENS THE PIRST AORTIC ARCH IS WELL PORMCD AND IN SPECIMEN 12RB

THERE IS INDICATION OP THC PORMATION OP THC SECOND AORTIC ARCH, THE

SECOND AORTIC ARCHES APPEAR TO ORIGINATE AS BUDS OPP THC DORSAL AORTAC

WHICH EXTEND AROUND THC PHARYNX TERMINATING BLINOLY BETWEEN THC PORC-

GUT AND THC VENTRAL SURPACC OP THC HCAO POLD. ASIOC PROM ITS WALL OP

SIMPLE ENDOTHELIUM THC VASCULAR SYSTEM WITH THC EXCEPTION OP THC

HEART IS UNMOO IP I CO. IN THC CEPHALIC REGION THC SOMITIC DIFFER­

ENTIATION IS WELL UNDER WAY AND IT IS POSSIBLE TO DISTINGUISH THC

SCLEROTOME, MYOTOME, AND DERMATOME. ?HC NOTOCHORD IS PRESENT AS A

CYLINDRICAL STRUCTURE EXTENDING TO THC LEVEL OP THE MIO-BRAIN.

T h e r e i s some in d ic a t io n op oippcrcntiation op th e c e p h a l ic

PORTION OP THC NEURAL TUBE INTO THREE BRAIN VESICLES (PORE-, MID-, 30

AND NINO-BRAINI (P IQ . 1 5 ). THC OPTIC VESICLES ANC ENLARGED ANO

CLOSELY APPROXIMATE THC HEAD ECTODERM. HOWEVER, THERE IS NOT YET

CVIOCNCC OP A THICKENED LENS PLACODE ( f lO . 16 )• OTIC PITS ARE

PRESENT IN THC HIND-SRAIN RCOION. ALTHOUGH SPECIMEN 12RB WAS SEC­

TIONED RATHER OBLIQUELY, IT IS POSSIBLE TO LOCATE THC OTIC PIT

SLIGHTLY ANTERIOR TO THC SECOND PHARYNGEAL POUCH (P IG . 17)«

F igure 17 represents another longitudinal s e c t io n through

SPECIMEN 12RA BUT 8 SECTIONS LATERAL TO M G . I5 . THREE CONSPICUOUS

GROUPS Of CELLS ARC NOTED IN THC REGION LATERAL TO THC HINO-SRAIN*

TWO Or THC CELL AGGREGATES ARE CLOSELY ASSOCIATED WITH THC OTIC P IT ,

ONE CLUSTER LYING CLOSE TO ITS ANTERIOR ASPECT, THC OTHER CLUSTER

LYING IMMEDIATELY POSTERIOR TO IT . THE FORMER CELL MASS COMPRISES

THC V II AND V I I I GANGLIA WHILE THE LATTER MASS IS DESIGNATED AS

THE IX GANGLION. ANTERIOR TO THC OTIC PIT ANO ITS ASSOCIATED

GANGLIA IS THE THIRD ANO LARGEST GROUP Or CELLS. THIS IS THC V

GANGLION.

IN THC CEPHALIC REGION THC EMBRYONIC COELOM ENLARGES ANO EN­

CLOSES THC DEVELOPING HEART. CAUDALLY, AT THC LEVEL Or THC MPTH

SOMITE THC INTRA-CMBRYONIC COELOM TRACT BECOMES CONTINUOUS WITH THC

EXTRA-EMBRYONIC COELOM. DIVISION o r THE COCLOMIC CAVITY IS IN PRO­

GRESS. The u n io n o r p a ir e d la t e r a l somataplcuric bulges c ar rying

THC COMMON CARDINAL VEINS, WITH SIMILAR BULGES Or SPLANCHNOPLCURC

PROM THC CAUOAL REGION o r THE HEART HAVE CREATED THE LATERAL MCSO-

CAROIA WHICH IN TURN HAVE PARTIALLY DIVIDED THC COCLOMIC CAVITY

INTO A PERICARDIAL AND PERITONEAL CAVITY. THESE LAST MENTIONED

COELOM1C CAVITY CAVITIES ARE STILL CONTINUOUS ABOVE ANO BELOW THE 31

LATERAL MC90CAR0IA RY WAY OF THC DORSAL ANO VENTRAL PARIETAL RECESS

or H is* I n t h is aoc group the ventral mesentery is under - going

RAPID LATERAL AND CEPHALIC EXPANSION* ITS GROWTH WILL EVENTUALLY

BRING IT INTO CONTACT AND FUSION WITH THE LATERAL AND VENTRAL BODY

WALLS AND DORSALLY WITH THC ALREADY ESTABLISHED LATERAL MESOCARDIA

FORMI NO THC SEPTUM TRANSVCRSUM* THE IMPORTANCE OF THIS DEVELOPMENT

is two- f o l d : F ir s t , it contributes to the further s e p a r a tio n o r

THE PERICARDIAL AND PERITONEAL COELOM. SECOND, THE MESENCHYMAL

CELLS PROLIFERATED IN THC SEPTUM TRANSVCRSUM CONTRIBUTE TO THC

MESODERMAL CLEMENTS OF THE LIVER. 32

Horizo n X II

Throughout the two prec cdino h o rizo n s there has been a rather

CLOSE CORRELATION BETWEEN THE DEVELOPMENTAL CHARACTERISTICS AS POUND

IN THE MINK AND THOSE DESCRIBED BY STREETER FOR THC HUMAN EMBRYO.

IN THIS AGE GROUP, HOWEVER, IT HAS BEEN FOUND NECESSARY TO DEVIATE

FROM THC CRITERIA ESTABLISHED BY STREETER TO A SMALL DEGREE. THE

VARIATIONS CAN BE EXPECTED TO INCREASE ANO MULTIPLY AS THE DEVELOP­

MENTAL ANATOMY OF THE TWO ORGANISMS UNFOLD IN SPECIFIC WAYS.

The two specimens (84m and 84p) selected to represent this

AGE GROUP WERE OBTAINED FROM THE SAME UTERINE HORN. IT MUST BE NOT­

ED HERE THAT THESE TWO SPECIMENS ILLUSTRATE THE MOST ADVANCED STATE

OF DEVELOPMENT FOR THIS PARTICULAR HORIZON. THIS OBSERVATION IS

FURTHER EMPHASIZED BY THC FACT THAT A THIRO EMBRYO, 84A, REMOVED

FROM THE SAME UTERINE HORN AS THC TWO SPECIMENS ALREADY MENTIONED

HAS BEEN PLACED IN THE SUCCEEDING HORIZON. THE FEMALE BEARING THC

TWO EMBRYOS OF THIS AGE GROUP HAD BEEN MATED TWICE, SO THC AGE OF THE

SPECIMENS IS COMPUTED TO BE EITHER 33 0R 0AYS (F ROM MATING TO RE­

MOVAL). Due to the extreme caudal t w is t in g of a ll the s p e c im e n s it

WAS NOT POSSIBLE TO MAKE AN ACCURATE COUNT OF THE SOMITES. THIS IS

THC FIRST INSTANCE IN THIS WORK THAT THE EMBRYO WAS REMOVEO FROM

THC UTERINE HORN FOR STUDY BEFORE SECTIONING. FIGURE l 6 ILLUS­

TRATES A CAMERA LUC IDA, WHOLE MOUNT DRAWING OF ONE OF THC SPECIMENS

(84D) ANO THE PLANE OF SECTIONING OF THIS SPECIMEN.

The presence of three p a ir s of b r a n c h ia l arches and CLOSURE

OF THE POSTERIOR NEUROPORE IN THE OLDER SPECIMENS AID IN CATCRGOR-

IZING THIS AGE GROUP. BIFURCATION OF THC FIRST BRANCHIAL ARCH INTO A MAXILLARY ANO A HANOISULAR PROCESS HAS TAKEN PLACE (F IG . l8 ) . AS

WELL AS THC ADDITION OF A THIRD PAIR OF PHARYNGEAL POUCHES# THC

PHARYNX HAS UNDERGONE IMPORTANT SPECIALIZATION* THERE IS A DORSO-

CRANIAD INVAGINATION OF THE ANTERIOR ASPECT OF THC ORAL CAVITY AS

Ra t h k c ' s POUCH. The a n te r io r end of THIS pouch l ie s immediately

CAUDAL TO THC DI ENCEPHALON. FURTHER CAUDAD IN THC PHARYNX# IM­

MEDIATELY POSTERIOR TO THE FIRST PHARYNGEAL POUCH IS THE THYROID.

IT EXISTS AS A UNILATERAL MEDIAN DEPRESSION OF THC FLOOR OF THE

PHARYNX. THE DEPRESSION IS SURROUNDED SY CELLS WHICH REPRESENT THE

THYROID PR IMOROIUM AND WHICH ARE DIFFCRCNTIATED FROM THC SURROUNDING

MESENCHYMAL CELLS (F IG . 1 9 ).

Other delineations of the d ig e s t iv e tract arc as follow s :

The development of the lungs is begun in t h is age group w it h the

FORMATION OF THC LARYNGOTRACHEAL GROOVE WHICH SUBSEQUENTLY B I­

FURCATES INTO RIGHT ANO LEFT BRONCHIAL STEMS (F IG . 2 0 ).

The l iv e r # in it s development # draws upon two types of t i s ­ s u e s . F ir s t # the mesodermal w hich was in vo lved in the form atio n of

THE SEPTUM TRANSVCRSUM AND WHICH CONTRIBUTES TO THC STROMA OR FRAME­

WORK OF THC LIVER. THIS PROCESS WAS INITIATED IN THE PREVIOUS

HORIZON. The SECOND TYPE OF TISSUE IS DERIVED FROM SPECIALIZED

AREAS OF GUT EPITHELIUM INVOLVED IN THC HEPATIC OIVCRTICULUM WHICH

MAKES ITS APPEARANCE IN THIS HORIZON. THE EPITHELIAL TISSUE IN­

VADES THC STROMA ENCIRCLING VASCULAR ELEMENTS WHICH HAVE DEVELOPED there ( f i g . 2 1 ). Conspicuously absent in t h is area is the g a ll

BLADDER AND CYSTIC DUCT PRIMORDIUM.

The CEPHALIC REGION of the neural tube d is p l a y s even greater 3*

REGIONAL DIFFERENTIATION. THE SURFACE OF THC NEURAL TUSC IS ABUNDANT­

LY SUPPLIED BY BLOOD VESSELS WHICH APPEAR TO HAVE ARISEN J_N SITU.

Mesonephric t u b u le s and ducts arc present but it was impos­ s ib le TO DETERMINE AT WHAT LEVEL THC TUBULES AROSE OR TO WHAT LEVEL

THEY EXTEND. THE MESONEPHRIC OUCTS EMPTY INTO THC CLOACA IMMEDIATE­

LY CAUDAL TO THC ALLANTOIC DIVERTICULUM (FIGS. 22 AND 23)*

I t has b e e n s t a t e d th a t w it h in t h is aoe group one o b s e r v e s

CHANGES IN PATTERN OF DEVELOPMENT WHEN COMPARING THC MINK AND HUMAN.

One of these differences is in the progress of otocyst FORMATION.

Streeter (19**2) describes the o tic vesicle of human embryos as re­ m a in in g OPEN THROUGHOUT T H IS AOE GROUP. ALTHOUGH THE M INK S P E C I­

MENS USED TO ILLUSTRATE THE SAME AGE GROUP REPRESENT THE OLDEST

MEMBERS OP THE GROUP IT APPEARS AS THOUGH THC OTOCYST HAD BEEN

CLOSED FOR SOME TIME. ANOTHER CHARACTERISTIC OF IMPORTANCE IS THE

PRESENCE OF THE ARM RIDGE IN THE MINK SPECIMENS OF THIS HORIZON

WHEREAS IN THE CORRESPONDING AGE GROUP FOR HUMAN EMBRYOS THE OLD­

EST MEMBERS OF THE GROUP POSSESS ONLY THC PRIMORDIUM OF THC ARM

BUD. 35

Ho r IZON XIII

THRCC EMBRYOS WERE USED TO ILLUSTRATE THIS AQC GROUP. THEY

WERE 8 4 a , 6LM, and 6la. It w ill se recalled that in the discussion

OP SPECIMENS 84m AND 84P OP THC PREVIOUS HORIZON, IT WAS NOTED THAT

A THIRD EMBRYO (a LITTER-MATC OP EMBRYOS 84C AND 840) WAS TO BE

PLACED IN A SUCCEED I NO HORIZON BY VIRTUE OP STRUCTURAL ADVANCEMENT.

Hence, embryo 84a represents a young member op th is new age group.

Embryos 6LM and 6la represent slightly older members.

Figure 24 illustrates a camera lucioa whole mount drawing op specim en 84a , and the plane op sectioning op this specimen. Figures

25, 26, 27, AND 28 ILLUSTRATE PHOTOGRAPHS OP THE RIGHT AND LCPT SIDES

OP SPECIMENS 6 LM AND 6LA RESPECTIVELY.

AS DEPICTED IN PIGS. 24 ANO 2 7, BOTH ARM AND LEG BUDS ARC

V IS IB L E . T he arm buds w h ic h appear ed e a r l ie r have d e v elo ped INTO

PRONOUNCED RIDGES WHICH EXTEND PROM SOMITES 8 TO 11 . THE LEG BUOS

MAKE THEIR APPEARANCE IN THJS ABC GROUP AS LATERAL SWELLINGS.

Ca u d al to the t h ir d p a ir op b r a n c h ia l bars is the d e p r e s s io n

KNOWN AS THE CERVICAL SINUS. THE ULTIMOBRANCHIAL BODY IS NOT CON­

SPICUOUS. The development o r the o t ic v e s ic l e c o n t in u e s a t it s

RAPID PACE AND IN THE YOUNGEST SPECIMEN OP THIS AGE GROUP THC D IP -

PCRENT I AT ION OF THE ENDOLYMPHATIC APPENDAGE HAS PROGRESSED TO THE

EXTENT THAT IT PROJECTS DORSAD, FORMING A DISTINCT RECESS ( r iG . 2 $ ).

The m e d ia n t h y r o id is conspicuous . I t is open to the floor o r THE PHARYNX BY THE TORAMEN CAECUM AT THC LEVEL OP THE SCCONO

PHARYNGEAL POUCH ( r iG . 3 0 ) . ALTHOUGH THE THYROID GLAND PRESENTS A

FAIRLY SOLID APPEARANCE, ITS NARROW CONNECTION WITH THC FORAMEN 36 caecum ( thc thyroglossal du c t) is s t il l c a n a l iz e d .

The pr im a r y bronchi are e v id e n t in t h is aoe group, h a v in g de ­ veloped PROM THC LATERAL LUNG OUTOROWTHS OP THC PREVIOUS HORIZON.

I n DISCUSSING THE DEVELOPMENT OP THC PRIMARY BRONCHI IN HUMAN EM­

BRYOS, Streeter (1 9 ^5 ) noted that the r ig h t p r im a r y bronchus as soon

AS IT ACQUIRES APPRECIABLE LENGTH, " iS OIRCCTCO MORE DOWNWARD

( caudalward ) whereas thc lc p t bronchus is more nearly transver se ."

I n s p e c im e n 84a no such dippcrcntiation is e v id e n t . The p r im a r y

BRONCHI APPEAR TO BE OP EQUAL LENGTH AND THEIR DIRECTION OP GROWTH

APPEARS TO BE SYMMETRICAL (P IG . 3 1 ).

I t appears as though the p a n c r e a t ic and h e p a t ic ducts and g a ll

BLADDER PRIMORDIA HAVE NOT YET DEVELOPED TO THC EXTENT THAT THEY ARC

DISTINGUISHED EASILY PROM ONE ANOTHER. HOWEVER, A SECTION OP SPECI­

MEN 6LA DISPLAYS THC CONTINUOUS LUMEN OP THE HEPATIC DUCT AND GALL

BLADDER PRIMORDIUM WITH A SLIGHT CONSTRICTION SEPARATING THE TWO

( p i g . 3 2 ). Although the growth op the trabeculae op the l iv e r is

EXTENSIVE IN THIS HORIZON, AS COMPARED TO THAT IN THC PRECEDING

AGE GROUP, IN NONE OP THC SPECIMENS EXAMINED DID THESE CORDS OP

EPITHELIAL CELLS, GROWING PROM THE SURPACC OP THE HEPATIC OIVCR-

TICULUM, EXTEND TO THC VENTRAL LIMITS OP THC SEPTUM TRANSVCRSUM.

The dorsal pancreas is a pronounced evagination op the

DORSAL ASPECT OP THC GUT TUBE AND IS CONNECTED TO THE LATTER

STRUCTURE BY A NARROW STALK. THE PLANE OF SECTIONING OP EMBRYO

6 L A allows thc dorsal pancreas to be shown to greater advantage

(riG. 3 3 ).

Thc circulatory system is d e te r m in e d more e a s il y in t h is AGC GROUP« T hc RIGHT AND LCFT UMBILICAL VC I hr ANO HCPATOCAROI AC

VC INS, ALL OF WHICH CMPTY INTO THC SINUS VCNOSUS, ARC PROMINCNT•

Thc ocvclopmcnt of thc h c p a t ic plc xu s i t rcmarkablc ano it i t pos -

tlO LC TO (0CNTIFY THC CNTRANCC OF THC VITCLLINC VCINt INTO T H It

PLCXUS.

Thc 01tCUttlON OF thc o p t ic cvagination , IN T H It agc group,

HAt tCCN WITHHCLO UP TO T H It POINT tCCAUtC OF IT t IMPORTANCC IN

DCTCRMININQ THC TRANtITION FROM THC PRCtCNT HORIZON TO THC ONC

WHICH I t TO FOLLOW. I n T H It AGC GROUP, IT I t POSSIBLC TO IOCNTIFY

THC MOtT D ltT A L tURFACC OF THC OPTIC CVAGI NAT ION A t THC RCTINAL O ltC .

Ha RKCO CCLLULAR PROLIFCRATION IN T H It ARCA HAt NOT ONLY THICKCNCO

THC RCTINAL DISC, tUT ALtO HAt CAUtCO IT TO BULGC INTO THC LUMCN OF

THC CVAGI NAT ION. THCRC IS A MARKCO TRANSITION FROM THC RCTINAL

O ltC TO THC THINNCR, AOJOINING SCGMCNT OF THC CVAG1NAT ION WHICH

IS TO OCCOMC THC PIGMCNT LAYCR OF THC RCTINA. ALTHOUGH THC LCNS

PLATC HAt THICKCNCO, THCRC IS NOT YCT ANY INDICATION OF THC FORTH­

COMING INVAGINATION OF THIS STRUCTURC ( r iG . 3 *0 . 38

Horizon XIV

S i x embryos were selected to represent this age group. They were 9 L A , 9 L M , 9 L P ( l i t t c r - m a t e s ) , 16RM, 7 L A , and 7 L P (the last two

ARE ALSO LITTER-MATCS).

F ig u re s 35 and 36 illustrate photographs op the right side op

SPECIMENS 9LM AND $ L P RESPECTIVELY. THE PLANE OP SECTIONING OP EM­

BRYO 9 L M IS INOICATED ON THC PHOTOGRAPH OP THIS SPECIMEN.

I n thc p r e v io u s age group there appcarco an alm ost im p e r c e p ­ t ib l e DEPRESSION IN THE DORSAL SURPACC OP THC EMBRYO AT ABOUT THC

LEVEL OP THE PIPTH SOMITE. THIS IS INTERPRETED TO BE THC EARLY AP­

PEARANCE OP THE NACKENGRUBE OP HlS DESCRIBED BY STRCCTCR (1 9 ^ 5 )*

Th is " neck d e p r e s s io n " a c q u ir e s prom inence in the young members op

THC PRESENT AGE GROUP, BECOMING LESS OBVIOUS IN THC OLDER SPECIMENS.

T he arm buds have develo ped in to d c p in it e p r o je c t in g APPEND­

AGES. Reference to the photographs op f ig s . 35 AND 3^ shows the

t a p e r in g structure op thc arm bud as w ell as thc tendency op it s

DISTAL PORTION TO CURVE VENTRALLY AND MEDIALLY. THE LESS. PRECO­

CIOUS LEG BUDS OP SPECIMEN (P IG . 3 6 ) APPEAR TO EXTEND LATERAL­

LY AS PIN -LIK E SWELLINGS, QUITE UNLIKE THC ARM BUDS IN ANY STAGE

OP THEIR DEVELOPMENT.

The MANDIBULAR AND HYOID BRANCHIAL BARS ARC PROMINENT. THE

THIRD BRANCHIAL BAR ALSO IS PRESENT BUT, IP ONE COMPARES THE RE­

LATIVE SIZES OP THE THIRD BRANCHIAL BAR IN THIS HORIZON AND THOSE

OP THC PRCCCDING AGE GROUP, IT IS POUND TO BE RELATIVELY SMALLER

AND DISAPPEARING INTO THC CERVICAL SINUS DEPRESSION. THERE IS A

PAINT INDICATION OP THE SUBDIVISION OP THC HYOID BARS INTO A DORSAL AND VENTRAL SEGMENT. BUT THERE IS NOT THE FURTHER SU BDIVI-

SION OF THE VENTRAL SEGMENT OF THE HYOID BAR WHICH STREETER NOTES

IN THE CORRESPONDING AGE GROUP OF HUMAN EMBRYOS.

RATHKE'S POUCH HAS ENLARGED INTO A PROMINENT FEATURE IN THE

ROOF OF THE MOUTH AND WITH THE EXCEPTION OF THE DERIVATIVES ALREADY

MENTIONED THE PHARYNGEAL POUCHES REMAIN RELATIVELY UNMODIFIED. THE

THYROID GLAND ALSO HAS GROWN CONSIDERABLY AND IT APPEARS THAT THC

CONNECTION BETWEEN THIS GLAND AND THE FLOOR OF THE PHARYNX IS LOST

IN THIS AGC GROUP. THE TRACHEA HAS BECOME DETACHED FROM THC ESO­

PHAGUS ( f i g . 3 7 ). The le ft primary bronchus is directed dorsally

TO A GREATER EXTENT THAN THC RIGHT, ITS DISTAL END LYING ALMOST

* • * LATERAL TO THC ESOPHAGUS. THE RIGHT PRIMARY BRONCHUS EXHIBITS A

LARGER LUMEN THAN THE LEFT AND IS DIRECTED LATERALLY. NO SECONDARY

BRONCHI ARE EVIDENT. THE SIXTH AORTIC ARCH IS PRESENT POSTERIOR TO

THC FOURTH PHARYNGEAL POUCH AND IS CONTINUOUS WITH THC AORTIC SAC

( f i g . 3 8 ). Immediately posterior to this point the aortic sac gives

RISE TO A PAIR OF MINUTE BLOOD VESSELS WHICH COURSE POSTERIORLY,

OCCUPYING POSITIONS LATERAL ANO SLIGHTLY VENTRAL TO THC TRACHEA

( f i g . 39 ). These blood vessels represent the in itia l appearance

OF THE PULMONARY ARTERIES. THERE ARE NUMEROUS OTHER SMALL BLOOD

VESSELS WHICH ALSO TRAVERSE THIS AREA AND WHICH SEEM TO BUD OFF THE

DORSAL AORTAC. FURTHER CAUDAD, IN THE REGION OF THE PRIMARY BRONCHI,

SOME OF THESE BLOOD VESSELS APPEAR TO ANASTOMOSE FORMING A LARGER

VESSEL WHICH BECOMES CONTINUOUS WITH THE ATRIUM THROUGH ITS DOR­

SAL roof ( f i g . H o ) , T h is is the i n i t i a l appear an c e of the p u l­ monary VEIN. Thc ventral p a n c r e a t ic diverticulum makes it s i n i t i a l a p pear ­ ance IN THIS AQC GROUP, ARISING FROM THE PROXIMAL END OF THC HEPATIC

DIVERTICULUM. SlNCC IT IS IMPOSSIBLE TO ILLUSTRATE THC VARIOUS D I­

VERTICULA WHICH ARISE FROM THC GUT TUBE IN THIS REGION IN ONE SEC­

TIONAL VIEW, A SCRIES OF PHOTOGRAPHS REPRESENTING SEVERAL LEVELS

OF SECTIONING IS P ICSCNTCO (FIG S. 4 1 , 42, 43, 44 , ANO 4 ^ ) .

The mesonephros is conspicuous and w ell DEVELOPED ANTERIORLY.

The secretory tubules are highly coiled and canalized. They expand mesially into a Bowman's capsule w ithin which develops a glomerular

tu ft. Laterally the secretory tubule joins the mesonephric duct

( f i g . 4 6 ) .

Some of the c r a n ia l g a n g lia are distinguished e a s il y . The

ACOUSTI CO-FACIAL GANGLIA EXTEND INTO THE HYOID ARCH. THE FIFTH

GANGLION REACHES VENTRALLY INTO THE MAX ILLO-MANDIBULAR SWELLING.

The GLOSSOPHARYNGEAL AND VAGUS NERVES ARC ALSO DEMONSTRABLE (F IG .

**7 ).

A . IN THC STAGING O f HUMAN CM.RYOS ST STRCCTCR ( l9 ^ 5 ) f ™ c

INDENTATION OF THE LENS VESICLE IS ESTABLISHED AS THE CRITICAL CRI­

TERION FOR THIS AGE GROUP. IN ALL SPECIMENS INCLUDED IN THIS HORI­

ZON THE LENS IS IN SOME PHASE OF INVAGINATION. THERE IS A REMARK­

ABLE AMOUNT OF CELLULAR.DEBRIS WHICH ACCUMULATES IN THE LENS PIT

AND MAY GIVE ONE THE ERRONEOUS IMPRESSION Or THC LENS VESICLE BE­

ING COMPLETELY INVAGINATED (F IG . 4 6 ) . THE SIGNIFICANCE, IF ANY, OF

THIS DENSE MASS OF CELLS IN THC LENS P IT HAS NOT BEEN EXPLAINED.

The THIN OUTER WALL OF THE OPTIC CVAGINAT ION SHOWS SOME EVIDENCE

OF PIGMENTATION. THIS LAST FEATURE IS RELATIVELY PRECOCIOUS WHEN 1+1

COMPARED TO HUMAN EMBRYOS OP THC CORRESPONDING AGE GROUP.

Aside prom an increase in the dorso-mesial growth op the en­ d o ly m p h a tic APPENDAGE; THERE IS NO 8 IGNIPICANT CHANGE IN THC MOR­

PHOLOGY OP THE CAR VESICLE. THE OLFACTORY PLATE EXHIBITS A DEFIN­

ITE DEPRESSION. THE MARGINS OP THIS DEPRESSION ARC SOMEWHAT RAISED;

AND; ALTHOUGH THEY DO NOT YET OVERHANG THE CONCAVITY; THEY PORM A

DEFINITE ENCIRCLING RIDGE (P IG . 1+9). kz

Horizon XV

Thrcc embryos were selected to represent the present aoe group. They are 15RA, IJRM (litter-m ates), and 13LP*-

Figures 50 and 51 illustrate photographs or the right ano lcpt sides or specimen 15 RM. The plane or sectioning or the same embryo

is inoicateo on riG . 5 1 .

One n o t a b l e a d v a n c e in development e x h ib i t e d b y m em ber s or

THIS HORIZON IS RELATED TO THE ARM BUD. IN THE PREVIOUS AGE GROUP

THE ARM BUDS WERE DESCRIBED AS DEPINITE PROJECTING APPENDAGES, BROAD

AT THE BASE AND TAPERING DISTALLY. IN THIS AGE GROUP A DEPINITE

CONSTRICTION IS EVIDENT IN APPROXIMATELY THE MEDIAN PORTION OP THE

ARM BUD (riO S . 50 AND ^ 1 ) . THIS CONSTRICTION DIVIDES THC ARM BUD

INTO A DISTAL HAND SEGMENT AND A PROXIMAL ARM AND SHOULDER SEG­

MENT. T he le g b u d s h a v e l o s t t h e ir p i n - l i k e a p p e a r a n c e , b e c o m in g

DEPINITE, PROJECTING APPENDAGES. HOWEVER, THE LEG BUDS DO NOT EX­

H IB IT THC REGIONAL DIrPERENTIAT ION POSSESSED BY THE MORE PRECOCIOUS

ARM BUDS.

The m an d ibu lar and h y o io b r a n c h ia l barb are s t il l p r o m in e n t .

The t h ir d b r a n c h ia l bar , in duplicating the tate o r the rouRTH

BRANCHIAL BAR, HAS VIRTUALLY DISAPPEARED INTO THC CERVICAL SINUS

DEPRESSION (riG S . JO AND ) . IT APPEARS THAT THC 0 1PPERCNTI AT ION

Or THC MANDIBULAR AND HYOID BARS IN THE MINK OCCURS SOMEWHAT LATER

THAN IN THE HUMAN EMBRYO. STREETER (1 9 ^5 ) REPORTED A SUBDIVISION o r THC HYOID BAR INTO A DORSAL AND VENTRAL SEGMENT AS EARLY AS ho rizo n X I I I . T h is p a r t ic u l a r development is not apparent in the

MINK UNTIL THIS AGC GROUP. 1*3

Thc growth op Ra t h k c ' s pouch has co ntinuc d to thc extcnt that

THC ANTERIOR ASPECT OP THC POUCH IS CONTIGUOUS WITH THC BRAIN*

Therc also appears to bc A DCPINITC m id - ventral cvagination op thc

0 1CNCCPHALON IN THIS RCGION OP CONTACT (PIG * 5 2 ).

Thc m c d ian th y r o id has incrcasco in s iz e ano it s d is t a l por­ t io n HAS ASSUMED A PRONOUNCED DORSO-VENTRALLY, FLATTENED APPEAR­

ANCE. T he p r im a r y bronchi have become consiocrably longer , t h c ir

DISTAL PORTIONS LYING LATERAL AND SLIGHTLY DORSAL TO THC ESOPHAGUS

IN SECTIONS POSTERIOR TO THAT SHOWN IN PIG* 52* SECONDARY BRONCHI

MAKE THCIR IN IT IA L APPEARANCE IN THIS AGE GROUP AS SMALL BUDS OPP

THE PRIMARY BRONCHI (P IG . 5 3 ).

The a r t e r ia l supply and venous dr a in a g e op thc d e v e lo p in g

LUNGS IS ESSENTIALLY UNCHANGED PROM THE PRECEDING HORIZON. IT MAY

BE 8 A 10 HERE THAT THE ESOPHAGUS, TRACHEA AND PRIMARY BRONCHI HAVE

BECOME EVEN BETTER DEPINED BY VIRTUE OP THC DIPPERENTI AT ION OP

CELLS IN THE SURROUNDING TISSUE INTO COMPONENT PARTS OP THE WALLS

OP THESE STRUCTURES.

There is no signipicant change to report in the r e l a t io n ­ s h ip s OP VENTRAL AND DORSAL PANCREAS AND THE GALL BLADDER DIVER­

TICULUM EXCEPT IN THE CASE OP THE LATTER. THE GALL BLADDER COM­

MUNICATES WITH THE COMMON BILE DUCT BY A SLENDER TUBE WHICH MAY

BE REFERRED TO AS THC CYSTIC DUCT. |T IS PROM COMMON BILE DUCT

THAT THC HEPATIC DUCTS AND THC VENTRAL PANCREAS APPEAR TO ARISE

(no., jit, 55 , 56, .no 57).

Development op the h in d - gut reg ion p a l l s short op the mark

ESTABLISHED BY STREETER ( l? ^ 3 ) FOR HUMAN EMBRYOS OP THE SAME h o r iz o n . Thc m in k embryos or t h ib aoc group e x h ib it thc urachub ab

AH OPEN COMMUNICATION BCTWCCN THC ALLANTOIS ANO THC CLOACA (FIG S.

58 and 59). I m m e d ia t e ly c e p h a lic to thc juncture or thc urachub

AND THC CLOACA ARC TWO LATERAL EXTCNBION8 OF THC LATTER (F IG . 6 0 ) .

E ach e x te n s io n appears to d iv id e in to two tubal structures , one the

MESONEPHRIC DUCT, AND THE OTHER, THC METANEPHRIC DUCT, OR URETER

( f i g . 6 l ) . E ach ureter t e r m in a t e s in a bulbous f a s h io n as the

RUDIMENT OF THC PELVIS OF THC METANCPHROS (F IG . 6l ) . THE ES­

SENTIAL DIFFERENCE IN STRUCTURAL ADVANCE MADE IN THIS REGION BE­

TWEEN THC HUMAN AND MINK EMBRYOS IS THAT IN THC HUMAN A SEPARATION

OF THC BLADDER FROM THE RECTUM HAS TAKEN PLACE, AND THIS SEPARA­

TION HAS PASSED CAUDAL TO THC URETERS. THERE WAS NO SUCH SEPARA­

TION NOTED IN ANY OF THC MINK EMBRYOS OF THIS AGE GROUP CXAMINCO.

ON THE OTHER HAND, THE DEVELOPMENT OF THE OLFACTORY PIT IS

SLIGHTLY IN ADVANCE OT THE PACE EXHIBITED BY HUMAN EMBRYOS. THE

NASAL SAC EXTENDS BACKWARD INTO THC PRC-OPT 1C REGION. THE VENTRAL

FLOOR Or THE NASAL SAC PROLIFERATES A PLATE-LIKE EPITHELIAL KEEL

WHICH MAINTAINS CONTINUITY BETWEEN THC NASAL SAC AND THC ROOF OF

THE MOUTH (riG. 6 2 ). THE POINT OF CONTACT BETWEEN THIS EPITHELIAL

KEEL, OR NASAL F IN , AS IT IS CALLED, ANO THC ROOF OF THC MOUTH IS

IDENTIFIED ON THC LATTER BY A SHALLOW GROOVE, THC PRIMITIVE PALA­

TINE groove of Pe t e r .

The invagination of the len s v e s ic l e has been co m pleted .

A s in S t r e e t e r 's (1 9 ^8 ) c la s s if ic a t io n o f human em bryos, t h is

CHARACTERISTIC IS PRESENTED AS ONE OF THC SIGNIFICANT EMBRYONIC

REQUISITES rOR INCLUSION IN THIS AGE GROUP (F IG . 63 ), *»5

H o rizo n XVI

TWO CMBRYOS REPRESENT THE PRESENT AQC GROUP. THEY ARE 11LH

ANO 11 LA (UTTER-MATES). PHOTOGRAPHS OP THE RIGHT AND LEFT SIDES

OF THESE SPECIMENS ARE SHOWN IN FIGS. 64, 65 , 66, AND RESPEC­

TIVELY* Development of the arm bud has progressed beyond a mere

DIVISION SY CONSTRICTION INTO A DISTAL HAND SEGMENT ANO A PROXIMAL

ARM AND SHOULDER SEGMENT. |N THIS AGE GROUP, THE HANO SEGMENT IS

DIFFERENTIATED INTO A CENTRAL OR CARPAL PART AROUND WHICH SPREADS

THE FLANGE-LIKE DIGITAL PLATE. ON THE BASIS OF PHOTOGRAPHIC EVI­

DENCE IT IS IMPOSSIBLE TO CITE SPECIFIC ADVANCES IN THC DEVELOP­

MENT OF THE LEG BU08 IN THIS AGE GROUP.

The HYOID AND MANDIBULAR BARS ARC STILL EVIDENT ALTHOUGH THC

FORMER IS MUCH MORE OBVIOUS. THE APPARENT ASCENDENCY OF THE HYOID

BAR OVER THE MANDIBULAR BAR IS DUE TO THE INCLINATION OF THC LATTER

TOWARO THC MEDIAN LINE. THE PROMINENCE OF THC HYOID BAR IS AUGMENT­

ED BY THE SUPERFICIAL CELL PROLIFERATION WHICH WILL ULTIMATELY FORM

THC AURICULAR HILLOCKS OF Hi S. THE APPEARANCE OF THESE HILLOCK PRE­

CURSORS IS MADE IN THIS AGC GROUP.

PROXIMALLY, RATHKE's POUCH IS JOINED TO THC ROOF OF THC MOUTH

SY A THICK STALK WITH MERELY THC REMNANT OF A LUMEN, WHILE DISTALLY

IT HAS BECOME SOMEWHAT ENLARGED AND PARTIALLY ENCIRCLES THC RATHER

OCFINITC CVAGINATION OF THE FLOOR OF THC BRAIN. THE LATTER DE­

VELOPMENT MAY BE REGARDED AS THE INITIAL APPEARANCE OF THC NEURAL

HYPOPHYSIS (FIG . 66) . IT MIGHT SC NOTED THAT THC DEVELOPMENT OF

THC HYPOPHYSIS IN THC MINK PROCEEDS AT A SIGNIFICANTLY GREATER PACE

THAN THAT OF ANY OTHER STRUCTURE CITED HERE. TO LOCATE A 4 6

REASONABLY COMPARATIVE DESCRIPTION OP TNE HYPOPHYSIS IN HUMAN EM­

BRYOS; ONE MUST ADVANCE TO STREETER'S 0 9 4 6 ) PRESENTATION OP HORI­

ZON XVIII AND OP THE TRANSITIONAL CONDITIONS WHICH EXIST BETWEEN

H orizons XVIII and XIX (Streeter, 1951)-

I n THE LUNGS, PURTHER SUBDIVISIONS OP THE SECONDARY BRONCHI

PROOUCE WHAT IS REPERRED TO AS THE BRONCHIAL TREE.

I t w il l be recalled that when the ventral pancreas made it s

INITIAL APPEARANCE (HORIZON XIV), ITS POSITION CLOSELY APPROXIMATED

THE DUODENUM PROM WHICH IT AROSE. A CHANGE IN THC RELATIVE POSITION

OP THC VENTRAL PANCREAS AND DUODENUM WAS PIRST NOTICED IN THC PRE­

CEDING AGE GROUPS ANO IS APPARENT IN THE PRESENT HORIZON* THE DIS­

TANCE BETWEEN THESE TWO STRUCTURES HAS BEEN INCRCABED CONSIDERABLY,

ALTHOUGH THEY ARC STILL JOINED BY THC LENGTHENING COMMON BILE DUCT

(p ig * 69). The ventral pancreas arises directly opp the common bile

DUCT BY A SHORT, VIRTUALLY IMPRCCCPTIBLC DUCT, WHEREAS THE DORSAL

PANCREATIC DUCT IS LONGER AND EASILY DISCERNIBLE (PIG . 70) • THE

COMMON BILE DUCT HAS UNDERGONE A DORSAD ROTATION WITH THC RESULT

THAT THE VENTRAL PANCREAS NOW OCCUPIES A POSITION QUITE CLOSE BUT

STILL SEPARATE PRON THE DORSAL PANCREAS (PIG . 7 0 *

An important structural aovancc w ith respect to thc h in d - gut

REGION WAS OBSERVED IN THIS AGC GROUP. THE SEPARATION OP THC BLAD­

DER PROM THC RECTUM HAS PASSED CAUDAL TO THC URETERS. THE MESO­

NEPHRIC DUCTS AND URETERS STILL ARISE AS BIFURCATIONS OF SINGLE

TUBES, WHICH IN TURN ARISE AS EXTENSIONS PROM THC RIGHT ANO LEFT

SIDES OP THC BLADOCR (PIS* 7 2 ). ASIDE PROM CONSIDERABLE FLATTEN- * 7

RELATIVELY UNMOO IF ICO.

The n a sa l sac has secome somewhat e n la rg e d , the caudal p o rtio n or ITS ventral floor s till maintaining continuity with the roof or

THE MOUTH BY MEANS OF THE EPITHELIAL KEEL* ANTERIOR TO THE KEEL

THERE IS EVIOENCE OF THE PROLIFERATION OF THE MESENCHYME OF THE PRE-

MAXILLARY GROWTH CENTER. THIS IS THE FIRST INDICATION Or THE ES­

TABLISHMENT Or THE PRIMORDIUM OF THE PRIMARY PALATE. ALSO OF SIG­

NIFICANCE IN THIS AGE GROUP IS THE FIRST INDICATION OF THE VOMER­

ONASAL ORGANS (JACOSOSON'S ORGAN). ALTHOUGH THE DEVELOPMENT OF

THESE STRUCTURES IS LIMITED TO A PAIR OF GROOVES, ONE ON THE MEDIAN

WALL OF EACH NASAL SAC, (F IG . 73)* ,T 19 *TILL FAR IN ADVANCE OF

THE SAME STRUCTURES IN THE HUMAN EMBRYO. AS IN THE CASE OF THE

HYPOPHYSIS, ONE MUST ADVANCE TO STREETER'S ( 1 9 ^ ) DESCRIPTION OF

Jacobson's organ in Horizon XVIII to meet a comparable situation.

The DEVELOPMENT OF THE CAR, AS IT WILL BE RECALLED, WAS

CHARACTERIZED BY ITS RAPID PACE IN THE EARLY STAGES. THIS PACE

DIMINISHED TO A SIGNIFICANT EXTENT IN THE PREVIOUS HORIZON, AND THE

MORPHOLOGICAL DESCRIPTION OF THE EAR OF MINK IN THIS ABC GROUP ONCE

AGAIN COMPARES FAVORABLY WITH THE DESCRIPTION o r THE EAR OF HUMAN

EMBRYOS OF THE SAME HORIZON. THE ENDOLYMPHATIC DUCT IS RELATIVELY

LONG. The REMAINDER OF THE OTIC VESICLE HAS UNDERGONE MARKED

DORSO-VENTRAL ELONGATION, THE VENTRAL PORTION Or THE VESICLE

BEGINNING TO TAKE ON THE APPEARANCE OF THE COCHLEAR POUCH AL­

THOUGH THERE IS NOT YET AN OBSERVABLE TRANSITION ZONE BETWEEN THE

POUCH AND THE VESICLE.

The lens of the eye is completely detached from the parent 4 8

ECTODERM BUT I t STILL CLOSELY APPLIED TO IT* THERE IS A SLIGHT DIF­

FERENCE IN THE THICKNESS OF THE OUTER AND INNER WALL OF THE LENS

VESICLE, THE LATTER SEINO SOMEWHAT THICKER* ALTHOUGH MIGRATION OF

NUCLEI OF THE INNER WALL CELLS OF THE LENS APPEARS TO SE UNDERWAY,

NO DEFINITIVE LENS FIBER FORMATION WAS OBSERVED IN THE SPECIMENS

STUDIED (FIG * 7 4 ). k9

Ho r iz o n X V II

TWO CMBRYOS WCRC SCLCCTCD TO RCPRCSCNT TMC PRESCNT AOC BROUB.

Tncy a rc 8 l p and 8 u (littcr-m atcs). That thcsc cmdryos arc thc

ONLY RCPRCSCNTATIVCS OF THIB HORIZON I t UNFORTUNATC SINCE, AT BCST,

THCY ARC ACCCFTCD A t CXTRCMCLY YOUNG MEMBERS. WITH RCSPCCT TO tOMC

CHARACTERI»TIC», THC 8PCCIMCNS MUST tC REGARDED A t TRANSITIONAL,

OCCUPYING A POtITION tCTWCCN T H It AND THC PRCCCDINB HORIZON.

PHOTOBRAPHt OF THC RIGHT AND LCFT tID C t OF 6tA ARC tHOWN IN

F IG t. 75 AN0 76 RCtPCCTIVCLY* ONC OF THC MOtT tTRIKING FCATURCt OF tPCCIMCNt PLACCD IN T H It ABC GROUP I t THC CHARACTCRItTIC FORMATION

CXHItITCD tY THC AURICULAR HILLOCKt OF H it . THCRC ARC t lX AURI­

CULAR HILLOCKS THRCC OF WHICH ARC FORMCO ON THC CAUDAL tURFACC OF

THC MANDIBULAR BAR, AND THRCC WHICH ARC FORMCD ON THC CRANIAL

AtPCCT OF THC HYOID BAR. THC INTCRVCNING GROOVE, BETWEEN THCBC

TWO ROWt OF HILLOCKt IS, OF COURSE, THC HYOMANOISULAR GROOVE. IN

THC PRCBCNT AGC GROUP SOME OF THCSC HILLOCKS APPCAR TO HAVC COALCS-

CCD AND HAVC TAKCN A FORM SIMILAR TO THAT DCSCRIBCD BY STRCCTER

(19*18) FOR HUMAN CMSRYOS IN THC SAMC ABC GROUP. THC HILLOCKS AND

HYOMANDISULAR GROOVE ARC REFCRRCD TO A t A KEY FLAfC. THC OPCNINB

FOR THC KCY (THC GROOVC) I t CNCROACHCO UPON BY THC ENLARGING BCCONO

AND FIFTH HILLOCK. THC KCY SLOT IS THUS SEPARATED INTO UPPCR AND

LOWCR PARTS LIKC A NUMERAL 8 (FIGS. 75 AN0 7 ^ )*

Thc d ig it a l platc of thc arm bud has cnlargcd but thc a c q u is i ­ t io n OF FINGCR RAYS I t ONLY FAINTLY INOICATCD. THC LCG BUD P O ttC ttC t

A ROUNDCD DIGITAL PLATC WHICH I t BARCLY DISTINGUISHED FROM AN CLCVA-

TCD TARSAL RCGION, WHILE BOTH ARC DCCIDCDLY SCT OFF FROM THC LCG 50

BY A CONSTRICTION.

Thc ventral, and a portion op thc lateral aspects or th c

NCURAL HYPOPHYSIS ARC COVCRCO BY THC TCRMINAL PORTION o r Ra THKC'S

POUCH. FlGURC 77 ILLUSTRATES THC COMMUNICATION OP THC NCURAL HYPO­

PHYSIS WITH THC 0 1CNCCPHALON ANO THC CLOSCLY APPLIED, CRESCENTIC,

Rathkc ' s pouch. Thc o is t c n s io n op Rath kc ' s pouch o is t a l l y is c v i -

DCNT IN THIS FIGURE, HOVCVCR, SEVERAL SECTIONS CCPHALAO, THC CAVITY op Rathkc ' s pouch d isappears and it s conncction w ith thc roop op

THC MOUTH ALL BUT DISAPPCARS.

Thc separation op the lungs prom thc parcnt d ig c s t iv c tract

IS CBSCNTIALLY COMPLETE. |T IS POSSIBLE TO RCCOGNIZC THRCC GROUP­

INGS OP BRANCHES OPP THC RIGHT STEM BRONCHUS AND TWO DISCRCTC

GROUPINGS OP BRANCHES OPP THC LCPT BRONCHUS. IT MAY BC INPCRRCD

THAT EACH OP THC ABOVE GROUPINGS WILL CONTRIBUTE TO AS MANY MAJOR

LOBCS OP THC DEFINITIVE LUNGS. OP ADDITIONAL INTCRCST IS THC

FURTHER DIFFERENTIATION OP THC WALL OP THC CSOPHAGUS WHICH LICS

BCHIND THC TRACHEA. |N EVCN THCSC YOUNGEST MEMBERS OP THIS AGC

GROUP ONE CAN RCCOGNIZC THC APPEARANCE OP A RCTICULAR COAT EN­

CLOSING THC CSOPHAGUS WHICH WILL CONTRIBUTE TO THC FORMATION OP

THC CIRCULAR AND LONGITUDINAL MUSCLE LAYCRS. THIS COAT IS ABSENT

IN THC TISSUC CONDENSATION SURROUNDING THC TRACHCA (PIG . 7&)*

Thc DORSAL AND ventral pancreas s t il l r e ta in th c ir separate

DUCTS BUT THC GLANOS THEMSELVES LIE CXTRCMCLY CLOSC TO ONE ANOTHER

( p ig . 79). It could not bc determined in thc spccimcns cxamincd

IP ACTUAL FUSION OF THC DORSAL ANO VCNTRAL PANCREAS HAD BCCN AC­

COMPLISHED. ON THC BASIS OP ITS LOCATION IN THC PLANC OP SCCTIONINQ, IT IS BC-

LICVCO TO REPRESENT THC IN ITIAL APPEARANCE OP THC CACCUM (F IG . 8 0 ).

Thcsc is l it t l c to add at t h is p o in t to thc description op

THC EXCRETORY SYSTEM, THE RECTUM AND BLADOCR WCRC SEPARATED CAUDAL

TO THC JUNCTURC OP THC URCTCRS IN THC PRCCCDINO HORIZON. THC BLAD­

DER IB STILL IN OPCN COMMUNICATION WITH THC ALLONTOIS. A NCW

PCATURC WORTHY OP MENTION* IN RESPECT TO THC RCNAL PELVIS, IS THC

APPCARANCC OP SCVCRAL DIVERTICULA WHICH ARISC PROM THC PELVIS

( p ig , 8l ). Thcsc diverticula arc intcrprctcd as thc in itia l ap-

PCARANCE OP THC COLLECT I NO TUBULES OP THC DCVCLOPINO MCTANCPHRIC

KIDNEY.

There arc c er tain notable advanccb obscrvco w ith rcbpcct to

THC POSTCRIOR EXTENSION OP THC NASAL SACS AND THCIR SURROUNDINO TIS­

SUES. Minute cleavage spaccd were apparcnt in the previously s o lid

EPITHELIAL KEEL (p|G. 8 2 ). I P ONC IS PERMITTED TO ASSUME THAT THC

PATC OP THC EPITHELIAL KEEL OP HUMAN CMBRYOS IS REPEATED IN THC

CASC OP THC MINK CMSRYOS, IT WOULD SC CXPCCTCD THAT THIS NASAL PIN

SCCOMCS TRANSFORMED PROM AN CPITHCLIAL PLATC TO AN CPITHCLIAL LIN -

CD PASSAGE AS A RESULT OP THE COALESCENCE OP ITS CLCAVAGC SPACES.

For a short PERIOO OP TIMC THIS passage is obstructed by thc in ­ complete CLEAVAGE OP THE EPITHELIAL KCCL AND IT CONSTITUTES A cul- dc- sac , the " hintercn Blin d s a c k ” op Pctcr (Streeter ,

1 9 ^8 ). The m axillar y growth ccnter c x h ib it s cvidcncc op markcd

PROLIFERATIVE ACTIVITY, FOLLOWING THE COURSC OP THC PRCMAXILLARY

GROWTH CCNTER, IN THAT BOTH CENTERS HAVC BLCNDCD BENEATH THC NASAL SACS ANTERIOR TO THCIR CPITHCLIAL KCCLS. THC DEFINITIVE PRIM­

ARY PALATC IS THCRCSY ESTABLISHED. THC VOMCRONASL OROANS CONTINUE

TO APPCAR AS MCOIAL PAGINATIONS OP THC NASAL SACS. ASIOC PROM AN

INCRCASC IN THCIR EXTENT* THCSC GROOVES CXHISIT ONLY A SLIGHT AO-

VANCC OVCR THOSE OP THC PRECEDING HORIZON.

Ma k in g a somewhat premature appcarancc is tnc primoroium op

THE NASOLACRIMAL DUCT WHICH ARISES AS A STRAND OP EPITHCLIUM DERIVED

PROM THC VCNTRAL SURFACE OP THC NASOMAXILLARY GROOVE. THIS CPITHC­

LIAL STRAHO COULD BC TRACCD POR ONLY A SHORT DISTANCE ANTERIORLY

THROUGH THC MCSCNCHYMC OP THC MAXILLA.

Thc EPIPHYSIS MAKCS ITS IN ITIAL APPCARANCC IN MEMBERS OP THIS

AGE GROUP (P IG . 83).

The ENDOLYMPHATIC APPENDAGE HAS DCVCLOPCD INTO A RELATIVELY

LARGC THIN WALLED PUSIPORM SAC DISTALLY* BUT BECOMES DUCT-LlKC NEAR

THC POINT o r JUNCTURE WITH THC OTIC VCSICLC. WHILE NO SEMICIRCULAR

OUCTS WERE IN EVIDENCE* AN INVAGINATING FOLD WAS OBSCRVCO ON THC MID­

POSTERIOR ASPECT OP THE OTIC VESICLE. THIS FOLD WAS INTCRPRCTCD AS

A DEMARCATION BETWEEN THC LATERAL VCBTIBULAR POUCH AND THE MEDIAL*

PRIMOROIUM OP THC POSTERIOR SEMICIRCULAR CANAL (P IG . 8 4 ).

There is remarkably l it t l e to rcport w ith rcspcct to the

DEVELOPMENT OF THC CYC. THE CONTRAST BETWEEN THC THICK INNER WALL

AND THE THIN OUTER WALL OP THC LENS IS MORE APPARENT BUT THERE IS

NOT YET THC MARKED INVASION OP THE LCNS CAVITY BY ITS OUTER WALL

CHARACTERISTIC OP HUMAN CYC IN THIS AGC GROUP. THCRC IS STILL NO d e f in it iv e lens f ib e r formation . The h ya lo id artery is prcscnt

IN THC REGION OP THC OPTIC CUP AND THCRC IS CVIDCNCC OP THC 53

FORMATION OF THC VASCULAR CAPSULC OF THC LCNS, DCRIVCD FROM SRANCHCS

OF THC HYALOIO ARTCRY (F IG . 83)* ONC OTHER CHARACTERISTIC OF THC CYC*

WHICH MAKCS ITS APPCARANCC RATHER PRECOCIOUSLY IN THIS ASC GROUP; IS

THC PCRIPHCRAL PROLIFERATION OF MCSOOCRMAL CLCMCNTS BETWEEN THC MAR­

GIN OF THC LCNS ANO SURFACC CCTODCRM. 5 "

Horn io n XVIII

TWO EMBRYOS VCRE •ELCCTCD TO REPRESENT THC PRCSCNT AGE GROUP.

Thcy a r c 16LA and 16LM (litter-m ates). Photographs op thc right ano

LETT SIDES OF THCSC SPECIMENS ARC SHOWN IN FIOS. 86, 8 f , 88, AND 89

RESPECTIVELY.

I t has bccomc increasingly d ip f ic u l t to as sig n human and m in k

CMSRYOS TO A COMMON AOC GROUP, CSPCCIALLY WITH RCSPCCT TO THC PRCSCNT

AND THE PRCCCDING HORIZONS. ISOLATED STRUCTURAL VARIATIONS BETWEEN

THC TWO FORMS WHICH WERE MODERATE IN PREVIOUS HORIZONS HAVC INCREAS­

ED IN MAGNITUDE. IN SOME INSTANCES THC POINT HAS BEEN REACHED

WHCRC A REORGANIZATION OF STREETER'S AGE GROUP CRITERIA FOR HUMAN

EMBRYOS APPEARS NECESSARY IF THEY ARC TO BC USED IN THC CLASSIFI­

CATION OF MINK EMBRYOS. THE PROBLEM IS TO DECIDE AT WHAT POINT

THIS REORGANIZATION MIGHT BE MOST BENEFICIALLY INTRODUCED. THIS

AUTHOR WAS AT FIRST TEMPTED TO RECLASSIFY THC HORIZON CRITERIA IN

PREPARATION OF THE PRESENT AGE GROUP BUT DCCIOCD AGAINST THIS MOVE

FOR SEVERAL REASONS. THE FIRST WAS THAT ALTHOUGH MANY DIVERGEN­

CIES WERE NOTED AT THIS LEVEL OF MINK AND HUMAN DEVELOPMENT, THERE

IS STILL A CONSIDERABLE DEGREE OF SIMILARITY IN EXTENT OF STRUCTUR­

AL ADVANCEMENT. SECOND, THE PRCSCNT HORIZON IS THE LAST OF THIS

s e r i e s . Ad d it io n a l o l d e r s p e c im e n s s h o u l d b e e x a m in e d for a more

DEFINITE INDICATION OF THC PRECISE POINT FOR THC INTRODUCTION OF

CRITERIA RECLASSIFICATION.

The SPECIMENS REPRESENTING THC PRCSCNT HORIZON MUST BE CON­

SIDERED AS THC OLDER MEMBERS OF THIS GROUP. SOME OF THC CHARACTER­

ISTICS THEY EXHIBIT ARC TRANSITIONAL BETWEEN THC PRCSCNT ANO A 5 5

HORIZON WHICH MAY FOLLOW.

Thc CARLY RUDIMENTS of thc cyclio r and thc rathcr sharp FRON­

TONASAL ANQLC ARC APPARENT IN THC PROF ILC PHOTOGRAPHS. THC AURICUL­

AR HILLOCKS OF HlS WHICH WCRC INDICATCD AS SCINO ONC OF THC MOST

STRIKING FCATURCS OF SPCCIMCNS PLACCO IN THC PRCCCDINO HORIZON AP-

PCAR TO SC LOSING THCIR INDIVIDUALITY IN THC PROCCSS OF FUSION.

According to Strcctcr (19^+6) thc uppcr two h illo c k s ( two and thrcc)

OF THC MANOISULAR SAR FUSC TO FORM THC CRUS HCLICIS. THC TWO UP­

PCR HILLOCKS (FOUR ANO FIVC) POSTCRIOR TO THC HYOMANDISULAR CLCFT

ALSO MCRGC TO FORM THC HCLIX. HILLOCKS ONC AND SIX PCRSIST TO SC-

COMC, RCSPCCTIVCLY, THC TRAGUS AND ANT I TRAGUS. WHILC THC FUSION

PROCCSS AS OCSCRIDCD ASOVC BY STRCCTCR FOR HUMAN CMSRYOS APPCARCD

TO HAVC SCCN DUPLICATED IN THC MINK. ONLY THC STUDY OF OLDER MINK

CMSRYOS CAN DEMONSTRATE THC DUPLICATION OF THC FATC OF SUCH FUSION.

Thc hands of a ll spccimcns examined in t h is agc group havc

DISTINCT FINGCR RAYS WITH INTCR-0IGITAL NOTCHES APPEARING ON THC

RIM OF THE DIGITAL PLATE. THC DIGITAL PLATC OF THC LCG EXHIBITS

TOC RAYS AND THC FAINT INDICATION OF INTCROIGITAL NOTCHES AS WELL.

Thc expansion of the d is t a l portion of Rath kc ' s pouch,

WHICH NOW MAY BC REFERRED TO AS THC ANTCRIOR LOBC OF THC PRE­

SUMPTIVE HYPOPHYSIS, CONTINUES IN THIS AGC GROUP. THC CONNEC­

TION OF THC ANTCRIOR LOBC WITH THE CPITHCLIUM OF THC ROOF OF THC

MOUTH STILL PERSISTS, BUT IS LIMITCO TO A SLENDER CPITHCLIAL STCM.

Each mctanephric kidncy possessesan attenuated ureter. As

THC URCTCR IS TRACED DI STALLY FROM ITS ORIGIN IN THC BLADDER THC

LUMCN IS CONSTRICTED ANO THE STRUCTURE ITSELF SCCOMCS EXCEEDINGLY 5 *

DIFFICULT TO IDENTIFY. THC PELVIS OF THC KIDNCY CXHIDITD BRANCHING

OF THC CALYCES ANO THCRC IS MARKED CONDENSATION OF MCTANCPHORGCN1C

TISSUE SURROUNDINO THC COLLECT I NO TUBULES. THCRC WAS NO CVIDCNCC o f M a lp ig h ia n CORPUSCLE FORMATION ( f10. 9 0 ).

The transformation of thc r ig h t and left nasal sacs lcads to

THC ESTABLISHMENT OF RIGHT AND LCFT RESPIRATORY PASSAGCS WHICH BY­

PASS THE MOUTH. THE FINAL STAGE OF THC TRANSFORMATION IS ACCOM­

PLISHED BY THC COMPLETE CLEAVAGE OF THC CPITHCLIAL KCCL AND THC

COALESCENCE OF THC NASAL SAC AND ORAL CAVITIES. THC SITE OF THIS

COALESCENCE MARKS THC POSITION OF THE CHOANAE (F IG . $\), THE ‘

VOMERONASAL ORGANS ARC STILL WELL IN ADVANCE OF THOSE CHARACTER­

ISTIC OF HUMAN EMBRYOS IN THIS AGC OROUB. WHAT WERE ESSENTIALLY

DEEP GROOVES IN THC PRECEDING HORIZON HAVC BECOME TRANSFORMED INTO

CAUOALLY DIRECTED, BLIND SACS WHICH OPEN BROADLY INTO THC MEDIAN

ASPECT OF THC NASAL PASSAGCS (FIGS. $2 AND 9 3 )*

There arc two important developments to report w ith respect

to the inner car . One is concerned w ith the establishment of the

SEMICIRCULAR CANALS. THIS DEVELOPMENT WAS FORESHADOWED IN THC PRE­

VIOUS HORIZON BY THC APPEARANCE OF AN INVAGINATINO FOLD ON THC MID-

POSTERIOR ASPECT OF THC OTIC VESICLE. IN ALL OF THC SPCCIMCNS RE­

PRESENTING THE PRESENT AGE GROUP, ALL THRCC OF THC SEMICIRCULAR

CANALS HAVE BEEN FORMED (FIGS. AND 95 )* ALSO, IN THIS AGC

GROUP, THE COCHLEAR POUCH EXHIBITS A NOTABLE ADVANCE IN THAT ITS

VENTRALLY DIRECTED, TAPERING TIP COMMENCES TO GROW AT RIGHT ANGLES

TO THC REMAINDER OF THC POUCH. THIS RESULTS IN AN "L M-SHAPEO

COCHLEAR DUCT. There is a remarkable change in the appearance or the le n s .

The meoial wall of the lens has undergone tremendous enlargement,

BULGING INTO THE LENS CAVITY. THIS EXTREME THICKENING OP ONC ASPECT

OF THC LENS BODY REDUCES THE LENS CAVITY TO A THIN CRESCENTIC AREA.

Lens fibers arc also evident. Some of the nuclei of the retina ap­ pear TO BC MIGRATING INWARD (F IG . $6) . WlTH RESPECT TO THE ABOVE

MENTIONED CHARACTERISTICS THC DEVELOPMENT OF THE CYC IN MINK IS

FAIRLY WELL IN PACE WITH THAT OF THE HUMAN EMBRYO. IN ANOTHER

RESPECT, HOWEVER, THE EYE EXHIBITS A DEFINITE ADVANCE, A CHARAC­

TERISTIC NOT INTRODUCED IN THC TREATMENT OF THC HUMAN EYE DEVEL­

OPMENT UNTIL THE FOLLOWING HORIZON. REFERRING AGAIN TO FIGURE $6,

ONC CAN DETECT A THIN LAYER OF MESODERMAL CELLS OCCUPYING A POSI­

TION BETWEEN THC EPITHELIUM OF THC LENS AND THE SURFACE ECTODERM.

IT MAY BE INFERRED THAT THIS LAYER OF TISSUE REPRESENTS THC

PR IMOROIUM OF THE CORNEA ANO THC FIRST STEP IN ITS FORMATION. DISCUSSION

Although it is not tnc purpose op t h is paper to TREAT TMOROUSH-

LY THE PHENOMENON OP DELAYED IMPLANTATION IN THE MINK, IT MIONT SE

WELL TO DISCUSS THC SITUATION, AT LEAST AS IT RELATES TO THC D IF­

FICULTY OP RECOVERINS STAOCD CMSRYONIC MATERIAL* DUE TO VARIATIONS

IN DELAY OP IMPLANTATION, FEMALES SUED ON THC SAME DAY MAY YIELD

EMBRYOS OP SI ON IPICANTLY DIPPCRCNT STASES OP DEVELOPMENT ON THC SAME

RECOVERY DATE. THERE IS NO ASSURANCE, THEREFORE, THAT FEMALES BRED

ON CONSECUTIVE OATES WILL YICLO PROSRCSSIVCLY STAGED EMBRYOS

(T able I ) .

Al l staocs op development prom the free blastocyst to Ho r izo n

X V III WERE RECOVERED PROM THC FIRST WEEK OP APRIL TO THC THIRD WEEK

OP THC SAME MONTH. DESPITE VARIATIONS IN THC DATE OP IMPLANTATION,

THESE VARIATIONS APPEARED TO BC SMALL ENOUGH TO ALLOW, WITH SOME

EXCEPTIONS, THC RECOVERY OP PROGRESSIVELY OLDER EMBRYOS ON PRO­

GRESSIVELY LATER DATES. THE ABOVE STATEMENT CAN BC MADE ONLY WITH

RCPCRCNCE TO THC MATERIAL EXAMINED IN THIS WORK. IT MAY WELL BC,

FOR EXAMPLE, THAT VARIATIONS OP IMPLANTATION NOTED HERE WERE UN­

CHARACTERISTICALLY SMALL, ALLOWING THC GENERALIZATION TO BC MA0C.

Th is is extremely l ik e l y when one considers that the m in k ex ­

h ib it s A GREATER VARIATION (pROM 40 TO 75 BAYS) OP LENGTH OP PREG­

NANCY THAN ANY OTHER MAMMAL REPORTED IN THC LITCRATURC (ENOERS

1952), AND THAT VARIATIONS IN PERIODS OP GESTATION ARC ATTRIBUTED

TO VARIATIONS IN IMPLANTATION DATES. SINCE THC ACTUAL DATE OP

IMPLANTATION FOR A GIVEN FEMALE MINK IS UNKNOWN, IT IS IMPOSSIBLE 59

TO DETERMINE WITH A DESIRED DEOREE O f ACCURACY THC AOC OP CMSRYOS

SHE IS CARRY INO. SINCE CMSRYONIC DEVELOPMENT PROCEEDS AT A RCOULAR

PACE FOLLOWI NO IMPLANTATION, THC SOLUTION OP THIS PROBLEM APPEARS TO

LIE IN THE ACCURATE DETERMINATION OP THE IMPLANTATION OATC POR EACH

MINK. Th is lead s to the desirability op a prconancy test for m in k .

Wit h in recent years proqs and toads have seen used as test a n im a l s

IN PRCONANCY DIAONOSIS. BRIEFLY, THC BASIS POR THC TEST RESTS UP­

ON THC PRINCIPLE OP QONADOTROPHIC HORMONE PRODUCTION BY CHORIONIC

TISSUES. The hormones arc pound in the urine, scrum, ano pre­ sum ably, EVEN IN THC PCCCS (BhADURI 1950) o r THE PREGNANT FEMALE.

After injection op any one op these substances (usually in con­ centrated form) into thc test animal, their oamctokinctic effect

is rcplcctco in thc oischaroc op spermatozoa by thc test animal.

While almost a ll op the tests developed have seen directed toward

THC DIAONOSIS OF PRCONANCY IN THC HUMAN, A PEW HAVC BEEN APPLIED TO

OTHER MAMMALS (BHADURI 1 9 5 1 ). THE RESULTS OP THC LAST MENTIONED

TESTS ARC PAR PROM CONCLUSIVE BUT NEVERTHELESS THEY ARC ENCOURAQ-

INO. The development op a dependable prconancy test for the m in k

WOULD BC A BOON TO THC CMSRYOLOOIST WORK INO NOT ONLY ON THE MINK,

BUT ON ANY FORM CHARACTERIZED BY DELAYED IMPLANTATION.

There is l it t l e to report in the comparison op the zy o o tic

STAGE OP MINK EMBRYOS WITH THOSE OP OTHER MUSTCLIDS AND CLOSELY

RELATED FORMS. MAINLAND (1931) AND HAMILTON (1 9 3 ^ ) PRCSCNT DATA

ON THC FERTILIZED COOS OP THE FERRET. HAMLCTT (1 9 3 2 ), UNFOR­

TUNATELY, LIM ITS HIS DESCRIPTION OP THC BADQCR COO TO A STAGE

PRIOR TO THC FIRST MATURATION D IVISIO N . THE DIAMETER OP THE 6o

FERTILIZED COO OF THC MINK (ENOCHS 193^) IS I I 1! MICRONS. THIS

FIGURC COMPARCS FAVORABLY WITH THAT O f THC MATUHC OVARIAN COO O f THC

BADOCR, 114 X 150 MICRONS (HAMLCTT 1932) ANO IS SOMCWHAT ORCATCR THAN

THAT O r THC CAT, 85 X 7^ MICRONS (HlLL 192*1). THC ZONA PCLLUCIDA Of

THC rC R TILIZC D MINK COO IS CONSIOCRABLY THtCKCR ( l l MICRONS) THAN

THAT riOURC OIVCN BY HAINLANO (1 9 3 & ) r o * ™ C ^SRRCT (*l TO 6 Ml CRONS)«

E n o chs ( 1 9 3 8 ) reports no tcnocncy toward polar accumulation o r

LIPOID MATCHIAL IN THC MINK COO, WHCRCAS BOTH MAINLANO ANO HAMILTON

OBSCRVCO POLARITY IN THC FCRRCT COO. ALL Or THC ABOVC INVCSTIOA-

TORS AORCC ON THC PRCSCNCC Or LIP O ID MATCH IAL IN THC CYTOPLASM ANO

A SHARPLY OCriNCO ZONA PCLLUCIDA.

I n a ll or thc larocr mink blastocysts cxaminco by thc prcscnt

AUTHOR, THC INNCR CCLL MASS OCCUPICS AN ANTIMCSOMCTRIAL POSITION.

T hc ZONA p c l l u c id a i s p r c s c n t ano t h i s m n o i n o is c o n s is t c n t w it h t h a t o r E n o c h s a n d P ca r s o n (1 9 *1 3 ), h c p o r t in o on thc b l a s t o c y s t o r thc n s H C R . I n f a c t , th c s c a u t h o r s d c s c r ib c thc zo n a o r th c f is h c r

AS BC I NO conspicuous , a v c r a o in o 1*1.*1 m ic r o n s i n t h ic k m c s s , a n d c o n-

SIOCRABLY THICKCR THAN THOSC OF THC MARTCN, WCASCL, OR BAOOCR. THC

CARLY MINK BLASTOCYST RCVCALS A LAYCR Or CCLLS OVCRLYINO THC INNCR c c l l m a s s (R a u s c h ' s l a y c r ) . Du r in g th c ocvclopmcnt o r th c m in k

BLASTOCYST, RAUSCH'S LAYCR DISAPPEARS. THIS LAYCR OISAPPCARS IN

THC FCRRCT (HAMILTON 1 9 3 7 ) AT A RCLATIVCLY CARLY STAOC OF BLASTO­

CYST FORMATION. |N ALL Or THC FORMS MCNTIONCO THUS FAR IN THC

DISCUSSION, THC NYPOBLAST CCLLS PROLIFCRATC FROM THCIR OR 10 IN

BCNCATH THC INNCR CCLL MASS, CVCNTUALLY FORMINO A COMPLCTC INNCR 61

I f THC OSSCRVATIONS O f VAN OCR STNICNT (1 923) ANO HlLL (l? 2 H ) ON THC

MANNCN Or HYPOBLAST fONMATION IN THC OOO ANO CAT, RCSPCCTIVCLV, ANC

CONNCCT, IT WOULD OCCN THAT THC MINK MOKC CLOSCLY RCtCMOLCO THC CAT

IN THIS NCOPCCT. THAT I t , THC DIFPCNCNTIATION O f THC CPIOLAOT ANO

INO o r THC INNCN CCLL MAOO ANO ITS INTCNCALATION WITH THC SURROUND-

INO TNOPHOOLAOT CLCMCNTO.

Thc FORMATION or MCOOOCRMAL CLCMCNTO IN THC MINK AND IN THC

rcRRCT (Hamilton 1937) 10 coocntially sim ilar* In doth cmsryonic

rORMO THC MCOOOCRMAL CCLLS APPCAR TO ARIOC FROM THC CCTOOCRM AND BC-

COMC OIOPCROCD SCTWCCN THC CCTOOCRM ANO CNTOOCRM OF THC FORMAT IVC

ARCA.

Wit h thc c x c c p t io n o r Ha m l c t t ' o (1 9 3 2 ) papcr on thc badgcr ,

A OCARCH THROUGH THC LITCRATURC RCVCALO NO ATTCMPTf PRCVIOUO TO THC

PRCSCNT WORK; TO OCSCRIBC THC CMSRYOLOOY Or A MUSTCLID BCYONO THC

PRIMITIVC STRCAK STAOC. CVCN THC PAPCR SY HAMLCTT, CITCO ABOVC,

NCQLCCTS THC ORSANOOCNCS10 OF THC CMORYO ANO STRCSBCS INSTCAO THC

RCLATIONSHIP ANO ABSOCIATION OF MATCRNAL ANO CXTRA-CMORYONIC

TISSUCB.

IT IS COMMON PRACTICC, IN THC OCSCRIPTIVC CMSRYOLOOY OF A

NCW FORM, TO INCLUDC OCCASIONAL, COMPARATIVC RCFCRCNCCS TO ANOTHCR

FORM (OR OTHCR FORMS) ALRCADY INVCSTIOATCO. In THC PRCSCNT WORK,

THC AUTHOR HAS LCANCO HCAVILY UPON THC OUTSTANDING PUBLICATIONS

PRCPARCO SY THC LATC Or . STRCCTCR ON HUMAN CMSRYOS. THC RCASONS

FOR THC SCLCCTION OF STRCCTCR'S WORK AS A GUIOC WCRC TWO-FOLOJ

F ir s t , thc papc r s prcscntco by Strcctcr rcprcscnt what is p r o b a b ly 6 2

THC MOST COMPRCHCNSIVC YCT PCTAILCO ACCOUNT OP MAMMALIAN DCVCLOPMCMT.

SCCOND, THC AUTHOR OP THC PRCSCNT WORK WAS IMPRCSSCO WITH THC MCTNOD sy w h ic h Strcctcr staoco h is m a t c r ia l , and dc c io c d to d u p l ic a t c , ip

POSSIBLC, THC PROCCOURC IN CLASSIPYINQ MINK CMSRYOS.

Thc fo llo w in o is a tasu la r summarization op human and m in k cm-

SRYONIC AOC OROUPS (TASLC I I ) • IT IS DCRIVCD PROM STRCCTCR*S WORK ON

MAN AND THC PRCSCNT WORK ON MINK.

IT IS HOPCD THAT THC PRCSCNTATION OP THCSC DATA WILL PULPILL

THRCC OBUCCTIVCS. FlRST, TO SUMMARIZC THC CONDITIONS POUND IN CACH

OP THC MINK CMSRYO HORIZONS, PLACING THCM CLOSC TO TNC CORRCSPONO-

I NO DCSCR IPTIONS OP HUMAN CMSRYO HORIZONS POR CASY CROSS RCPCRCNCC.

SCCOND, TO ILLUSTRATC THC PARTIAL NATURC OP THC PRCSCNT INVCSTIGA-

TI ON SY COMPARISON WITH THC COMPLCTC WORK OP STRCCTCR ANO THCRCSY

RCVCAL THC CXTCHT OP WORK YCT TO BC DONC ON MINK CMSRYOLOGY. FINAL­

LY, THIS SUMMARY SHOULD AID IN SRINO INO INTO SHARPCR FOCUS THC D IF -

PCRCNCCS AS WCLL AS THC SI MlLARITICS WHICH CXIST BCTWCCN THC TWO

FORMS.

Throughout thc coursc op m in k dcvclopmcnt covcrcd in t h is

WORK ONC IS IMPRCSSCD WITH THC RCMARKASLC SIMILARITY SCTWCCN THIS

FORM AND THC DCVCLOPING HUMAN CMSRYO. THIS SIMILARITY IS NOT ONLY

STRIKING IN THC APPCARANCC OP THC CMCRGING ORGAN SYSTCMS SUT, TO A

LARGC CXTCNT, WITH THC SCOUCNCC OP SUCH CMCRGCNCC. 63 TABLE II

Age Group Characteristics Age Group Characteristics por Human E mbryos (Str e e t e r ) for Min k EN sryos (Kis s e n )

Ho r izo n I Zyqote ; one c e l l . Zygote ; one c e l l . (E nders ) H o r 1zon 11 Blasto m eres ; ear ly cleavage Early blastocyst, segmentation cav­ i t y . No INNER CELL MASS. Ho r izo n I I I Segmentation c a v it y ; free F ree bla s t o c y s t ; antimesometrial BLASTOCYST. INNER CELL MASS; ENTOOERMAL CELLS LINE INNER CELL MASS; ZONA PEL- LUCIDA. Ho r izo n IV At t a c h in g ovum ; cytotropho - Appearance of mesoderm ; s i la m in a r SLAST ANO SYNCYTIUM. BLASTOCYST. Ho r izo n V E gg im bed d ed ; d is t in c t c h o r i­ o n ic c a v it y ; no v i l l i ; em - SRYONIC AXIS INDICATED. H o riz o n VI Be g in n in g v i l l i ; a x is d e f i ­ n it e YOLK SAC. H o riz o n VII De c io u a capsular is c o m plete ; BEGINNING OF PRIMITIVE NOOE; BRANCH ING V IL L I. Ho r izo n VI11 Germ d is c f l a t ; no neural fo ld s ; notochordal process ; CHORD CANAL. Ho r izo n IX Neural fo ld s ; s o m it e s s e g in ­ Neural groove; heao fold; fore­ n in g to form; fore - gut i n d i ­ g u t ANO ORAL MEMBRANE FORMED; c ated ; PERICARDIAL CAVITY SE- APPROXIMATELY k SOMITES; PAIREO GINNING TO FORM CRANIAL TUBULAR HEART BUT FUSION IS IM­ FLEXURE. MINENT. H o r 1zon X Fu s io n of neural folds im ­ Fusion of neural folds in cranial m in e n t or in progress ; 4 to TWO-THIRDS OF EMBRYO; 10 TO 13 12 PAIRS OF s o m it e s ; o p t ic PAIRS OF somites; anlage of h in d- su lc u s ; m a n d is u la r and h y o id g u t; PRONEPHRIC TUBULES AND DUCTS SARS VISIBLE EXTERNALLY. FROM 6th to 12th somites; OPTIC VESICLES. jFORM I NO. DIFFERENTIA­ TION OF SOMITES. 6 4

TABLE I t (C o n t in u e d)

Aac Group Characteristic# Age Group Characteristics po r Hu m a n E m b r y o * (St r e e t e r ) po r M in k E m b r y o * ( K is s e n )

Ho r izo n XI Form ation and closure op an te ­ D isappearance op an te r io r neuro - r io r neuropore ; presence op pore AND PERSISTENCE OP POSTERIOR HYOID AND MANOIBULAR BAR*; NEUROPORC; 2 PHARYNGEAL POUCHES; presence op V, V11-V111 nerves ; MANDIBULAR AND HYOID BARS VISIBLE; 13 TO 20 SOMITES. PRESENCE OP V, VI I-V I I I GANGLIA. Ho r izo n XII I n older spec im en s pour bran ­ Three b r a n c h ia l bars ; bifurcation c h ia l bars arc v is ib l e ; c e r v i ­ OP FIRST BAR INTO MAXILLARY AND cal s in u s depressions ; closure MANDIBULAR PROCESSES, CLOSURE OP op po ster io r ncuropore ; p r im - po ster io r ncuropore ; Rath ke ”* ORDIUM OP ARM BUD AT 8th TO pouch; th yr o io d epr essio n ; 10th somite; omental bursa marks l iv e r oivcrticulum ; r ig h t and LOCATION op stomach; near closure l e f t br o n c h ial stcmb ; otocyst op otocyst; solid mass op live r closed ; pr im o r o iu m op arm buo ; tissue; thyroid diverticulum. mesonephric ducts empty in to clo aca . Ho rizo n XIII Arm and leg buds v is ib l e ; 4 Arm and leg buds v is ib l e ; arm bud a o r t ic arches ; otocyst com­ A PRONOUNCEO RIDGE; MEDIAN THY­ p le t e ly closed ; m ed ian thyro io ROID conspicuous ; pr im ar y b r o n c h i ; AND ULTIMO-BRANCHIAL BODY CON­ PRIMORDIUM OP GALL BLADDER; PRO­ SPICUOUS; MESONEPHRIC DUCT NOUNCED DORSAL PANCREAS; THICKEN­ OPENS TO U .-G . s in u s ; l iv e r ING OP RETINAL DISC ANO BEGIN­ INCREASES IN SIZE AND COM NING OP OPTIC cup; differentia ­ p l c x it y ; pig m en t layer of t io n OP ENDOLYMPHATIC APPENDAGE. r e t in a ; t h ic k e n in g op lens PLATC. Ho r iz o n XIV Arm BUDS ROUNDED AND TAPER­ Arm buds d e f in it e pr o je c tin g ap ­ ING; LEG BUDS P IN -LIK E ; DIS­ pendages ; leg buds p in - l i k e ; d is ­ APPEARANCE op t h ir d bran ­ appearance OP THIRD BRANCHIAL BAR c h ia l bar ; subdivision op INTO THE CERVICAL SINUS; PAINT h y o id bar ; o t ic v e s ic l e w ith SUBDIVISION OP HYOID BAR; DISAP­ WELL MARKED ENDOLYMPHATIC AP­ PEARANCE OP THYROQLOSSAL DUCT; PENDAGE; INDENTATION OP LENS TRACHEA DETACHED PROM ESOPHAGUS; VESICLE; OLFACTORY PLACODE PULMONARY VEINS; VENTRAL PAN­ th ic k e n e d ; trachea DETACHED CREAS, WELL DEVELOPED MESONEPHROS; PROM CSOPHAGUS; INITIATION INDENTATION OP LENS; BEGINNING OP INTESTINAL COILS; D IF­ PIGMENTATION OP RETINAL LAYER. FERENT I AT ION OP MESONEPHROS. 6 5 TABLE I I (Continued)

JU l QROUP CHARACTERIITICO AOC group Characteristics por Human E morvoo (St r c c tc r ) por Min k E mbryos ( K io o c n )

Hor ISON XV Clo sur e op lc n s v e s ic l e ; forma - Arm DUO DIPPERCNTIATCD INTO D IS ­ TION OP OLFACTORY P IT S ; ARM DUO TAL HAND SCOMCNT AND PROXIMAL ARM DEVELOPS OISTAL NANO OCOMCNT AND SHOULDER SCOMCNT; LEO SUDS AND PROXIMAL ARM ANO SHOULDER DEFINITE PROJECTINO APPCNOAOES; r c s io n ; seconoary and t c r t ia r y inpundisulum ; secondary b r o n c h i ; SRONCNl ; VENTRAL PANCREAS! c y s t ic d u c t ; f o r m a tio n op u r e ­ caec uh ; s e p a r a t io n op blaodcr te r s ; RUOIMENT OP PELVIS OP PROM RCCTUM CAUDAL TO URCTERSj k id n e y ; e p it h e l ia l k e e l j o in s CCRCSRAL EVAS INAT IONS SECOM- VENTRAL ASPECT OP NASAL SACS INO DCPINITE. WITH ROOF OP MOUTH; LCNS VESICLE COMPLETELY CLOSED. ISON XVI Na s a l w in s ovcrnanos ploor op Ha NO SCOMCNT DIPPERCNTIATCD INTO OLPACTORY DEPRESSION; PRECUR­ CARPAL PART AROUND WHICH SPREADS SORS OP AURICULAR HILLOCKS; THC OIOITAL PLATE; PRECURSORS OP HAND RES ION DIPPERCNTIATCD INTO AURICULAR HILLOCKS; POSTERIOR CENTRAL CARPAL PART SURROUNO- LOSE OP h y p o p h y s is ; subdivision EO SY o io it a l p l a t e ; e p ip h y s is ; op seconoary b r o n c h i ; dorsad NCURAL h y p o p h y s is ; o c p in it c MlORATION OP VENTRAL PANCREAS; PIOMCNTATION OP RETINA; LCNS SEPARATION OP BLADDER PROM RECTUM VESICLE PRCE PROM OVERLY INO CAUDAL TO URETERS; PRIMOROIUM OP cctoocrm; le n s p is c r forma ­ PRIMARY PALATE; VOMERONASAL OR­ t io n ; PRIMOROIUM OP LCNS SONS AS PAIR OP OROOVCS; ELOMOA- BODY; COCHLEAR POUCH; CPITHC­ TION OP OTIC VESICLE. LIA L KEEL SCLOM THC NASAL SACS. Hor ISON XVII D is t in c t nasofrontal oroovc , o l ­ Oc p in it c a u r ic u l a r h il l o c k s ; p a in t fac to r y P IT OPENS VCNTRALWARO INDICATION OP PINOER RAYS; LEO BUD ANO NOT SEEN IN PROFILE; DEF­ WITH OIOITAL PLATC BARELY D I8TIN- INITE AURICULAR HILLOCKS; FIN­ OUISHED PROM TARSAL RCSION; LUMEN DER RAYS AND INTEND ISITAL NOTCHES OP HYPOPHYSEAL STALK ONLY PARTIAL­ ON OIOITAL PLATC OP OLDER SPECI­ LY p r e s e n t ; caecum ; i n i t i a l a p ­ MENS; ROUNDED OIOITAL PLATC ON pearance OP COLLECTINO TUBULES IN LEO BUS; TRANSFORMATION OP C PI­ k id n e y ; clcavao c s p a c e s in e p i ­ THCLIAL KEEL TO INCOMPLETE C P I- t h e l ia l KEELS OP NASAL SACS; THCLIAL-LINCO PASSAOC OPEN TO PRIMOROIUM OP NASOLACRIMAL DUCTS; VENTRAL PLOOR OP NASAL SACS; e p ip h y s is ; b c s in n in o op SEMI­ VERMIFORM APPENDIX; FUSION OP CIRCULAR duct f o r m a t io n ; h y a l o id DORSAL ANO VENTRAL PANCREAS OUT a r t e r y ; proliferation op mesoderm NOT OP DUCTS; NCURAL HYPOPHYSIS; SCTWEEN MARSIN OP LCNS AND SUR­ CRESCENTIC CAVITY OP LCNS FACE ECTODERM. VESICLE. 6 6

TABLE I I (C o n t in u e d)

Aac Group Characteristics Aac Group Characteristics pon Human E mbryos (Strcctcr) por Min k E mbryos (K is s e n ) ____

E ar ly rudimcnt op c y c lid s ano Ruoim cnts op c y c l id s ; fu s io n op oroovc in it ia t in o conjunctival somc au r ic u lar h il l o c k s , hands sacs ; pu s io n op somc a u r ic u lar w it h d is t in c t pino cr rays and h il l o c k s ; d is t in c t pin o c r rays intcrdioital notches ; TOC RAYS WITN INTCRDIOITAL NOTCNCS; TOC ANO PAINT INDICATION OP INTER* RAYS OUT NO INTCROIttlTAL NOTCNCS; OIOITAL NOTCHES; HYPOPHYSEAL CHOANAC; VOMERONASAL OROANS AS STALK IS A SLCNOCR CPITHCLIAL SHALLOW POLOS; INITIATION OP STCM; BRANCHINO OP CALYCES, NASOLARCRIMAL DUCT; BIFURCA- FORMATION OP CNOANAC; VOMCR- TION OP SCCONOARY BRONCHI; SEP- ONASAL OROANS ARC CAUDALLY ARATION OP RIGHT AND LCPT VCN- DIRCCTCD, SLINO SACS; 3 « M I - TRICLC; CAVITY OP LCNS VCSICLC CIRCULAR DUCTS; "L * SMAPCO REDUCED TO S LIT ; BRANCH INO COCHLEAR DUCT; INVAOINATION CALYCCS; PRIMORDIA OP KIDNCY OP PORTION OP LCNS WALL; LCNS TUBULCS; 3 BENI CIRCULAR DUCTS. PIBCR CVIDCNT. H o r iz o n XIX E x TRCMITICS CXTCND NEARLY STRAIOHT FORWARD; TOC RAYS PROMINENT BUT NO INTCRDIOITAL notches ; p r im it iv c corneal body ; sm all slcndcr o p t ic ncrvc ; TIP OP cochlca turns up; in it ia t io n op renal VCSICLCS, THICK HYP0PHY8CAL STALK WITH REMNANT OP LUMCN (Rath kc ' s pouch) ; vomer ­ onasal OROANS CSSCNTIALLY OROOVCS OR PITS. H o r iz o n XX Arms s l ig h t l y scnt at e l ­ bows ; HANDS WITH SHORT STUBBY PINOCRS; LONG SLCNDCR HYPO- PHYSCAL s t a l k ; DIPPCRCNTIAT- I NO CORNCAL BODY; BROAD OPEN­ ING OP VOMCRONASAL ORGAN AT ORAL CND, CAUDAL BLIND SAC. 67

TABLE I I (Co n t in u e d)

Aoc Group Characteristics Aac Group Cnaractcridticd por Human E moryos (Strcctcr) por Min k E mbryos (Kis s e h )

Horizon XXI FINOCRS LONG WITH SWOLLEN TCRMINAL PHALANGES; HANDS SLIONTLY PLCXCO AT WRIST; CORNCAL SODY COMPACT, 2 TO 5 CCLLS THICK, RCTURN DOWN* CURVC OP cochlca ; thread * LIKC HYPOPHYSEAL STALK] ORAL OPCNINO OP VOMCR- ONASAL OROANS RCDUCCD IN SIZC, SHORT NARROW NCCK, CAUOAL CNO EXPAND INO• H orizon XXII Be g in n in g c y c lio closure ; TRAGUS AND ANT I TRAGUS BE- comino d e f in it e ; Dcscemet ' s enoothclium ; a pew large g lo m eruli ; remnant op in ­ complete HYPOPHYSEAL STALK AT EITHER END. H orizon XXIII Th ic k corneal sooy ; second TURN UPWARD OP COCHLEAR TIP; LONG SECRETORY TUBULES IN KIDNEY. 66

I n THE EARLIER STAGES OF DEVELOPMENT DIVERGENCIES ARC MODERATE.

For example, in Horizon X II of this work the otocyst is closed and

THE ARM BUD APPEARS AS A RIOOC, WHILE THC HUMAN CMSRYOS OF A COMPAR­

ABLE AOC GROUP THC OTOCYST IS STILL OPEN AND THCRC IS ONLY THC PRIM-

ORDIUM OF AN ARM BUD PRCSCNT. THE DEVELOPMENT OF THC NASAL PASSAGCS,

INCLUDING THC VOMERONASAL ORGANS AND NASOLACRIMAL DUCTS, THC HYPO­

PHYSIS, AND THE CORNEA OF THC LCNS IN MINK APPEAR TO BC PRECOCIOUS

IN COMPARISON WITH HUMAN EMBRYOS. ON THE OTHER HAND, IN STREETER'S

(19^5) H o rizo n XIV the subdivision of the hyoid bars into ventral

AND DORSAL SEGMENTS IS ONE OF THC FEW INSTANCES IN WHICH THC APPEAR­

ANCE OF A STRUCTURAL ADVANCE IN HUMAN EMBRYOS IS PRECOCIOUS IN COM­

PARISON TO THC EMERGENCE OF A SIMILAR SITUATION IN THE MINK. AGAIN, in Streeter's (19^6) H o riz o n XV the rectum is separated from the

BLADDER CAUDAL TO THC URETERS. THIS IS IN ADVANCE, IN TIME, OF THC

SAME CONDITION ARISING IN MINK EMBRYOS IN HORIZON X V t.

AS DEVELOPMENT PROCECOS, THC DIFFERENCES BETWEEN COMPARABLE

STAGES OF MINK AND HUMAN EMBRYOS ARC MORE MARKED.

The last horizon in which there remains sufficient sim ilari­ t i e s TO JUSTIFY ASSIGNMENT OF THE SAME SET OF CRITERIA FOR MINK AND

HUMAN EMBRYOS IS HORIZON XVIII. THEREAFTER, MINK AND HUMAN EMBRYOS

DIVERGE IN THC TIME OF APPCARANCC OF STRUCTURES TO THC EXTENT THAT

APPLICATION OF CRITERIA FOR ANY PARTICULAR HORIZON OF HUMAN EMBRYOS

IS NOT APPLICABLE IN THC MINK.

The developmental anatomy of the m in k as presented in t h is

PAPCR IS CONSIDERED ADEQUATE FOR A GENERAL ACCOUNT BUT IS OB­

VIOUSLY NOT A COMPLETE TREATMENT OF THC AGE GROUPS CONSIDERED. 6 9

Mo r e o v e r , t h e r e io s t i l l considerable w o rk r e m a in in o i n t h e r e a l m o r

EARLY EMORYOLOOY (FROM IYQOTE TO PRCCHOROAL STAGE) • THE LAST HORIXON

(XVI I I) INCLUDED IN THIS PAPER FALLS SHORT, BY PERHAPS THREE OR POUR

STAOES, OP COMPLETINQ THE EMORYOLOOY OP T H IS PORM. IT IS UNOERSTOOO

THAT WHILE THE LAST STAOE OP SUCH A SERIES WOULD TECHNICALLY COM­

PLETE THE EMORYOLOOY, SUBSEQUENT FETAL DEVELOPMENT WOULD INVOLVE

CONSIDERABLE INCREMENT IN VOLUME BEFORE B IR T H . HOWEVER, IT IS BE­

LIE V E D THAT THC MATERIAL PRESENTED HERE W IL L CONSTITUTE THC FRAME­

WORK TO WHICH MAY BC ADOCD FURTHER INFORMATION CONCCRNINO NOT ONLY

THC MINK, BUT ALSO OF OTHER MUSTCLIDS.

T h e HORIZON NUMBERS WHICH HAVE SEEN ASCRIBED TO THC VARIOUS AOC

GROUPS OF MINK EMBRYOS A 1C FULLY ASSUMED TO SC TENTATIVE ONES, SUB­

JECT TO CHANCE ON THE B A S IS OF A D D ITIO N AL D ATA. THESE NUMBERS WERE

u s e d b y St r e e t e r to c l a s s i f y t h e v a r io u s ao c g r o u p s o r h u m a n e m b r y o s ,

ANO, AS A MATTER OF CONVENIENCE, WERE EMPLOYED SY THC AUTHOR OF THC

PRCSCNT WORK AS A MEANS OF ID E N T IF Y IN G MINK EMBRYOS IN COMPARABLE

STAGES Or DEVELOPMENT. I t IS FORCSCCABLC THAT WITH ADDITIONAL STUDY

OF SOMC CARLY EMBRYOLOGY STAGE OF THE M IN K , NO COMPARABLE S ITU A TIO N

IN THC CASE OF HUMAN EMBRYOS MIGHT BC FOUND (OR VIC E VERSA) , IN

WHICH CASE A READJUSTMENT OF ALL SUCCEEDING HORIZON NUMBERS WOULD

HAVC TO SC MADE. IN T H IS PAPCR, THEN, THC ASSIGNMENT OF HORIZON

NUMBER X IS MADE TO A PARTICULAR GROUP OF MINK EMBRYOS WHICH COM­

PARE FAVORABLY TO A GROUP OF HUMAN EMBRYOS S IM ILA R LY NUMBCRCO. IT

IS NOT INTENDED THAT THE PRCSCNT SYSTEM OF NUMBERING MINK EMBRYO

AOC GROUPS W ILL HAMPER FUTURE IN V E S T IG A T IO N , BUT SHOULD SC CON- SUMMARY

THC PRESENT p a p e r i t b e l ie v e d to t c th c i n i t i a l a t t e m p t a t

DCSCRIBING THC DEVELOPMENTAL ANATOMY OF THC MINK.

ADVANTAGE WAS TAKCN OF THC BICORHUATC STRUCTURE OF THC MINK

UTCRUt IN THAT TWO tCFANATC CHTKICt ON DIFFCKCNT DAYS WCRC MADC IN­

TO CACH FEMALE FOR THC RCMOVAL OF CMBRYOt. T H IS DOUBLED THC NUMtCR

OF RCCOVCRCD tT A O C t FROM A LIM ITC O STOCK OF BRCD FEMALE*.

BCCAUSC OF DELAYED IMPLANTATION IT WAS NOT POSSISLC TO RE­

COVER OCSIRCO STAOCS ON THC BASIS OF COPULATION AOC SINCC THIS

FACTOR IS CONTINGENT UPON THC INTERVAL OF TIME BETWEEN FERTILIZA­

TION OF THC COO ANO ITS IMPLANTATION. THC DATE OF IMPLANTATION IS

THC LOGICAL POINT AT WHICH TO COMMENCE THE STAOINO OF MINK EMBRYOS.

T h e r e is g e n e r a l a g re e m e n t in t h e l it e r a t u r e t h a t development p r o­

c e e d * AT A RCOULAR PACE FROM TH IS POINT TO PARTURITION. THC SUG­

GESTION IS MADE THAT THE DEVELOPMENT OF A DEPENDABLE PREGNANCY TEST

FOR MINK, WHICH WOULD ESTABLISH THE DATE OF IMPLANTATION, WOULD PRO­

VIDE THE MOST SUITABLE MEANS OF ACCURATELY STAGING MINK EMBRYOS.

RCCOVEREO EMBRYOS WCRC C LASSIFIED WITH RCFCRCNCC TO THC

PRCSCNCC AND STATE OF DEVELOPMENT OF STRUCTURAL CHARACTERISTICS.

Sp e c im e n s w h ic h , to a l a r g e e x t e n t , p o s s e s s e d a s e t o f c h a r a c t e r ­

i s t i c s IN A S IM ILA R STATE OF DEVELOPMENT WCRC PLACED IN A COMMON

AGC GROUP OR "HORIZON**. THIRTEEN SUCH HORIZONS WCRC DESCRIBED

FOR THC MINK AND ARC PRESENTED IN THIS PAPCR.

T he m in k a p p e a r s to d e v e l o p in a t y p ic a l l y m a m m a l ia n

FASHION. No DIFFERENCES HAVC BEEN DETECTED IN THC MODE OF 70 ORGANOGCNCTIC DEVELOPMENT IN MINK VERSUS MAN. DIFFERENCES INVOLVE

RELATIVE TIMINQ IN DIFFERENTIATION OF PARTICULAR STRUCTURED. FOR

EXAMPLE, OTOCYST CLOSURE, DELINEATION OF ARM-DUD RIOOE, ANO CORNEAL

SODY, AMONG OTHERS, APPEAR EARLIER SY AT LEAST ONE HORIZON IN TNE

MINK THAN IN MAN. CONVERSELY, THE SUBDIVISION OF THC HYOID SARD

AHD THC SEPARATION OF THE RECTUM FROM THC BLAOOCR CAUDAL TO TNE

URETERS OCCURS EARLIER IN MAN THAN IN M IN X . IN THC LAST HORIZON

(XVI 1 1 ) , DIVERGENCIES BETWEEN THCSC TWO FORMS IN THC MATTER OF

THC SEQUENCE OF EMERGENCE OF CERTAIN CHARACTERS APPEAR TO WIDEN

AND INDICATE THC NECESSITY OF ESTABLISHING SEPARATE SETS OF

CRITERIA FOR SUCCEEDING MINK HORIZONS.

A TABLE IS INCLUDED WHICH SUMMARIZES THE AGE GROUP CHAR­

ACTERISTICS FOR BOTH THC MINK ANO HUMAN EMBRYOS, THC LATTER AS

DESCRIBED BY STRCCTCR.

IN THC INTEREST OF STANDARDIZATION IT IS HOPED THAT THC

SYSTEM EMPLOYED IN TH IS PAPCR OF STAGING MINK EMBRYOS, AFTER THC

METHOD ESTABLISHED BY THC LATE OR. STREETER FOR HUMAN CMSRYOS,

WILL BC PERPETUATED IN THC CMBRYOLOGICAL STUDIES ON OTHER

MUSTELIOS. ABBREVIATIONS USED IN PLATES

A I * - ANTERIOR INTESTINAL PORTAL HG - HINO-GUT ALO - ALLANTOIC DIVERTICULUM HS - HEPATIC STROMA AM - AMNION NY - HYPOBLAST AMC - AMNIONIC CAVITY HYA - HYALOID ARTERY AN - ANTERIOR NEUROPORE IM - IMPLANTATION CRESCENTS AO - AORTIC ARCH INT - INTESTINE AS - AORTIC SAC INTM “ INTERMEDIATE MESODERM ASC - ANTERIOR SEMICIRCULAR CANAL L - LENS AT - ATRIUM LCB - LENS BODY B - BLASTOCYST LEP “ LENS PI BCR BC - BLASTOCOEL LO - LUNG OUTGROWTH BL BLADDER LSC - LATERAL SEMICIRCULAR CANAL bo - Bowman ' s capsule LT - LARYNGOTRACHEAL GROOVE BUL - BULBUS LV - LENS VESICLE CA - CAECUM MB - MID-BRAIN CAL - CALYCES MCO - MESONEPHRIC DUCT CBO - COMMON B ILE DUCT MCP - MESODERMAL PROLIPERATION CCD - CELLULAR DEBRIS MET - MCTANCPNROGCNIC TISSUE CH - CHOANAC MTD - MCTANCPNRIC DUCT CL - CLOACA NC - NEUROCOCL CO - COLLECT INO TUBULES NHY - NEURAL HYPOTHYSIS CP - CORNEAL PRIMORDIUM NHYP - NEURAL HYPOPHYSEAL CY - CYSTIC DUCT PRIMORDIUM DA - DORSAL AORTA NS - NASAL SAC DI - DI ENCEPHALON OC - ORAL CAVITY OP - DORSAL PANCREAS OD - OLFACTORY DEPRESSION DPD - DORSAL PANCREATIC DUCT OLP - OLFACTORY PLACODE DU - DUODENUM ON - ORAL MEMBRANE EA - ENDOLYMPHATIC DUCT OP - OTIC PIT CK - E P ITH E LIA L KEEL OPV - OPTIC VESICLE EKC - E P IT H E LIA L KEEL CLEAVAGE OS - OPTIC STALK CP - CP IBLAST OV - OTIC VESICLE CPI - EPIPHYSIS P - PHARYNGEAL POUCHES CS - ESOPHAGUS PB - PRIMARY BRONCHI PB - PORE-BRA IN PD - PRONCPHRIC DUCT PG - PORE-GUT PE - PELVIS OP KIDNEY POC - PORAMCN COCCUM PCRC - PERICARDIAL CAVITY GB - GALL BLAODCR PH - PHARYNX GLO - GLOMERULAR TUPT PN - POSTERIOR NCUROPORE GT - GUT TUBE PR - P R IM ITIV E STREAK H - HEART PSC - POSTERIOR SEMICIRCULAR HB - HIND-BRAIN CANAL HO - HEPATIC 0 1VERT ICULUM PSCP - POSTERIOR SEMICIRCULAR HED - HEPATIC DUCT CANAL PRIMORDIUM

72 ABBREVIATIONS (Continued)

PT - PRONEPHRIC TUBULE SU - SUBCEPHALIC POCKET PUA - PULMONARY ARTERY T - TROPHOBLABT PUV - PULMONARY VEIN THY - THYROIO R - RECTUM TR • TRACHEA RAT - RATHKE' s POUCH U - UTERINE TISSUE RD - RETINAL DISC UO - UTERINE OLANDS RP - PIOMENT LAYER OF RETINA UL - UTERINE LUMEN PRIMORDIUM UR - URETERS S - SOMITE URA - URACHUS S8 R - SECONOARY BRONCHI YEN - VENTRICLE SET - SECRETORY TUBULES VO - VOMERONASAL GROOVE SO • SOMATOPLEURE VP - VENTRAL PANCREAS SP - SPLANCHNOPLEURE VS - VOMERONASAL SAC ST - SEPTUM TRANSVERSUM Z - ZONA PCLLUCIDA STOM - STOMACH EXPLANATION OF PLATE I

(Un i t scale - 0.1 mm. u n le s s o th er w ise in d ic a t e d )

F igure Cleavage stag e , transitional setwecn p e r io d op MORULA AND INNER CELL FORMATION. (HORIZON 11)* F igure I nINER CELL MASS OF A FREE BLASTOCYST. (ill' F igure FRfcc blastocyst in the lumen of a uterus. ii M). F igure Fo r m a tiv e area of a free b la s to c y s t , ( i l l ) . FIGURE Me SODERM FORMATION. (lV). F igure He AD FOLD, SHOWING SUBCEPHALIC ROCKET AND s i LAMINAR EMBRYONIC DISC. (IX). FIGURE Fore - gut f u s io n w it h ven tral ectoderm ( oral membrane ) . (IX). FIGURE An te r io r in t e s t in a l p o r t a l . (IX). F igure So m atic and s p la n c h n ic lay er s of mesoderm . (IX). F igure Sa g it t a l s e c t io n showing fore - gut, a n t e r io r in t e s t in a l p o r ta l , and h in d - g u t . (X).

7 * PUTS I EXPLANATION OF PLATE I I

(Un i t s c a le * 0 .1 mm. u n les s o th er w ise in d ic a t e d )

F ig u r e 11. A pronephric tusule and duct at level op SEVENTH SOMITE. (X). F ig u r e 12. Sa g it t a l s e c t io n s h o w in o f i r s t a o r t ic ar c h AND DIFFERENTIATION OF THE HEART. (X). F ig u r e 13. Anterior extrem ity ^ f neural tuse showing IN ITIATIO N OF OPTIC VESICLES. (X). F ig u r e 1 4 . H in d - g u t a n d a l l a n t o is o c c u r r in g a s a s in g l e CHAMSER CAUDAL TO POSTERIOR INTESTINAL PORTAL. (XI). F ig u r e 15. Sagittal section showing oivisions of heart and FORE-, M ID -, ANO HI NO-BRAIN. (Xl). F ig u r e 16. De v e l o p m e n t o f o p t ic v e s ic l e s . (XI). F ig u r e 17* Sa g it t a l S E C tio N s h o w in g o t ic p i t , V, VIl-VIII, AND IX GANGLIA. (XI)• F ig u r e 1 6 . Camera lucida whole mount drawing of a specimen 84P SHOWING PLANE OF SECTIONING. 3*0 MM. GREAT­ EST LENGTH (APPROX.) • (XI I )• F ig u r e 19* P r im o r d iu m o f t h y r o io g l a n d . (XII). F ig u r e 20. L aryngotracheal g roove a n d i t s bifurcation in t o RIGHT AND LEFT BRONCHIAL STEMS. (XI I )• F ig u r e 21. He p a t ic diverticulum a n d a s s o c ia t e d s t r o m a . (XII).

76 PLAYC II. 77 EXPLANATION OF PUTE H I (Unit scale ■ O .t unless otherwise indicated)

F ig u r e 2 2 . Me s o n e p h r ic d u c t s e m p t y in g in t o c l o a c a . (XII)• F ig u r e 23* Communication between hino-but and allantoic DIVERTICULUM. (X I I )• F ig u r e 2k, Camera lucida whole mount drawing or specimen SHOWING PLANE OF SECTIONING. 3 *6 ***• a*S *T“ EST LENGTH. (XII I ) • FIGURE 25. Photograph or right side or specimen 6l m . (X111). F ig u r e 2 6 . Photograph o r l e f t s id e o r s p e c im e n 6 L M . (X III). FIGURE 2 7 . Photograph o r r ig h t s id e o r sp e c im e n 6LA. (XIII). F ig u r e 2 o. Photograph o r l e f t s id e o r s p e c im e n 6LA. (X M l). F ig u r e 2$, Otic vesicle ano in itia tio n or endolymphatic APPENDAGE. (X I I I ) • F ig u r e 30. Foramen caecum marking point or origin or median THYROID. (XIII). F ig u r e 3 1 . P r im a r y b r o n c h i . (XI11) • F ig u r e 32. Hepatic diverticulum and initiation or gall BLADDER PRIMORDIUM. (X I 11)• FIGURE 33* Dorsal pancreas . (XI11)•

7 8 PUTE I I I EXPLANATION OF PLATE IV

(U n i t s c a l e ■ 0*1 u n l e s s o t h e r w is e i n d ic a t e d )

F ig u r e 3 ^ . I n i t i a t i o n o p o p t i c c u p a n d t h i c k e n i n g o p l e n s PLATE. (XIII). F ig u r e 3 5 * P h o t o g r a p h o p r ig h t s i d e o p s p e c im e n 9LM show ­ in g PLANE OP S E C T IO N IN G . 6 . 8 NM. CROWN-RUMP. , (XIV). F ig u r e 3 ° * P h o t o g r a p h o p r ig h t s i d e o p s p e c im e n

6 o

EXPLANATION OF PLATE V

(U n i t s c a l c ■ 0.1 mm. u n l e s s o t h e r w is e in o ic a t c o )

F iq u r c 4 4 . Du o d e n u m , v c n t r a l p a n c r e a s , a n d f o r m a t io n o f DORSAL PANCRCAS. (XIV). FIQURC 4 j . Du o d e n u m a n d d o r s a l p a n c r c a s . (XIV). F iq u r c 4 6 . D ifferentiation o f mcsoncphros , s h o w in o Bo w m an ' s CAPSULE, GLOMERULUS, SECRETORY TUBULCS, AND MCSONCPHRIC DUCT. (XIV). F iq u r c 47. Cr a n ia l q a n o l ia . (XIV). FiauRc 48. I nvagination o f l c n s . (XIV). F iq u r c 4 9 . Ol f a c t o r y d c p r c s s io n . (XIV). FIQURC 50 . P h o t o g r a p h o f r iq h t s i d e o f s p c c im c n 15RM. 6.0 MM. CROWN-RUMP. (XV). F iq u r c 51 . P h o t o g r a p h o f l c f t s i d e o f s p c c im c n 15RM. (XV). F iq u r c 52. Ra t h k c ' s p o u c h a n d p r im o r d iu m o f neu r a l hy p o t h y s is . (XV). F iq u r c 53 . I n i t i a t i o n o f s e c o n d a r y b r o n c h i . (XV). F iq u r c 5 4 . St o m a c h , common b i l e d u c t , a n d hcpaticouct . (XV).

8 2 PLATE V. fl3 I

EXPLANATION OF PLATE VI

(U n i t scale - 0.1 mm. u n le ss o th er w is e in d ic a t e d )

F iq u r e 55. Stomach, point or common bile duct, and ventral PANCREATIC OR 10 IN S. (XV). F ig u r e j> 6 . V e n t r a l p a n c r e a s . Du o d e n u m a n d d o r s a l p a n c r e a s SEPARATED BY CONSTRICTION. (XV). F ig u r e 57. I n t e s t i n e , d o r s a l a n o v e n t r a l p a n c r e a s . (XV). F ig u r e 5 0 . Ur a c h u s a n d c l o a c a a b o u t to ruse. (XV). F ig u r e 59* F u s io n or u r a c h u s a n d c l o a c a . (XV). F io u r e 6 0. Me s o n e p h r ic d u c t s a s l a t e r a l e x t e n s io n s or t h e c l o a c a . (XV). F ig u r e 6 l . D i v i s i o n o r l a t e r a l e x t e n s io n s o r c l o a c a in t o MESONEPHRIC AND METANEPHRIC DUCTS. BULBUS e n d in g ( p e l v i s ) or metonephric d u c t ( u r e t e r ) (XV). F ig u r e 62. Na s a l s a c a n d e p i t h e l i a l k e e l . (XV). F ig u r e 6^. Co m p l e t io n or l e n s invagination . (XV). F ig u r e 6 5 . Photograph or the rig h t side or specimen 11LM. 7 *3 MM. CROWN-RUMP. ( XVI)• F ig u r e 6 5 . P h o t o g r a p h o r t h e l e f t s i d e or s p e c im e n 11LM.

84 PLATE V I EXPLANATION OF PLATE V II

(U n it scale ■ 0 *1 mm. unless otherw ise in d ic a t e d )

F igure 66 . Photograph or the right side or specimen 11 LA• 7 .8 MM. CROWN-RUMP. (XVl). FIGURE . Photograph or the l e t t s id e or s p e c im e n 11 LA. (XVI). F igure • Ra t h k e - s pouch and the p o s t e r io r lo be o r THE HYPOPHYSIS. (XVI). F igure 6 9 . Duodenum, common b i l e d u c t, and h e p a tic d u c t. (XVl). F igure 7 0 . Intestine, ventral pancreas, and dorsal pancreatic duct* (XVI)• F ig u r e 71* Intestine, ventral, and dorsal pancreas. (XVI). F igure 7 2 . Bladder, rectum, mesonephric duct, and ureter. (XVI). FIGURE 73* Vomeronasal o rg an . (XVI). F ig u r e 7 4 . Detached le n s v e s ic l e . (XVI). FIGURE 7 5 . Photograph or the right side or specim en 8la. 9.1 MM. CROWN-RUMP. (XVIl). F ig u re 7 6 . Photograph or the le ft side or specimen &LA. (XVII).

66

EXPLANATION OF PLATE VI11

(Un it scale ■ 0*1 mm. u nless otherw ise in d ic a t e d )

F ig u re 77 . Neural h y p o p h y s is and Ra t h k e ' s pouch. (XVIl). F ig u re 7° » Further diffcrent 1 at ion op trachea and esophagus, pulmonary arteries and vagus nerves. (XVIl). F ig u re 79 . Pr o x im it y o r ventral and dorsal pancreas . (XVIl). F ig u re 00 * I n i t ia l appearance of the caecum . (XVIl). F ig u re 8 l . I n it ia t io n o r c o lle c tin g tubules in the k id n e y . (XVII). F ig u re 82 . Coalescence o r cleavagc spaces in e p it h e l ia l K E E L. (XVIl). F igure 83 . I n it ia l appearance or the e p ip h y s is . (XVIl). F igure 84 . Prim o r dium or po ster io r semicircular c an al , ENDOLYMPHATIC DUCT. (XVIl). F ig u re 85 . Hy a l o id artery , in it ia t io n or mesodermal fo r m atio n BETWEEN THE MARGIN O r THE LENS AND THE SURFACE ECTODERM. (XVIl). F ig u re . Photograph o r the le ft s id e o r specim en 16LA. I I . ^ mm. crown- rump. ( X V I I I ) . F 1gure 87• Photograph o r the le f t s id e o r specim en 16LM. 11.0 mm. c r o w n - r u m p . (XVIII)*

8 8 PLATE VIII EXPLANATION OF PLATE JX

(Un i t scale • 0.1 mm. u n less o th er w ise in d ic a t e d )

F ig u re 6 8 . Photograph or the right side or specim en 16LA, (XVIII). F iq u rc 8 9 . Photograph or the right side or specim en i 6l M. (XVIII). F iq u rc 90 * Branching calyces, malphigian corpuscle PRIMORDIUM. (XVI II). F igure • Coalescence o r the nasal sac and oral c a v it ie s rORMING THE CHOANAE. ( XVIII). F ig u re s 92, 93* Sa c u la t io n o r the vomeronasal grooves . (XVIII). F ig u re s 9^» 95 . Formation or semicircular canals. (XVIII)* F ig u re 9 6 * Reduction or the lens cavity migration or retinal NUCLEI, LENS riDER TORMATION, CORNEAL PRIMOROIUM. (XVIII).

90 P L A T E I X . BIBLIOGRAPHY

Bh a d u r i , J. L. 1950. Ma l e toad ( Bufo melanostictus ) t e s t for b o v in e PREGNANCY BY F IL T E R RARER D IA L Y S IS OF HORMONE EXTRACTION FROM FAECES. S C I, AND C U L T ., l 6 : 2 6 4 .

______1951-* The role of Sa l ie n t ia in human and m a m m a lia n RREONANCY TESTS. PROC. 38 TH I NO. Sc I. CONG., PT. II: 1 7 1 •

Cnders, R. K. 1952. Reproduction in the mink. Proc. of the Amer. Philos. Soc., 9 6 :( 6 ) 91 -7 5 5 *

______and 0. P. Pearson. 19^3* The blastocyst of the fisher. The Anat. Rec., 8 5 1 ( 3 ) 285 .

19^8. The ovum of the mink (Mustela vison). The Anat. R e c ., 7 2 : ( 4 ) 4 6 9 .

H a m ilto n , W. J. 193^* The early stages in the development of the ferret. Trans, of the Roy. Soc. of Coin., 5 8 : 251 .

1937* The ea r ly stag es in the development of the FERRET. Y r ANS. OF THE ROY. SOC. OF COIN., 59* (0 1^5.

Hamlett, G. W. 1932* Observations on the embryology of the badger. Anat. Rec., 5 3 : 286 - 303.

Hansson, A. 1947 . The physiology of reproduction in mink (Mustela vison Schreb.) with special reference to delayed implantation. Acta. Zool., 2 8 : 1- 136.

H ill, J. P . and M. Tribe. 1924. The early development of the cat (Felis domestica). Quart. Jour, or Micros. S ci., 681(272)5 ^3*

Johansson, J . and 0. Vengi. 1951. Relation of the mating interval TO THE OCCURRENCE OF SUPERFETATION IN THE M IN K . ACTA. Z O O L., 32: 255 .

M a in la n d , D . 1931. The early development of the fe rre t: The cytoplasm. Jour, of Anat., 6 5 :(4) 4 1 1 .

92 93

Mainland, D. 1932 * The early development o r thc ferret: The zona GRANULOSA, ZONA PELLUCIDA, ANO ASSOCIATED STRUCTURES. JOUR. or An a t . , 66:(4) 586.

Ne l s o n , 0. E. 1953* Co m p a r a t iv e e m b r y o l o g y or t h c vertebrates . T hc B l a k is t o n Co . I n c . , N . Y , a n d T o r o n t o .

SHACKCLrORO, R. M. 1952. SUPCRfCTATION IN THE RANCH MINK. ANCR. Nat., LXXXVI : 830.

S t r e e t e r , G. L. 19**2. Developmental horizons in human cmsryos. Description o r aoc g ro u p XI, 13 t o 20 som ites and agc group X II, 21 t o 29 somites. Contris. to Emdryol., XXX:( 187- 197) 211- 21*5 .

19^5* Developmental horizons in human embryos* Description o r age g ro u p X III, embryos about k o r 5 mm. lo n o > AND AGE GROUP XIV, PERIOD OP INDENTATION Or THE LENS VESICLE. Contris. to Embryol., XXXI: (198-206) 27*63.

_ _ _ _ _ 19^* Developmental horizons in human embryos* Description o r age g ro u p XV, XVl, XVIl, and X V III, b e in g the THIRD ISSUE o r A SURVEY Or THC CARNEGIE COLLECTION. CONTRIS* to Embryol., XXXII: ( 207- 212 ) 133- 203.

_ _ _ 1951* Developmental horizons in human embryos* &CSCRIPTION o r AGE GROUPS XIX, XX, XXI, XXII, XXIII, BEING the rirTH issue o r a s u rv e y o r THE C a r n c g ie COLLECTION. Contris. to Embryol., XXXIV:(222-230) 1 6 5 - 196*

van OCR S tricht, 0. 1923. The blastocyst o r the dog. Jour, or A n a t . , LV111:(1) 52. AUTOBIOGRAPHY

I , A b b o t t T h e o d o r e K is s e n , w a s born in New Y o r k C i t y ,

New Y o r k , No v e m b e r 24, 1922. I r e c e i v e d m y s e c o n d a r y s c h o o l

EDUCATION IN THE PUBLIC SCHOOLS OP BROOKLYN, NEW YORK,

GRADUATING PROM JAMES MADISON HlGH SCHOOL IN 19*^ • ^

UNDERGRADUATE TRAINING WAS OBTAINED AT BROOKLYN COLLEGE,

B r o o k ly n , New York, prom which I received the Bachelor op

Arts degree in 1950- I entered graduate school at The Ohio

State University in 1950 and received the degree op Master op Arts in 1952. The past pour years have been u tilize d in

COMPLETING THE REQUIREMENTS POR THC DEGREE DOCTOR OP

P h il o s o p h y . Du r in g m y s i x y e a r s i n th e Gr a d u a t e Sc h o o l o p

T h e Oh io St a t e Un i v e r s i t y , I a c t e d a t v a r io u s t im e s i n t h e c a p a c it y o p g r a d u a t e a s s is t a n t a n d a s s i s t a n t in s t r u c t o r .

94