STUDIES IN THE REPRODUCTION~ GROWTH AllD DEVELOP;MEN'r OF THE CHI)lCHILLA
FORREST DONALD TIBBITTS
A 'J."'dESIS
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OREGON STATE COLLEGE
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Dr. Howard n. Uillemann o£ the ~artment of Zoology, I. Introduction • • • • . . • ...... 1 II. Materials and Uethods ...... 1 III. General Remarks on Chinchilla Reproduction . • • 3 Su.~ •••• • • • •• • • • • • • • • • • • • 7 rv. The Ovarian Cycle • • • • • • • • • • • • • 8 ~ .... ' .• . . • • • • • • • • • . . . • • 13 V. Reproductive Potential of the Chinchilla . . 14 s'll:nl.!'nacy • • • • • • • • • • • • • • .. • .. • • 20 VI . Early Ege Loss in the Chinchilla 21 s~ ...... ' . . 24 VII. Development of the Ova!:"'J • ...... - - 25 s Ul:illllary • • • • • • • • • • il .. 29 VIII. Postnatal Gr{)wth in the Chinchilla . . . .30 Gr-aph 1 • • • • • • • • • • • • . . . . .31 Gr nph 2 ...... 32 Summtllj" ...... ~ ...... 33 Platel ...... ~ . . 35 Bibliography • • I . Introduction Because of the chinchilla•s gain as an important £'Ill' bearer, COllllllercial breeders have become increasingly interested in obtain· :illg more and better qu.al.ity animals . This of course implies a thorough i:tJVestigation of the reproductive habits o£ this animal. A survey of the literature indicates that1. with the axception of the guinea pig, there is a. dearth of information e.oncerning the reproduction of' eystricomorph rodents in general and the chinchilla in particular. .Asdell (l) summarizes what work bad been done on the ohincllilla and other Hy'strieomorpha up to 1946 and in l948-49 Pearson (6), (7) contributed bis studies on reproduction of the lUOuntain visca.cha, I.asf:di~ peruanum !!eyen. 'thus, the present - $ . . series of studies '7ere begun to .form a basis upon which could be developed later more comprehensive in~estigations on chinchilla reproduction. II. Materials and Methods The ebine-hillas used in these studies were donated p:rilnarily by west coast commercial breeders. Upon receipt of the animals thei.r weieht and crown- rump measurements were taken, the ova.ties and uteri rem~ed at sacri!iee, weighed and fiXed in Bollin ' s fiuid. In addition, if the !em.Ues were pregnant, the er.1bryos vrerE!t 2 removed, weighed and measured and prepared by the Spalteholtz technique for future skeletal studies. Amniotic fluid volumes were taken 'When practical and the placentae inject-ed and preserved in Bouin •s fluid. OVaries ere embedded in paraffin, sectioned at 10-12 microns and stained with hematoxylin and eosin. The slides were examined by projecting them onto a screen using a 35 mm. projector With a glass plate holder adapted for 2 x 3 inch slides. By using the 35 mm. slide projector the entire slide could be examined at once, thus reducing the possibility of counting the same corpus luteum. or follicle more than once. Uore specific accounts of the methods used in the analysis of the ovaries for the various studies Will be given under the appro priate discussions! In. General Remarks on Chinchilla Reproduction Among the Hystricoidea the estrous cycle and gestation are quite long in comparison i'fith other rodents. .Also a smaller litter size and fewer young per year seem to be the general rules although mucn variation is noted even w1thin genera. In the chinchilla the length of the estrous cycle is thirty six ~s (30-42) while gestation varies from 109 to 115 days de pending upon the breed of animal. In captivity the females are polyestrous and are capable of breeding at any time of the year1 but apparently breed most often during the winter months (Dec . . ar. ) • Usually females will undergo tr1o successive cycles during the winter and then lapse into a non-cycling or anestrous condition for the remainder of the year. However, litter records from breed ers indicate that some good breeding females may eycle during the entire year or1 on the other hand, poorer breeding ani.Jnals may cycle onl.y once per year or not at all. In the female chinchilla the external vaginal orifice is nor- mally closed by a thin, non-vascular epithelial plate except for a period of a fn days during estru.s. During the course of this study it became evident that the time of vaginal plate rupture and the time of ovulation were not precisely s.ynchronized; however, vaginal opening may be used as a relative indication that ovula tion he.s occurred within several fu\vs prior to rupture of the vaginal plate. Records kept of vaginal opening and closing indicate 4 that the vagina remains open for an average of one to five c:laya normally, but rn.ay remain open through the twentieth · ~ of the cycle. In two animals the vagina was observed to be open during pree.nancy, in one of which it remained open for three dqs. No explanation is offered for this abnormal behavior. ting is nocturnal (early evening) when the animals are most active, and a copulatory plug is formed of male accessory gland elements. This plug remains in the vaeina of the female for sev eral hours after mating and i ·s covered with a layer of cornified V'aginal epithelium when extruded (Asdell, 1). The uterine horns are filled with a 1lhiti.sh, mucilaginous fluid during estrus. This is uterine nrl.lk which presumably serves as an embryotrophic substance for the nourisbntent of the embryo before .implantation in the uterine mucosa occurs. A similar dis tension of the uterine horns with fluid at estrus is described by Pearson {6) in the mountain viscacha, Lagidium peruanum Meyen, a South American rodent closely related to the chinchilla. However, Pearson does not give measurements of the amount of fluid present, merely stating that it is quite voluminous and can easily flow fl"om one uterine horn to the other. In the chinchilla the volume of fluid was about two or three cc. for both uterine horns, al though in one case eleven cc. were found. This particul.ar animal as,. horrever,. injected with an estrogenic hormone four days pre vious to sacrifice. 5 The testes were removed from a total of thirty-seven males (ranging in age frCJn 131 days to five years) over a period of six months J weighed, measured and examined microscopically for the presence or absence of active epididymal sperm. Active sperm were found in all epididsmides and, judging from the relative constancy of the wights and measurements, the testes are uniform in size throughout the entire year. This confirms the state ments of ccmnercial breeders to the effect that males can and do successfully mate t he year around. The testes reach mature size at about nine months of age although it is said by breeders that males become sexually mature at four months. The average weieht of an adult testis is 2.5 gm. (0.9-3.6)., the average width 15 mm. (lo-18), and the average length 22 mm. (14•26). The copulatory plug which is characteristic of mating in lil8ey' rodents is thought to be conducive to fertilization since it par mits a gradual release of the spermatozoa. as it disintegrates.. The chemistry of its formation is rather involved and has not been thoroughly worked out but it is thought that the coagulation of the semen is due to an enz}'me, vesic'IU&se, produced by the pros tate gland (Asdell, 1). In the chinclUlla coagulation was ex perimentallY induced by bringing together semjnal vesicle fluid and prostatic fiuid. The bulbourethral glands were found to play no part in copulatory plug formation, at least as far as semen 6 eoagtilation was eoncernecl, since a combina.ti,on o£ oither setllinal vesiele fluid or prostatic fluid alone With btllbouretbral gland nuid failed to produce coagulation. 7 1 . In captivity female chinchillas are polyestrous, usually under going one o:r two estrous cycles during the winter months and remaining in an anestrous state for the remainder of ·t:.he year. 2. The vaginal orifice is closed by a thin epithelial plate ex cept for a short period (l-5 days) during estrus. Rupture of the vaginal plate i.s not precisely synchronized 1d.th ovula.. tion but mq be used as a relative indication that ovulation has taken place within several days prior to vaginal opening. 3• Active epididymal sperm were found in thirty-seven males of varying ages during a six month surve;y.. Also there was found to be no seasonal fluctuation in the size of the testes. 4. ·ating is nocW:rnal and a copulatory plug is formed of male accessory gland elements. Coagulation of the semen into a copulatQry plug was experimentally induced by bringing to gether seminal vesicle fluid and prostatic fluid. Fluid from the bulbourethral glands ·as found to play no part in semen coagulation in the chinchilla. 8 IV. The Ovarian Cycle For this phase of the investiga.tion twenty-six p.a.irs of seri ally seetioned ovaries from :f'et1ales in var.i. ou.s stages of the ~s trous cycle were examined. From this series it was possible to construct a provisional picture of the normal estrous cycle in the chinchilla, although certai,n critical s ta.ges couJ.d not be obtained" Observation of the time o1' vaginal openi.ng gives an appro;x:imation or the time of occurrence of ovulation,. .as stated previously1 and this method wns used to date most of the estrou.s cycle stages. Those vlbi.ch cquld not be dated by this method (i•.e • ., animals shipped deep fro~en from commercial ranchers). were compared 'With ovaries from females of known estrous stages. ll.nimals were sacrificed at one day inte"als for the first ten davrs following vaginal opening and at .five day intervals thereafter until ten d.ayls before the end of the cycle when they lfere again sacrificed at one da.y intervals. This gave a canplete spread over a forty-two day cycle emphasizing the beginning and end. Young ova were seen in every ovary examined, lying just be neath the germinal epithelium, sometimes 1.n groups of three or f'our (so-called aenninal nests). Primary and secondary follicles were llkemse observed in all ovaries indicating that follicular devel opment up to the antrum stage was a constant process during the course of the c,role. 9 Toward the mid Fig. 4) and the ovum and follicular fluid are apparently expelled fd.th little or no hemmorhage . The follicles do not lose their .shape after ovuJ.ation,; instead. the granul.osa cells of the follicle~ ha;lte begun to luteinize and in their expansion invade t he vacant follicular cavity. In some in stances herniated corpora lutea were observed 'Which resulted fran granulosa cells everting through the rupture point and then be coming luteinized (Plate 1, Fig. 2) . The luteinizing cells of the granulosa encroach in a regular fashion upon the antrum resulting in a gradual reduction of the latter to a small eavity which usual ly contains extravasated blood cells. This csvity eventually either disappears in the later maturation of the corpus luteum or persists as a vascular sinus. The ovaries of females sacrificed immediately after vaginal 10 opening shovted newly forming corpora lutea thus indicating that ovulation takes place shortly before the vaginal plate ruptures. The method of dating the estrous cycle from the time of vaginal opening therei'ore prevented the acquisition of ovaries at the precise time of ovulation tram the limited number of females available. Because of this release of ova prior to rupture of the vaginal plate it is clear that ovulation occurs epontaneously in the chinchilla and does not depend upon mating stimuli as in the cat, rabbit and ferret. The mature corpora lutea apparently persist tbrougbout the greater part of the non-pregnant cycle and from their histological appearance they remain activaly secretory. It is thought that involution of the corpora lutea occurs rather rapidly near the terminus oi the cycle since these bodies were normally not .found persisting into the succeeding cycle. In a few cases, however, the corpora lutea were observed to persist into the succeeding cycle as degenerate structures With irregular outlines and fatt,y / infiltration. In the chinchilla there are two types of corpora lutea pre sent in the ovaries; the large corpora lutea of ovulation or preg... nancy which result from follicles which have ovulated, and the smaller so-called accessor.y corpora lutea which develop from follicles which tapparently did not ovulate. Such accessory cor pora lutea have been described for the horse (Cole, et al., 2) and the m.ountai.n viscacha (Pearson, 6) in association with the 11 cycle of pregnancy. In the chinchilla the accessory corpora lutea are normally present in both tbu estrous and preenancy cycles. They appear at about the same time as the large corpora lutea of ovulation or pregnancy. Immature unovulated follicles of a ttide range in size m.ay become luteinized since accessory corpora lutea exhibit consider able size variation which could hardly be attributed entirely to their intrinsic expansion. Cole# et al. (2) state that in the pregnant mare some of the accessory corpora lutea fo~ frcn follicles which have ovulatedJ they noted areas of dehiscence in nswly luteinized follicles. HovfeVer, in the chinchilla, the ac cessory corpora lutea are normally separated from the periphery of the ovary by such an extent of compact ovarian stroma as to preclude the possibility of their havine ovulated. It is proposed that the term "accessory corpora lutea" be restricted to those eorpora lutea ~ich form from unovulated fol licles during estrus or pi egnancy and that the ten1l "secondary corpora lutea" be applied to follicles Which ovulate and luteinize during precnancy as in the mare. The egg cells of the unovula.ted f ollicles disappear shortly after luteinization begins. Only one instance wao noted so far in which the egg could be positively identified and this was shortly before ovulation in an animal sacrificed at twenty- four hours fol lOWing parturition (Plate 1, Fig. 3). l2 Reeords kept of the n'Uiber of accessory corpora lutes. in the OVtu;'i.es of pregnant females indicate that there is a progressive incre,ase in. the nU!'.Ilber of accessories present dw."'::.ng the cou.'t"se of eest6t.ion rangip,g from 2~8 in early pregnancy to as numy as 16 shortly be£ore term. ·This inctease has been similarly noted in t 1e ovaries of the pr~gnant taare by Cole) et a:t. (2) although the increase in number is not as great as in the chinchilla~ In general the aeeessor.y corpora lUtea parallel the actiVi• ties o;f t he corp.Qra lutea of ovulation o.r pregnancy an.d att.er for mation cannot be histologi.cally differ-entiated £rom the latter ex<;Apt by size. :Physiolog:i..cally they are thGugbt to be supple men.~t--aey sour-ces or the lutea.t homnone progesterQne and aid in maintai..ldng it in high concentration to act as a n.... . factor o:f safety in insuring implantation•. (Cole, et al.,.., 2) . lJ 1 . Follicular development up to the antrum stage is a constant process in the chinchilla avary, the formation or antral staces beginning about the middle of the cycle. 2. Ovulation occurs spontaneously and is not dependent upon mating stiJaUli • Following ovulation the follic1es do not lose their shape; the luteinizing granulosa cells expand into the antral cavity in a regular fashion. 3. 'liro types of corpora lutea may be distinguished in the wary of the chinchilla: corpora lutea of ovulation or gestation and ~~ller accessor,y corpora lutea. 4. It is suggested that the term ttaccessory corpora lutea" be applied only to those corpora lutea which form from unovulated follicles, restricting the term "secondary corpora lutea" to those corpora lutea Which result !ram the luteinization of follicles which have ovulated during pregnancy• .. V. Reproouctive Potential of the Chinchilla The reproductive potential of an animal may be defined as the number of young Which theoretically could be produced providing all the ova shed at a. g;iven ovulation were fertilized and not lost in subsequent development prior to birth. This theoretical number may be compared with the number of yO'Ullg born per pregnancy to determine what percentage of the potential reproductive capacity is actu~ realized. In the chinchilla the avert.tge number of young born and sur vivine for thirty days is 1. 6 per year according to the 1953 sur vey of the chinchilla ranchers conducted by the National Chinchilla Breeders A8sociation. This indicates that with an average of two litters per year, an average of only 0. 8 young are born per litter, while it is known that a female m:q easily accommodate J-6 (possi bly 9) nomal embryos in utero per pregnancy. Thus one sees that there is normally a lw realization of the full reproductive potential in the chinchilla. When, in the course of investigations into methods or in creasing the average litter size, it became necessary to have a thorough knowledge of the theoretical potential the present stuey was initiated. The results of this inquiry then £om a reference point from which further research maw be projected. A good indication of the reproductive potential can be gained by counting the number of large follicles and corpora lutea present 15 in the ovaries. Such large follicles and corpora lutea were care ful~ counted in the serially sectioned ovaries !rom seven~-one mature virgin, non-pregnant mature and pregnant mature female chinchillas.. The large follicles are not as good a criterion as corpora lutea upon hich to base reproducti.ve potential since not all large follicles seen in an ovary prior to ovulation l'li.ll actually ovulate. This is in part due to follicular atresia and perhaps ~~e luteinization of follicles into accessory corpora lutea may also play a role. Nevertheless it was thought advisable to include the counts of large follicles in the tables for pur poses of comparison, A closer approximation of reproductive potential may be ob tained by counting the number of corpora lutea of ovulation or gestation in a given pair of ovaries since each corpus luteum represents a follicle which has ovulated. The results of these corpora lutea counts are recorded for each female categor.y in Tables 1 through 4 and for the combined categories in Tables 5 and 6. From Table 11 which includes twenty- tJro virgin females, one sees that not all the anjmals have both corpora lutea and large follicles. Since the sample of females in this instance was random it 'laS inevitable that some of the females i'fere in an anestrous condition, accounting for the absence of large folli cles and/or corpora lutea. This, however, does not imply that these females would not have corpora lutea or large follicles at 16 some other phase of the cycle, but to include them into the repro ductive potential calculations would give a reproductive potential average only for a random sample of chinchilla females at a given time. Therefore, if those twelve females 'Which showed neither large follicles nor corpora lutea of ovulation are discounted on the basis of their non-cycling condition, the average of the ten re maining animals , as seen in Table 2, illustrates more accurate avera~es upon which to interpret the overall reproductive potential of actively cycling virgin females. Si.mi.larly, this interpretation holds for the pregnant and non~regnant mature females so that Table 4 represents the more accurate averages upon Tihich to base the reproductive potential of this group. All the mature female categories are grouped together in Table 6, from which it m~ be seen that the average number of large corpora lutea per female is 4.2 per cycle. Then if two pregnancies per year are possible, an average of 8.4 eggs per year could be ovulated. Therefore it appears that, if the national average of 1 . 6 young born per year per female is considered, there is a realization of only about one-fifth of the theoretical repro dueti.ve potential in the chinchilla. 17 Table l VI RGIN {ATURE m.t.ALES {U'it h or rithout large follicles and/or corpora lutea of ovulation) Larie Follicles Totals Averages Right Corpora Lutea of Ovulation Totals Averages 'ght ovaries (22) 24 1 . 0 Left ovaries (22) 24 1 . 0 Right and left ovaries 48 Average for 22 a.nimals 2. 1 Table 2 VIRGil TURE FE.t!.ALES (With large follicles and/or corpora lutea of ovulation) Larse Follicles Totals Averages Right ovaries (8) 27 Lett ovaries l3 Right and left ovaries 40 Average for 8 a.nima.ls Corpora Lutea of Ovulation Totals Averages Right ovaries (10) 23 2.3 Left ovaries (10) 22 2. 2 Right and left ovaries 45 Average for 10 animals l8 Tabl.e .3 PREGNM.ttr MID ~jQlf...PREGNANT l!ATURE FEMALES (With or lvithout la~ge follicles and/or corpora lutea) ~ar"r; . F;?~!?-.?le~ Totals Avera.ge.s Right cwaties (49) 78 1.5 Left ovaries {49) 65 l·3 P~ght ar~ lett ovaries 143 Average for 49 ~mls 2..9 ~oz:eora Lutea of Ovulation or PrefS!lan& Right ovaries (49) 89 1.8 Left ovaries {49) 87 lr7 Bight and left ovaries 176 Average for 49 animals .3·5 Table 4 PREGNA!iJT AND NON- PREGN.AN'f MATtJRE FEMALES ( Vith large .follicles and/or corpora lutea) .Large. Follicles. Totals Right ovaries (33} 78 J.eft ovaries ( .33) 65 .Right and left ovaries 143 Average for .33 animals CoteoJ:>a !.utea o£ Ovulation o:r Presnans.r Bight ovaries {45) 89 Le!t ovaries C45) 87 Right and left ovaries 176 Average for 45 animals 19 Table 5 VIBGI lJATORE, PRE lWIT 'AND W~f-PnEGNANT lJ.ATURl:: FEMALES ( U. th or 11:i.thou.t large follicles and/or corpora lutea) La:rce Follicles Totals Averages Right ovaries (71) 105 1·5 Left ovaries ( 71) 78 1.1 Rieht and left ovs.ries 183 Average foF 71 animals 2.6 Corpora Lutea of Ovulation or Prefffi~Cy ght ovaries (71) ll2 1.6 Left ovaries (71) 109 1.5 · ght and left ovaries 221 Average for 71 animals • Table 6 VIRGIN l!ATURE, PREGNANT AND liOll-PREGNANT MATURE FEM.AI.ES (With large follicles and/or corpora lutea) Large Follicles Totals Avera~es Right ovaries (41) 105 Left ovaries {U) 78 Right and left ovaries 183 Average for .41 animals Corpora Lutea of Ovulation or . Pregnancy Right ovaries (55) ll2 Left ovaries (55) 109 · ght and left ovaries 221 Average for 55 animals 20 1. A close approx::i.mation of the reproductive potential in the chinchilla msy be gained by counting the munber of corpora lutea of ovulation present in the ovaries since each mature corpus luteum represents a follicle vthich has ovulated a potentially fertilizable ovum. 2. Corpora lutea counts made on sectioned ovaries from forty ... ive females shmred that, although the national average num ber of young born per year and surviving thirt,y days is 1.8, it is t.neoretically possible for the average female to ovu late 8.4 ova per year. 3. The actual realization of the reproductive potential in the chinchilla was found to be only about one-fifth of the poten tial \Vhich is theoretically possible. 21 VI . Early Egg Loss in. the Chinchilla_ The data from the investigation o£ reprochlctive potential indicate that the theoretically possible number o.f young per year is tar £rom realized in the chinchilla. This is due £or the most part to the pre- implantation loss of wa in early stages and fetal necrosis in later gestation. Pre-implantation egg loss is not unique i .n the chi.nehilla, however, since it .is knmm to be a gen eralization for mammals as such. Perhaps the most extreme case of egg loss occurs in certain species of Ele,phantulusl in l'rhich one hundred or more eggs ~ be shed and only one or two implant. (Asdell, 1). Hammond (5) has reported that in swine the loss of t:;Na and fetal atropbiY' account for a loss of at least 33 per cent of the young in utero, and Corner and Bartelmez (3) have shown that about .30 per cent of the ova which are shed in the rhesus monkey do not become fertilized and fail to implant because of genetic defects. A close association is seen then between the degree of reproductive potential realization and the pre-implanta tion loss of ova, due primarily to defective germ (i. e ., intrinsic lethal genetic factors) . For the purposes of this stu~ ~ pre-implantation egg loss was dealt with. A total of thirty pregnant females were used for 1 . Elephant shrew (Order Insectivore.) 22 this i.nvesti.cation. The number af large corpora lutea (1 mm. and av-er) vre.re counted in each pair of ovaries and the totals for each animal compared with the number of pregnancy reactions, or locuJ.i (whether nomal or abnormal) found in the uteri at sacrifice. Since the uterus of the chinchilla is duplex, with the two uteri opening by separate cervices into tho vagina, it could be assumed With relative certainty that the embryos in one uterus resulted from ova 'Which had been shed from the corresponding ovary. Any ova passing through the cervices would presumably be lost in the vagina and could not gain entrance to the other uterus. The transmigration of ava !rom one ovary across the coelom to the opposite oviduct cannot be ruled out, however, since th.e ovaries are not fully encapsulated. Nevertheless, this possibility is so slight and of such rare occurrence that for the purposes of this present study it was discounted. Fro . Table 7 it 1JJJlY be seen that the number of ova which were unaccounted as pregnancy reactions, loculi or embryos was 24 per cent of the total ova for the average left CTV'ary, 31 per cent for the average .right ovary and an average of 28 pel" cent per animal . Presumably then an estimated one-fourth to one-third of all eggs ovulated in the chinchilla fail to implant. 2.3 Table 7 EARLY EGG W8S IN CHINCHILLA Left Ov& (30) Totals Aver.yes Umber of Corpora Lutea of Pregnancy • 54 1. 8 Number of Embr.yos or Loculi in Left Uterus ... Number of Ova Unaccounted ... Percentage Loss of Ova Ri@t Ovary (.30) Totals Averag~e 1Iumber of Corpora Lutea .of Pregnancy - 63 2. 1 Number of Embryos or Loculi in Right Uterus 4.3 Number of Ova Unaccounted 20 . 6 Percentage Loss of Ova ,31. 0% Averages per Animal tiumber of Corpora Lutea of Pregnancy - 117 Number of Embryos in Both Uteri 84 'umber of Ova Unaccounted 3.3 1 . 1 Percentage Loss of Ova per Animal 28 . 0% 24 1 . Pre- implantation loss of ova accounts in large part f'or the low realization of the reproductive potential in the chin chilla. 2. The umber o£ larGe corporD. lutea present in the OV"aries of thirty preenant £em.ales were counted and compared vti:t;h the nuntber of preenancy reactions, loculi or embryos present in the corresponding uteri. 3· An average o! 28% of the ava shed at a given ovulation could not be accounted for as pi--egnancy reactions, loculi or embryos indicating that about one ourth to one- third of all eggs ovulated in the chinchilla fail to implant. , VII •· Development of the OvarJ It is known that th.rough. the addition o£ interstitial tissue and the presence of large corpora lutea and follicles there is a progressive weight increase of' the CN.B-'7"• The present studf was inituted to ascertain the amount of increase in ovarian weight for femele chinchillas Of varying age ana repeated rep.rodu.ctive pel."fomance. A total or 130 . r~ .esh ovaries ~rere removed .from 65 £eXila1es at sacrifice, weighed, and divided into four categories depending upon whether the .female· was imlilature, mature virgin, first preg nancy mature or mature 'With a record of two or more pregnancies., Table 8 illustrates the ave~age weights of the ~aries o! ~1ese tour groups and show:s the progressive increase in \'teight which acc.ompanies maturity and increasing reproductive perf'omance. In the three categories o£ mature animals; which haVe a vtide overlap in age, the increa.se in average weight is preS'ijijlably due in large part to the increase in interstitial ti.ssue resulting trom successive estr.ous .o,x- pregnancy cy¢les and to the presence oi follicles and corpora lu.tea,. Actual growth by the additio:n or eonnective tissue st.r~ to the <:Nary is probably' only of minor ~ortanee among the mature :!elilale categories but it presumably plays the most important role in ovarian weight increase in the immature female oategory. A comparison of the Table 8 A COJPAR.ISOU OF OVARIAN WEIGHTS BE'rnEEN CATEGORIES OF VARYI1lG AGE AND REPRODUCTIVE PERFOmiANCE Immature Females Average eieyt (12, ages 1 to 126 days) Right ovaries Left ovaries Right and left ovaries Mature Vir~n Females (2o, ages ~3 to 1163 ~s) Right ovaries Left ovaries Right and left ovaries Females 'With One Pregnancy (10, ages 267 daiS toca. 3 years, either pregnant or non-pregnant) Right ovaries 27.2 mg. Left ovaries 24.4 mg . Right and left ovaries 25.8 mg. Femnles with Two or More Pre~cies (2), ages 367 to 348~ ~s, ~er pregnant or non-pregnant) Right ovaries ,31.4 mg . Lett Table 9 A CC!!PARISON OF OVARIAN EIGHTS OllG FEMALES WITH A Rr~CORD OF ONE PREGNANCY TO SHOW THE EFFECT OF THE PRruL.lWE OF CORPORA LUTEA OF PREGN1U~CY ON THE OVARIAN lEIGHT Average Rig!}t Ov& . reisht Pregnant (4 females, 9 corpora lutea) 32. 7 mg. Non...Pregnant ( 6 females, 3 corpora lutea) 23 . 5 mg . Left Ovary . . Pregnant (4 females, 4 corpora lutea) on.- egnant (6 .females1 6 corpora lutea) 21 . 1 mg. Right and Left Ovaries Combined Pregnant (4 females) )1. 0 ·:mg. on-Pregnant (6 females) 22 . ,3 mg. 29 l . OVarian weight increase in the chinchilla is due primarily to the increase of ovarian strGWa; inte.l~ati tial tissue and the pri:.lsence of cot>po1·a lutes. and follicles. 2. Pres.umably the increase in ovarian connective tissu~ stroma accounts tor the greater part o£ the weight increase in immat~ :t'emal,res. while amorl8 mature .females the weight gain is p;~,;1 obably due mainly to the accumulation of inter.st.itial . tissue .from successive rep~oduetive perfc;rmanee and the presence of corpora lutea and follicles• .3. :fue weights of avaries from pregnant females were found to average higher than those o! .non-pr.egnant !'ewes probably du.e to the additional weight of the cowora lutea of pregnanqr. 30 VITI. Postnatal Growth in the Chinchilla 1m interestine sidelight of tho studies on reproduction of the chinchilla relate to the relative grorlth ra·t;es of animals fro 'oirth to seven years of age. At birth young chinchillas are p.recocial but they require more time to reach adult size than do certain other rodents (rat, mouse) ll'b.ose young are born after a shorter period of gestation. r eight and crown-rump meaa~em.enta were taken on 1.33 animals received during the course of these studies and plotted graphi cally.. These measurer.1ent curves for jtNenile animals raneine in age fro~ one to 177 days are shova1 in Graph 1 accordine to age in days; sii!ril.e.rl;r grorrth Cl.ll'"Ves for I!l.ature animals are indicated in Graph 2, arranged according to age in months. From these two curves it m~ be seen that there is a pro gressive increase in weight and crown-rump during the first 125 dayD or so after birth w1th a gradual leveling off as the nonnal adult weight and crown-rump are approached. The average adult weight and Crown-ru.l!lp are reached at about two years after which there is a gradual inc:retlSe for the next two or three years f"ol 10Q!(t ~J a sharp decline in weight. CHINCHILLA eoSTNADL GRQWTH ·• I~' -~ . ~- .fW, rrn 1 I [ID l'l .tf1tl _1:1!If'" (gAl:§ 1-IZZ) lit .. f~ 1 ""':)" I - ·~U1 mJ rm: 't'J -~:t J f4#1#E '' I - IH#11m flA r- I ' '· ll!f. . t. ~ ail 1 ·' J ~-· lffil Em ±t+i +~• t:-r if+V' I;: j I + ---1 f-.. - f-· . . r . ~ ,I-\· -.1-- + r, i eo - t - - f I 1 . ' .4 I I I t - -j f·- t eo J ! A / I I . ; I ; ___,__ J., -\.I+'- If - ~L ~ -:--1- - - I - I ' - ~/ . ,\ . ~· I 1 \ I I I i l - - + t \ 1 I \ + 1 - I t/· 1-- . J.,. - ~ t-: ~ r ~---- - t ... . l I I ' '' - . . L I t ' ' - I I H I I " I 1 I I t I I A I ' I' I I\ I + • I t I I: -~ I I I I I I I I I I I I I I -I I I I I I I ,, I I !\' ,! v 10 40· eo AGE IN DAYS ------..1---. - t -r 1 -1 ~- --t- -· ~_J I I 1~~+--+--+--+---+-----lf----+--i )3 1. Growth curves plotted on the chinchillas used in these studies indicate a rapid eain in weight and crovm- rump l ength during the first 125 days following birth, with a proeressive leveling off as the adult weight and crawn rump ·length are reached at about two years of age. 2. A plateau at the average adult might is aintained for about t\'To or three years follovted by a rather sharp de cline associated With senility. 34 Explanation of Pl.ate Fig. 1 Longitudinal section of an ovary .shOWing large corpora lutea., mature follicles, and the s.'l!aller accessory co:rpol"a lutea. X 20. Fig. 2 Secti.on through a newly formins corpus luteum ehawing the old follicular lumen being inVaded by the lutai.lP.z.ing granulosa layer and the her niated condition resulting from the protrusion of part of the granulosa layer through the follicular rupture point. I 100. Fig. .3 Seetion through a luteinizing follicle {accessory corpus luteum) showing the rema.:ins or the ovum in the lcnrer part of the follicular lumen. X 100.. Fig. 4 Section through a recently ruptured follicle show ing the small rupture point. X 5o. 36 BIBLIOORAPH.Y 1. Asdell, s. A. Patterns of mammalian r-eproduction. New York, COI:lStock, 1946. 437 P• 2. Cole, H. H., c. E. Howell, and G. H. Hart. The changes occurring in the CNfJ.r'Y o£ the mare durin~ pregnancy. Anatomical record 49 tl99-209. 1931. ,3. Corner, G. \ ., and G. W. Bartelmez . Ear:cy abnormal embcyos of the rhesus mo!'.key. Carnegie institution of Washing ton. publication 60.3, Contributions to embryology. 35:1-10. 1954. 4. Eva.nst H. u., and 0 . SVrezy. Ovogenesis and the nonnal follicular cycle in adult Mammalia. emoirs o:r the University of California 9 :116-224. 1931. 5. Ha.mmond, John. Further observations on the factors control ling fertility and foetal atrophy. Journal of azricul tural science 11:3.37-368 . 1921. 6. Pearson, Oliver P. Reprod:t.t.ction of a South American rodent, the mountain viscaeha. .American journ 1 of anatomy 84tl43-l73. 1949. 7. ------Life history of the mountain viscacha in Peru. Journal of mam.malogy 29:345-374• 1948. 8. Results of the 19.53 chinchilla census. Uational chinchilla breeder October, 1954.- pp. 23-26, 26.