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The Chondrocranium of an Embryo Pig, Sus Scrofa

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The Chondrocranium of an Embryo Pig, Sus Scrofa THE CHONDROCRANIUM OF AN EMBRYO PIG, SUS SCROFA. A CONTRIBUTIONTO THE MORPIIOLOGYOF TIIE MAMMALIANSKULL. BY CHARLES SEARING MEAD. WITH 11 TEXTFIGURES AND 4 PLATES. CONTENTS. PAGE Introduction .......................................................... 1~7 The Skull as a Wliole ............................................... .1G9 Planum Basale ........................................................ 170 Regio Occipitalis .................................................... 175 Regio Otica ........................................................... 178 Auditory Capsules ................................................ 150 Sound-Conducting Apparatus ...................................... .1S5 Nerve Foramina in the Region of the Ear-Capsules. ................ .1SS Regio Orbitotemporalis ................................................ 1~2 Regio Ethmoidalis .................................................... 199 (!onelusions ........................................................... ZOC; Bibliography .......................................................... 20s INTRODUCTION. The study of the chondrocranium of Sus is of value not only in assisting us to understand the structure of the adult skull in this form, but also on account of its bearing on the general morphology of the mammalian cranium. Owing to the generalized dentition and the structure of the feet, Sus has becn placed relatively low in the ungulate series. Benee, we would expect many primitive char- acters to be retained in its cartilaginous cranium, and, indeed, this is the fact, fomr a number of reptilian characters arc present. This chondrocranium is also valuable for comparison with those of pri- mates and insectivores. THE AHERICANJOURNAL OF ANATOIfY.-vOL. Is, NO. 2. 168 Charles Searing Mead. The Sus embryo studied was 30 mm. long (head-rump measure- ment), and the length of its head I2 min. The head was cut trans- versely into ‘795 sections, each 0.015 inm. thick. The cartilage in all the even-numbered sections was drawn with the aid of a projec- tion apparatus, the drawings being enlargements of 25 diameters. In making the reconstruction, Born’s wax plate method was used. For comparison I have had Ziegler’s wax models of the chondro- crania of man, Gallus, Lacerta and Rana. In addition, Professor Eugen Fischer, of the University of Freiburg, loaned me his re- constructions of the Semnopithecus, Xacacus, Tarsius and Talpa skulls, and the series of sections from which his Talpa reconstruction was made. Also a reconstruetion of the primordial cranium of Lepus, prepared by Dr. Max Voit, has been of service. Of the niodels and reconstructions used for comparison, the one of Talpa resembled moi-e closely that of Siis than dicl any of the others, and, hence, has been referred to most frequently in the com- parisons. The literature dealing with the development of the mammalian skull is very extensive. The publications of Parker, Spandli, KZiilliker and Decker, on the chondrocrania of the mammals, are the principal works bclonging to the old school, in n-hich the skulls wem prepared principally by the maceration method. The introduction of Born’s wax plate method of reconstruction has made possiblc not only a inore exact study of the embryonic cranium itself, but has also enabled one to study the snrrounding tissues as well. A more fundamental view of the skull is thns obtained. Among the more recent papers on the embryonic skull may be mentioned Gaupp’s “Die Entwickelung des Kopfskelettes” in IIertwig’s ITandbueh (1905 b), valuable on account of its general survey; also his ex- tensive contribution on the developnient of the sliall of Echidna (1908). The papers of Fiseher on Talpa (1901 b) and the apes (I 903) and a forthcoiniiig paper of Yoit on IJelms (1909) likewise contain valuable results. This investigation was undertaken at the suggestion of Professor Ernst Gaupp. The ~vorlcwas conducted in the laboratory of the Comparative Anatomical Institute of Freiburg in Eaclen. The Choiidrocraniuin of an Embryo Pig. 169 I will here take the opportunity to express to Professor Gaupp my sincere appreciation iiot only for the ~iseof his ex&iisive series of sections, but also for his very valuable help and suggestions. Dr. Max Voit I wish to thank for the use of his Lep~isreconstruction aiid for the assistance which he has rendered me. And for the loan of the reconstrixctions of the Semnopithecus, Macacus, Tarsius and Talpa skulls, and also for the series of sectioiis froin which the Talpa reconstriiction was inacle I wish to express to Professor Eugen Fischer my siiicere thanks. TI-IESKULL AS A WIIOLE. The form of the priniordial cranimii of the pig represents well the generalized nianiiiialiaii type of chonclrocranium. No part is inarliedly underdeveloped and no part is greatly overderelopetl at the expense of the snrroundiiig portions. As a whole, the chondrocraniiiiii at this stage of development, i. e., in an embryo 30 mm. long, resembles a pear iii shape, with the small end anterior, forming the nasal region, and the large end posterior. The large brain-box is widely opeii dorsally, a feature characteristic of the amniotes. Later this region of the skull is completed by the formztion of the large roofing membrane bones, the frontals and parietals. Ventrally the geiieral contour is completed by the nian- dibular, hyoicl aiid branchial arches. The brain-case is large and extends forward OTW the posterior half of the nasal capsules, forcing the fenestra: cribrosa: froiii their priiiiitive vertical position into :3 horizontal one. Its side walls are formed by a broad continuous plate of cartilage on each side, thus standing in sharp coiitrast to the coiidition in siinilar eiiibryos of mail and the primates, in which this region is very rudimentary. The nasal capsules are of inoclerate length, not long as in Talpa, nor short as in nian. Taken as a whole, the chondrocraniuin is complete except for some minor details. None of the cartilaginous bones (Ersafzlcnochen, Gaupp) have as yet begun to ossify. Some of the membrane bones have not yet appeared aiid the others are only in the very early stages of their formation ; con- seq~~entlythey have not been inclutled in niy reconstruction (Pls. I-IV). With tlie exception of the iiianclibular, hyoicl and braaach ial 170 Charles Searing Mead. arches (the visceral skeleton), the different parts of the skull are not separated by sutures, the cartilage composing the neural cranium forming one continuous unit. Kolliker has divided the skull into the posterior part, the pars chordalis, through which the notochord runs, and the pars prechordalis, situated anterior to this. Each of these divisions Gegenbaur has again divided into two regions, which Gaupp has named, from behind forward, as follows: regio occjpitalis, regio otica, regio orbitotemporalis and regio ethmoidalis. For de- scriptive purposes it will be convenient to follow these divisions, ex- cept that the basal portions of the two posterior regions will first be considered together, since they form a fairly complete unit, the planum basale (Gaupp). PLANUMBASALE. The planum basale comprises the basal portions of the two poste- rior regions of the skull. It extends from the foramen magnum forward to the posterior LorJer of the hypophysial fossa and iq perforated for nearly its entire length by the notochord. Its anterior three-fifths, i. e., the portion belonging to the regio otica, is narrow and rod-like (Pl. I and Figs. 1, 3, 4 and 5), while its posterior por- tion is spread out into a broad quadrangular plate, the basilar part of the occipital region. The anterior portion of the basal plate in Sus differs strikingly from that in other mammals on account of its narrow, rod-like shape and also because of the character of its union with the ear-capsules. Here it is separated from the aar-capsules except for a thin connect- ing lamella of cartilage, while in the other mammals that I have examined it is firmly united with the capsules. In the other mam- mals, also, the planurn basale forms a broad plate of cartilage, which passes over without sharp demarcation into the broad basal cartilage of the orbitotemporal region. Tarsius offers an exception to this statement, since in it the portion between the cochlear spheres is even narrower than in Sus. This, however, is due to the extremely large size of the ear-capsules, which approach the middle line and compress the cartilaginous plate which lies between them. Back of the auditory bulk the planum basale spreads out into Thc Choncirocraniuin of an Embryo Pig. 171 a broad quadrangular plate, limited posteriorly by the foramen mag- num, anteriorly by the foramen jtigulare or fissura metolica and the auditory bull=, while laterally it passes over into the pars lateralis, regio occipitalis. On the boundary between the basal plate and the Z. pa a.s.c. VIII utr. Z.S.C. m.st. j.v. x. T. uw. .X - xrr n.c. 6.p. FIG.1. Section shoiviug the broad basal plate b.p, the uotochord n.c. near its dorsal surface, and tlie large auricular brmcli of the vagus nerve X r.aur. x 10. VZZZ, nervus acusticus ; mst., stapedial iiiuscle ; j.v., jugular vein ; I.S.C., lateral semicircular caiial : wtr., utricwlus : a.s.c., anterior seiiiicircular canal ; 1. par., lamina parietalis ; X, nervus vagus : XIZ, nervus hypoglossus. pars lateralis the proccssus paroccipitaZis arises and extends forward. The basal plate is perforated just median to this process by the foramen nervi hypogZossi (Pl. I, f. hyp.). Three branches of the hypoglossus nerve have been demonstrated in a number of mammalian 172 Charles Searing Mead. embryos, but the anterior one disappears and the other two usually leave the cranial cavity through a single foramen, the foramen hypoglossi (foramen spinokipitale). Voit found that in Lepus embryos two foraniiiia hypoglossi are pyseiit on each side, ant1 these may persist even in the adult. Fischer described in an embryo of Semnopithecus pruinosus two hypoglossal foramina on the left side and three on the right. On the other hand, his reconstruction of Semnopitkecus ninurus shows but a single foramen on each side. This shows what variations may occur within the limits of a single genus or even species.
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