xin PHYLUM CHORDATA 337 



Crocodilia and Chelonia, instead of the copulatoiy sacs there is a 

 median solid penis attached to the wall of the cloaca, and a small 

 process or clitoris occurs in a corresponding position in the female. 

 Though fertilisation is always internal, most Reptilia are ovi- 

 parous, laying eggs enclosed in a tough, parchment-like or calcified 

 shell. These are usually deposited in holes and left to hatch by 

 the heat of the sun. In the Crocodiles they are deposited in a 

 rough nest and guarded by the mother. In all cases development 

 has only progressed to a very early stage when the deposition of 

 the eggs takes place, and it is only after a more or less prolonged 

 period of incubation that the young, fully formed in every respect, 

 emerge from the shell and shift for themselves. 



Many Lizards, however, and also many Snakes are viviparous, 

 the ova being developed in the interior of the oviduct, and the 

 young reaching the exterior in the completely formed condition. 



Development. In all the Reptilia the segmentation is 

 meroblastic, being confined to a germinal disc of protoplasm 

 situated on one side of the yolk. This divides to form a patch of 

 cells which gradually extend as a two-layered sheet, the blasto- 

 derm, over the surface of the ovum. As the blastoderm extends 

 (Fig. 949) it becomes distinguishable into a central clearer area- 

 area pellucida (a. pd^} and a peripheral whitish zone area opaca 

 (((-. op.). On the former now appears an elliptical thickened patch 

 -the efoCbryonic shield (emb; s.) which is formed by the ectoderm 

 cells in this region assuming a cylindrical form while remaining 

 flat elsewhere. On the embryonic shield, in a direction correspond- 

 ing to the long axis of the future embryo, appears a thickening 

 due to a proliferation of the ectoderm cells, and here the upper 

 and lower layers coalesce (primitive streak) ; this is the preliminary 

 to the formation of the blastopore and neurenteric canal. In front 

 of this the lower layer develops a thickening which is the rudi- 

 ment of the notochord and the central portion of the mesoderm. 

 A depression appears on the surface of the ectodermal thickening 

 and this grows inwards, giving rise to an imagination the blasto- 

 pore (blp.). The formation of the archenteric cavity and of the 

 definite enteric cndoderm layer is, in Reptiles with a more primitive 

 mode of development, subsequent to, and dependent on, this 

 process of invagination ; in others, the process of imagination 

 is delayed, and takes place only after the endoderm and the 

 beginnings of the enteric cavity have become established. In either 

 case the invagination communicates with the primitive enteron, 

 forming a neurenteric passage which persists for some time. 



In front of the blastopore a longitudinal depression bounded by 

 a pair of longitudinal folds (med.f.) is the beginning of the medul- 

 lary groove. As this becomes closed, it encloses in its posterior 

 portion the blastopore or dorsal opening of the neurenteric canal. 

 At the sides of the medullary groove appear the protovertebra? 



VOL. II Z 



