Reproduction 



265 



the blastodermic vesicle — very suggestive of an abortive blastopore. It 

 is along the mammalian primitive streak, as in the similar sauropsidan 

 structure, that rapid proliferation of cells produces a mesoderm (Fig. 

 221) which progressively interpolates itself between the already sepa- 

 rated ectoderm and endoderm and spreads eventually into all regions 

 of the embryo. The mesoderm is at first a continuous layer — unseg- 

 mented — and devoid of cavity. 



Fig. 219. Early stage of the blasto- 

 dermic vesicle of the hedgehog. (EC) 

 Ectoderm; (EN) endoderm; (L) lacunas, 

 spaces occupied by maternal blood; 

 (T) trophoblast (trophoderm). (After 

 Hubrecht. Courtesy, Neal and Rand: 

 "Chordate Anatomy," Philadelphia, The 

 Rlakiston Company.) 



s---^--- 



Fig. 220. Embryonic 

 area or "shield" of the 

 blastodermic vesicle of the 

 rabbit after about 172 

 hours' development. (PS) 

 Primitive streak; (S-S) 

 position of section repre- 

 sented in Fig. 221. (After 

 Assheton. Courtesy, Neal 

 and Rand: "Chordate 

 Anatomy," Philadelphia, 

 The Blakiston Company.) 



In a rabbit embryo the embryonic shield is established ordinarily 

 by the fifth day of development, the entire blastodermic vesicle then 

 having a diameter of about 1.5 mm. 



The early development of the placental mammal presents many 

 perplexing features. It could be expected that the minute egg, unem- 

 barrassed by yolk, would revert to the relatively simple and direct 

 methods of early development which characterize Amphioxus. But it 

 does not. Mammalian stages precisely comparable to the blastula and 

 gastrula of Amphioxus or amphibians cannot be recognized. When it 

 comes to the formation of mesoderm, the laying out of the germ layers, 

 and the early shaping of the embryo, the behavior of the mammal is 

 closely similar to that of a reptile or bird. This similarity exists in spite 



