78 



COMPARATIVE ANATOMY 



However, both in mode of origin and in further history the mammalian 

 embryo at this stage shows perplexing discrepancies as compared to the 

 gastrula of a lower vertebrate. 



As stated above, the material which constitutes the definitive embryo 

 is within the thick and solid cell-mass (Fig. 51, /) of the early blastodermic 

 vesicle. As development proceeds, the behavior of this cell-mass is very 

 much like that of the blastoderm of the embryo of a reptile or bird. If the 



cavity of the vesicle were occupied by yolk 

 instead of by a watery fluid the whole 

 embryonic complex would resemble closely 

 an early reptilian embryo. The thick cell- 

 mass, lying in relation to the vesicular 



Fig. 52. — Early stage of the Fig. 53. — Embryonic area or "shield" of the 



blastodermic vesicle of the Hedge- blastodermic vesicle of the Rabbit after about 172 



hog. EC, ectoderm; EN, endo- hours development. PS, primitive streak; 5-5, 



derm; L, lacunae, spaces occupied position of section represented in Fig, 54. (After 



by maternal blood; T, trophoblast Assheton.) 

 (trophoderm). (After Hubrecht.) 



cavity much as the reptilian blastoderm lies upon the surface of the yolk, 

 flattens and thins out to form the "embryonic shield" (Fig. 53) in the 

 axis of which appears an elongated thickening similar to the primitive 

 streak of a sauropsidan embryo. At the anterior end of this mammahan 

 streak is usually found a small pit or even a perforation extending through 

 the "shield" into the cavity of the blastodermic vesicle — very suggestive 

 of an abortive blastopore. It is along this mammalian primitive streak, 

 as in the similar sauropsidan structure, that rapid cell-prohferation pro- 

 duces a mesoderm (Fig. 54) which progressively interpolates itself 

 between the already separated ectoderm and endoderm and spreads 

 eventually into all regions of the embryo. The mesoderm is at first a 

 continuous layer — unsegmented — and devoid of cavity. 



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. 



