CHORDATE ANATOMY 



equivalent of a blastula of a lower vertebrate. The term, blastodermic 

 vesicle, is applied to this stage of the mammalian embryo. The definitive 

 embryo is developed entirely from the thick cell-mass of the vesicle. The 

 thin region (trophoblast Fig. 47, T) of the wall of the vesicle becomes 

 concerned with the early attachment of the embryo to the wall of the 

 uterus. 



The fluid-filled cavity of the blastodermic vesicle rapidly enlarges 

 and meanwhile the thick cell-mass splits off a thin layer adjoining the 

 cavity (Fig. 48). This inner sheet of the thick mass then extends over 



the inner surface of the thin wall of the 

 vesicle and ordinarily completely lines it. 

 The vesicle as a whole thereby becomes 

 two-layered throughout, a condition which 

 characterizes a gastrula stage. The fur- 



S - 



Fig. 48. — Early stage of the Fig. 49. — Embryonic area or "shield" of the 

 blastodermic vesicle of the hedge- blastodermic , vesicle of the rabbit after about 

 hog. EC, ectoderm; EN, endo- 172 hours development. PS, primitive streak- 

 derm; L, lacunae, spaces occupied 5-5. position of section represented in Fig. 50. 

 by maternal blood; T, trophoblast (After Assheton.) 

 (trophoderm). (After Hubrecht.) 



ther history of the two layers identifies them as embryonic ectoderm and 

 endoderm. However, both in mode of origin and in further history the 

 mammalian embryo at this stage shows perplexing discrepancies as com- 

 pared 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. 47, /) 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 b>- a watery 

 fiuid the whole embryonic complex would resemble closely an early 

 reptilian embryo. The thick cell-mass, lying in relation to the vesicular 

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

 flattens and thins out to form the embryonic shield (Fig. 4q) in the axis 

 of which appears an elongated thickening similar to the primitive streak 



