264 



Basic Structure of Vertebrates 



cv_ 



q>-T® 



Fig. 218. Early stages in development of a rabbit. (A) Morula stage, 47 hours 

 after coitus. (B) Early blastodermic vesicle, 80 hours. (C) Blastodermic vesicle at 

 83 hours. The investing layers of the embryo are not shown. (CV) Cavity of blasto- 

 dermic vesicle; (I) inner cell-mass; (T) trophoblast. (Magnified about 285 diam- 

 eters.) (After Assheton. Courtesy, Neal and Band: "Chordate Anatomy," Phila- 

 delphia, The Blakiston Company.) 



that the stage is not the equivalent of a blastula of a lower vertebrate. 

 The term blastodermic vesicle is applied to this stage of the mam- 

 malian embryo. The definitive embryo is developed entirely from the 

 thick cell-mass of the vesicle. The thin region (trophoblast: Fig. 218, 

 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. 219). This inner sheet of the thick mass then extends over 

 the inner surface of the thin wall of the vesicle and ordinarily com- 

 pletely lines it. The vesicle as a whole thereby becomes two-layered 

 throughout, a condition which characterizes a gastrula stage. The fur- 

 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 compared to the gastrula of a lower vertebrate. 



As stated above, the material which constitutes the definitive em- 

 bryo is within the thick and solid cell-mass (Fig. 218, I) 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 in- 

 stead of by a watery fluid, the whole embryonic complex would resem- 

 ble 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 em- 

 bryonic shield (Fig. 220), in the axis of which appears an elongated 

 thickening similar to the primitive streak of a sauropsidan embryo. At 

 the anterior end of this mammalian streak is usually found a small pit 

 or even a perforation extending through the shield into the cavity of 



