X.] THE MEDULLARY PLATE. 327 



General growth of the embryo. We have seen 

 that the blastodermic vesicle becomes divided at an 

 early stage of development into an embryonic area, and 

 a non-embryonic portion. The embryonic area gives 

 rise to the whole of the body of the embryo, while the 

 non-embryonic part forms an appendage known as the 

 umbilical vesicle, which becomes gradually folded off 

 from the embryo, and has precisely the relations of the 

 yolk-sac of the chick. It is almost certain that the 

 Mammalia are descended from ancestors, the embryos 

 of which had large yolk-sacs, but that the yolk has 

 become reduced in quantity owing to the nutriment 

 received from the wall of the uterus taking the place 

 of that originally supplied by the yolk. A rudiment of 

 the yolk-sac being thus retained in the umbilical vesi- 

 cle, this structure may be called indifferently umbilical 

 vesicle or yolk-sac. 



The yolk which fills the yolk-sac in Birds is re- 

 placed in Mammals by a coagulable fluid ; while the 

 gradual extension of the hypoblast round the wall of 

 the blastodermic vesicle, which has already been de- 

 scribed, is of the same nature as the growth of the hy- 

 poblast round the yolk-sac in Birds. 



The whole embryonic area would seem to be em- 

 ployed in the formation of the body of the embryo. Its 

 long axis has no very definite relation to that of the 

 blastodermic vesicle. The first external trace of the 

 embryo to appear is the medullary plate, bounded by 

 the medullary folds, and occupying at first the anterior 

 half of the embryonic area (Fig. 103). The two me- 

 dullary folds diverge behind and enclose the front end 

 of the primitive streak. As the embryo elongates the 



