MAMMALIA. 227 



While these changes have been taking place the rudiments 

 of a vascular area become formed, and it is very possible that 

 part of the hypoblastic mesoblast passes in between the epiblast 

 and hypoblast. immediately around the embryonic area, to give 

 rise to the area vasculosa. From Hensen's observation it seems 

 at any rate clear that the mesoblast of the vascular area arises 

 independently of the primitive streak: an observation which is 

 borne out by the analogy of Birds. 



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-sack of the Sauropsida. It is almost 

 certain that the Placentalia are descended from ancestors, the em- 

 bryos of which had large yolk-sacks, 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-sack being retained in the 

 umbilical vesicle, this structure may be called indifferently um- 

 bilical vesicle or yolk-sack. 



The yolk which fills the yolk-sack in Birds is replaced in 

 Mammals by a coagulable fluid ; while the gradual extension of 

 the hypoblast round the wall of the blastodermic vesicle, which 

 has already been described, is of the same nature as the growth 

 of the hypoblast round the yolk-sack in Birds. 



The whole embryonic area would seem to be employed 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. 141). The two 

 medullary folds diverge behind and enclose the front end of the 

 primitive streak. As the embryo elongates, the medullary folds 



15-2 



