188 GENERAL GROWTH OF THE EMBRYO. 



established. The epiblast (on the view adopted above) clearly origi- 

 nates from epiblastic segmentation cells. The hypoblast without 

 doubt originates from the liypoblastic segmentation spheres which 

 give rise to the lenticular mass within the epiblast on the appear- 

 ance of the cavity of the blastodermic vesicle ; while, thougii the 

 history of the mesoblast is still obscure, part of it appears to originate 

 from the hypoblastic mass, and part is undoubtedly formed from the 

 epiblast of the primitive streak. 



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 ap- 

 pendage, 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 embryos 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 umbilical 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 ex- 

 ternal trace of the embryo to appear is the medullary plate, bounded 

 by the medullary folds, aiid 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 nearly meet behind and so 

 cut off the front portion of the primitive streak, which then appears 

 as a projection in the hind end of the medullary groove. In an 

 embryo rabbit, eight days after impregnation, the medullary groove 

 is about 1'80 mm. in length. At this stage a division may be clearly 



