CH. LVIII.] 



THE FffiTAL MEMBRANES. 



819 



nutriment thus absorbed from it passes to the foetal heart by two 



veins called the omphalo-mesenteric 



vein*. The blood-vessels are first 



formed at the circumference of a 



clear area surrounding the embryo ; 



and the place where they are situated 



is called the vascular area. This is 



shown about the natural size in the 



hen's egg in fig. 632. 



In birds the yolk-sac affords 

 nutriment till the end of incuba- 

 tion, and the omphalo-mesenteric 

 vessels are developed to a corre- 

 sponding degree ; but in mammalia 

 the office of the umbilical vesicle 



Fig. 632. Diagram showing vas- 

 cular area in the chick, a, area 

 pellucida ; b, area vasculosa ; 

 c, area vitellin.a. 



Fig. 635. 



Figs. 633, 634, and 635. Diagrams showing three successive stages of development. Trans- 

 verse vertical sections. The yolk-sac, yi. is seen progressively diminishing in size. 

 In the embryo it-self the medullary canal and notochord are seen in section, a, in 

 Fig. 621, the alimentary canal, becoming pinched off from the yolk-sac; a', in 

 lower figure, alimentary canal completely closed ; a, in last two figures, amnion ; "<. 

 cavity of nrnnion filled with anmiotic fluid ; ;>/>. space between amnion and chorion 

 continuous with the pleuro-peritoneal cavity inside the body ; vt, vitelline membrane, 

 or rona pellucida ; ys, yolk-sac, or umbilical vesicle. (Foster and Balfour.) 



ceases at a very early period, as the quantity of the yolk 

 is small, and the embryo soon becomes independent of it by 



