LATER DEVELOPMENT OF CHICK AND RABBIT 437 



sinus venosus. In the adult their embryonic portions form the 

 hepatic portal veins. 



In man the yolk sac is very different : it is completed at a fairly 

 early stage, and is quite small. It never reaches the far side of the 

 blastodermic vesicle, and so appears as a small bladder suspended 

 in the exoccel by the yolk sac stalk. It then diminishes in size, and 



V. 



AS 



N 



FIG. 153. Generalised diagram of the foetal membranes of a mammal, 

 adapted from Hertwig. 



A., amnion ; A.C., amniotic cavity ; Al., allantois ; Al.C., allantoic cavity ; Al.S., allantoic 

 stalk; B., brain; C.,chorion ; E.,enteron ; E. E., embryo nic ectode rm ; En. , embryonic entoderm ; 

 M., embryonic mesoderm ; N., notochord ; N.C., neural canal ; T., trophoblast ; V., villus ; 

 Y., yolk sac ; Y.S., yolk sac stalk. 



as the expansion of the amniotic cavity brings about the formation 

 of the umbilical cord, the yolk sac becomes first of all reduced to a 

 small blind ccecum and finally disappears altogether. 



Allantois. 



The allantois is also subject to considerable variation among 

 various groups of the Mammalia, but on the whole its relations are 



