DEVELOPMENT OF THE FECUNDATED EGG. 501 



duct is continuous with the intestine is the intestinal 

 licus, and the walls of the body which close around this duct 

 form the cutaneous umbilicus, or navel (see p. 231, Fig. 

 64). 



The umbilical vesicle contains a fatty albuminous fluid 

 which represents all the extra-embryonic portion of the 

 vitellus. This liquid serves for the nutrition of the mam- 

 malian fetus until the placenta is developed. The absorption 

 of the fluid of the umbilical vesicle is accomplished by a 

 system of blood-vessels (primary circulation, see farther on), 



Fig. 138. Blastodermic Fig. 139. Egg with its umbilical vessel 



vesicle.* fully developed.! 



which are developed in the external wall of the vesicle 

 (omphalo-mesenteric vessels) ; these absorb the contents of 

 this cavity by the medium of the internal epithelial surface 

 of the vesicle, just as in adult life the mesenteric vessels 

 (venaporta) absorb the contents of the intestinal canal by 

 the medium of the villous epithelium, (and indeed fine vas- 

 cular villi are often found on the internal surface of the 

 umbilical vesicle). 



* D, Yellow, 8, Vitelline membrane. , Membrane at the external layer of 

 the blastoderm, a, Middle layer, y, Internal layer. 



t 1, Vitelline membrane. * 2, External layer of the blastoderm. 3, Middle 

 layer of the blastoderm. 4, Internal layer. 5, Body of the embryo. 6, 7, 8, 9, 

 As in Fig. 137. o, Umbilical vesicle, al, Allantoid" pouch or protuberance, a, 

 Amniotic cavity. 



In this Figure, as in Figs. 137, 140, 141, the interrupted lines show the parts 

 belonging to the internal layer of the blastoderm ; the black lines belong to the 

 middle layer ; the dotted lines belong to the external layer. 



