FETAL MEMBRANES OF THE PIG EMBRYO 



69 



embryo, its functions then being transferred to the allantois. Branches of the 

 vitelhne vessels ramify in its wall, as in that of chick embryos, but soon degener- 

 ate. The trunks of the vitelline vessels, however, persist within the body of the 

 embryo. The allantois, developing as in the chick from the ventral wall of the 

 hind-gut (Fig. 70 A-D), appears when the embryo is still flattened out on the 

 germinal disc. In an embryo 3.5 mm. long it is crescent-shaped and as large as 

 the embryo. It soon becomes larger and its convex outer surface (splanchnic 

 mesoderm) is appKed to the inner surface (somatic mesoderm) of the chorion. 



Entoderm of primitive 



Amnion 



atic and 



nchnic mesoderm 



Yolk sac 



Entoderm 



-Chorionic mesoderm 

 -Choi ionic cclodcrm 

 ^Uterine epithelium 

 ^Tunica propria of uterus 



Fig. 73.— Diagram of the fetal membranes and allantoic placenta of a pig embryo in median sagittal sec- 

 tion (based on figures of Heisler and Minot). 



These surface layers fuse more or less completely. A pair of allantoic veins and 

 arteries branch in the splanchnic layer of the allantois. These branches are 

 brought into contact with the mesodermal layer of the chorion and invade it. 

 The outer ectodermal layer of the chorion in the meantime has closely appKed 

 itself to the uterine epithelium, the ends of the uterine cells fitting into depressions 

 in the chorionic cells (Fig. 73). When the allantoic circulation is estabUshed, 

 waste products given off from the blood of the embryo must pass through the 

 epithelia of both chorion and uterus to be taken up by the blood of the mother. 



