FKTAL MEMBRANES OF THE PIG EMBRYO 



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 applied to the inner 

 surface (somatic mesoderm) of the chorion. 



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 cho- 



Entodcrm of primitive gut 



Hind-gut 



A in n ion 



Ectoderm 



Fore-gut 



Somatic mesoderm 

 Splanchnic mesoderm 



Yolk sac 



Enloderm 



Chorionic mesoderm 

 -Chorionic ectoderm 

 ^Uterine epithelium 

 ^Tunica propria of uterus 



FIG. 73. Diagram of the fetal membranes and allantoic placenta of a pig embryo, in median 

 sagittal section (based on figures of Heisler and Minot). 



* 



rion and invade it. The outer ectodermal layer of the chorion in the mean- 

 time has closely applied 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 established, waste products given off from the 

 blood of the embryo must pass through the epithelia of both chorion and 

 utervs to be taken up by the blood of the mother. In the same way, 

 nutritive substances and oxygen must pass from the maternal blood 

 through these layers to enter the allantoic vessels. This exchange does 

 take place, however, and thus in Ungulates the allantois has become im- 



