FUNCTIONS OF THE PLACENTA 925 



In certain areas of the chorion, speciaUzed structures or areolae, containing 

 small villi, appear to slightly invade the uterine glands (fig. 378A'). How- 

 ever, the epithelium is not destroyed, and at all times the maternal and fetal 

 aspects of the greatly expanded placental area (see figure 378 A) may be 

 separated without injury either to the chorionic or to the uterine epithelium. 



6. Fate of the Embryonic Membranes 

 a. Yolk Sac 



The yolk sac of teleost and elasmobranch fishes is withdrawn gradually 

 toward the ventral body wall and intestine. The contribution of the yolk sac 

 differs considerably in the two groups. In the teleost fishes, the somatopleuric 

 portion of the yolk sac contributes much to the body wall while the splanch- 

 nopleuric tissues of the yolk sac form a considerable part of the latero-ventral 

 region of the intestine. In the elasmobranch fishes, the somatopleuric layer of 

 the yolk sac forms only a small area of the ventral body wall in the anterior 

 trunk region, and the splanchnopleuric tissue of the yolk sac is withdrawn 

 inward toward the duodenal area. This withdrawal of the splanchnopleuric 

 tissue is a complex affair, for as the external yolk sac is withdrawn an internal 

 yolk sac is developed as an evagination from the yolk stalk (vitelline duct) 

 near the duodenum (fig. 296A). While the external yolk sac gets smaller 

 the internal yolk sac increases in size, and after the external yolk sac has 

 been entirely withdrawn a considerable part of the internal yolk sac remains. 

 Ultimately the splanchnopleure of the internal yolk sac forms a small area 

 of the duodenal wall. 



In the chick the yolk sac is still large as hatching approaches. During the 

 eighteenth and nineteenth days the yolk sac containing a considerable amount 

 of yolk is withdrawn into the body cavity through the umbilicus. Here the 

 yolk is absorbed rapidly and the yolk sac tissues are taken up into the wall 

 of the intestine about 5 or 6 days after hatching. 



The yolk sac of the higher mammals does not contain yolk substance. One 

 of its main functions is the formation of the first blood cells (see Chapter 17). 

 The yolk stalk and yolk sac increase somewhat in size during the early phases 

 of development. Ultimately the yolk stalk becomes greatly elongated and 

 separates from the yolk sac. The proximal portion of the yolk stalk is taken 

 up into the wall of the intestine. In the human embryo, the area of yolk stalk 

 inclusion into the intestinal wall is about 18 to 24 inches proximal to the 

 ilio-caecal area. 



b. Amnion and Allantois 



The amnion and allantois of the Amniota function until birth. During par- 

 turition the amnion generally ruptures, but may remain intact around the 

 offspring. For example, in a litter of six puppies, half of the amnions may 

 be ruptured and half may be intact. The intact amnion must then be ruptured 



