FCETAL NUTRITION: THE PLACENTA 489 



placental layer proliferates irregularly round the lumen, and 

 degenerates. In the bat, according to Nolf, the vessels in which 

 the change occurs are the venous capillaries,, in which the blood, 

 returning from the placenta charged with foetal excretory products, 

 stagnates and produces the hyperplasia and simultaneous degenera- 

 tion. Hubrecht, however, states that an endothelial proliferation 

 occurs in arterial and venous capillaries alike in the hedgehog. 



At the embryonic pole the plasmodiblast undergoes a marked 

 thickening. It gradually replaces the superficial decidual cells, 

 and surrounds the vessels as in the rabbit. Then it attacks the 

 endothelial sheath and replaces it, so that lacunse of maternal blood 

 come to be surrounded by foetal tissue. At the same time the 

 cytoblast sends out cellular buds, which project into the plasmodial 

 mass. Under the cytoblast is the double layer of mesoblast, the 

 thin somatopleur, and the splanchnopleur in which the area 

 vasculosa is developed. A yolk-sac placenta is thus formed in 

 the same region as is subsequently occupied by the allantoic 

 placenta. Nutritive exchanges between maternal and fcetal blood 

 are now possible. 



In the further development of the placenta there is very little 

 or no penetration of maternal tissue by the trophoblast (Duval i). 

 Degenerative changes occur in the cells of the epithelioid layer in 

 the placental hemisphere. They lose their outlines, and form a 

 symplasma which is absorbed by the adjacent cells of the couche 

 paraplacentaire (Nolf). Superficially the paraplacental layer remains 

 until the end of pregnancy. The blind ends of the glands are still 

 distended, but their epithelium degenerates and is cast off into 

 the lumen. 



In the non-placental trophoblast, retrogressive changes also occur. 

 Its cells lose their phagocytic power and contain no granules. In 

 the placental area, as already mentioned, the allantois replaces the 

 yolk-sac. The " villi " resemble the tubes of the rabbit. They form 

 a series of arches whose meshes are occupied by allantoic vessels; 

 there are no villi hanging free. As the placenta develops, the 

 thickness of the arches surrounding maternal blood is reduced, and 

 the two blood-streams lie close together. The cytoblast almost 

 entirely disappears. 



Pteropus edulis. — In Pteropus the placenta is attached to a large 

 mushroom-shaped outgrowth of the uterine wall which grows nearly 

 round the ovum to form a decidua capsularis. As pregnancy advances, 

 the outer wall of the bell-shaped decidual mass is pressed against the 



' Duval, " Etude sur I'embryologie des Cheiroptferes," Jour, de I'Anat. et de la 

 Phys., 1895-97. 



16 A 



