462 THE PHYSIOLOGY OF REPRODUCTION 



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 de- 

 cidual cells, and surrounds the vessels as in the rabbit. Then it 

 attacks the endothelial sheath and replaces it, so that lacunae 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 splanchno- 

 pleur 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 be- 

 tween maternal and foetal 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 *). 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 ad- 

 jacent 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 sur- 

 rounding 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 



1 Duval, " ]tude sur I'embryologie des Cheiropteres," Journ. de I'Anat. 

 etdelaPhys., 1895-97. 



