FCETAL NUTRITION: THE PLACENTA 391 



sac or omphaloidean placenta reaches its full development at 

 the time when the allantois comes in contact with the tropho- 

 blast (see p. 451). Then the yolk-sac is gradually separated 

 from the wall, more and more of its viUi being peeled out from 

 the trophoblast as the separation increases. The vitelhne 

 circulation at the same time diminishes, though it never ceases 

 entirely (Hubrecht ^). 



AUantoidean region 

 of trophosplierc 



Ornphaloidean 



region of 



tropkospliere 



Decidua reflexa 



Uterine 

 luinen 



- Mesometi'ium 



Fie. 85.— Diagram to lUastrate the fcetal membranes of Erinaceus. (From 

 Hubrecht's " The Placentation of Erinaceus europcms," Quar. Jour. 

 Micr. Scl, vol. XXX., 1889.) 



In the shrew, the yolk-sac adheres by a zonary strip to 

 lateral cushions of proliferated mucosa, but the resulting yolk- 

 sac placenta is avillous (Hubrecht ^). The trophoblast is again 

 thickened, and in its spaces maternal blood appears, but at a 

 later date than in the hedgehog. The maternal blood is bodily 



' Hubrecht, " The Placentation of Erinaceus europceus," Quar. Jour. 

 Micr. Set., vol. xxx., 1889. 



" Hubrecht, " The Placentation of the Shrew," Quar. Jour, Micr. Sci., 

 vol. XXXV., 1894. 



