FCETAL NUTRITION: THE PLACENTA 391 



sac or ompkaloidean 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 villi being peeled out fi'om 

 the trophoblast as the separation increases. The vitelline 

 circulation at the same time diminishes, though it never ceases 

 entirely (Hubrecht ^). 



AHantokJcan region 

 ^'^' of troplixmyhcre 



Mesometrium 



FiC4. 85.~Diagram to illustrate the frntal membranes of Erinaceiis. (From 

 Hubrecht's " The Placentation of Erinaccus curopwiis," Quar. Jour. 

 Mia: Sci., vol. xxx., 18S9.) 



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

 lateral cushions of prohferated 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 



1 Hubrecht, "The Placentation of Erinaceus europceiis," Quar. Jour. 

 Micr. Sci., vol. xxx., 1889. 



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

 vol. XXXV., 1894. 



