rCETAL NUTRITION: THE PLACENTA 459 



of the ring runs a deep groove which is crossed by branches of 

 the allantoic vessels to reach an epithelial ridge of cells. 



Cheiroptera. — The mode of embedding in the bat is centric, 

 and the allantoic placenta is discoid. Before segmentation is 

 completed, the fertilised ovum reaches the uterus and invariably 

 enters the right comu (Ercolani ^). The zona peUucida is 

 already thinned and soon disappears, the spherical blastodermic 

 vesicle lying free in the uterine cavity. 



At the beginning of gestation, according to- van Beneden,^ 



All. ■ 



Fig. 118. — Replacement of omphaloidean by allantoidean placenta in Tupaia. 

 (From Hubreoht's " Ueber die EntwicHung der Placenta von Tarsius 

 und Tupaja," Intemat. Congr. of Zool., Cambridge, 1898.) 



TO.u.,mesodermic villi; Tr., trophoblast ; Ta. , trophospongia ; 4M., allantois : 



y.8., yolk-sac. 



the mucosa is composed of a richly cellular connective tissue, 

 covered by a non-cihated epithehum. Of the glands some are 

 simple tubes, and others divide dichotomously. None open on 

 the mesometrial aspect where the blastocyst later becomes 

 fixed. There also the cellular tissue is not so thick. 



Before fixation of the blastocyst, important changes occur 



•* Ercolani, " Nuove ricerche suUa placenta nei pesci cartilaginosi e nei 

 mammiferi," Mem. dell' Accad. d. Sc. delV Institut. di Bologna, vol. x., 1879. 



' v. Eeneden, " De la formation et de la constitution du placenta chez 

 le murin," Gomp. Bend, de la Soc. de Biol., vol. v., 1888. 



