486 



THE PHYSIOLOGY OF REPRODUCTION 



allows t^vo layers, plasmoililjlast and cytolilast. The outer relay fuses 

 so closely with the decidual tissue as to be indistinguishable from it. 

 The capillaries dilate and new vessels are formed, especially in the 

 layers next the ovum. Wheu theii' endothelium is destroyed, 

 maternal Ijlood enters the tropholjlastic lacunte and soon circulates 

 through them. The inter-vascular connective tissue cells proliferate 

 and form the trophospongia. The decidual layers outside it become 

 fibrillar, and soon are extremely attenuated. The trophospongia 

 remains longer, liut finally it also thins, and at the end of pregnancy 

 there is only a thin rim of maternal tissue left. 



Over tlie " Haftflecke " the tropholjlast is first \'ascularised fiy the 



All. ' 



■Ta. 



^Tr. 



Fig. 145. — Replacement of DUiphaloidean by allantoideaii placenta in Tupaia. 

 (From Hubrecht's " Ueber die Entwicklung der Placenta von Tardus und 

 Tupaja," Inieniat. (_'ongr. of Zoo/., Cambridge, 1898.) 



in.t\, Mesodermic villi ; 7V., troplioblast ; ZVr., trophospongia ; AIL, allantois ; 



i/.s., yolk-sac. 



vitelline vessels, and a tempcjrar)' y(.)lk-sao placenta is formed. Later 

 the allantois displaces the yidk-sac, and its vessels ^'ascularise the 

 same part of the tropholjlast (Fig. 14.5). " The permanent placenta 

 replaces the omphalic placenta both physiologically and t(.ipographi- 

 call}' " (Hubrecht). Tn this respect Tupaia differs from the hedgehog 

 and the shrew. 



Cfiitctrs. — A pecrdiar foiin of placentatiou has heen described ))y 

 StrahP in the tenrec (Centctea ecaudatus). A large effusion of maternal 

 Ijlood destroys the centre of the allantoic placenta, and leaves only 

 a peiipheral ring. Round the margin of the ring runs a deep groove 



1 Strahl, "Beitrage zur vei-gleichenden Anatomie der Placenta," Ahh. Senck- 

 rDherg. jVatnrf.-d'es., 1905. See also Rolleston, "On the Placental Structures of 

 the Tenrec {Centetes ecaudatus), etc.," Traux. Zool. ,Soc., London, vol. v., 1863. 



