486 THE PHYSIOLOGY OF REPRODUCTION 



shows two layers. plasmodiblast and eytohlast. The outer relay fuses 

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

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

 layers next the ovum. AVhen their endothelium is destroyed, 

 maternal blood enters the trophoblastic lacuna- 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 tvophospongia 

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

 there is only a thin rim of maternal tissue left. 



Over the " Hafttieeke " the trophoblast is first vasculariscd by the 



FIG. 14"). Replacement of omphaloidean by allantoidean placenta in Ttipaia. 

 (From Hubrecht'd " Ueber die Entwicklung der Placenta von Tnr^ins und 

 T'if,'ij,i" li-,ti;-n<it. ('<i(f,: <>f Z< <>!., Cambridge, 189*.) 



m.v.j Meaodermir villi ; TV., trophoblaat ; To., troplio.spongia ; .I//., allantois ; 



//..<., Vnlk-sJlr. 



vitelline vessels, and a temporary yolk-sac placenta is formed. Later 

 the allantois displaces the yolk-sac, and its vessels vascularise the 

 same part of the trophoblast (Fig. 145). "The permanent placenta 

 replaces the omphalic placenta both physiologically and topographi- 

 cally" (Hubrecht). Tn this respect 7' />///</ differs from the hedgehog 

 and the shrew. 



Centetes. A peculiar form of placentation has been described by 

 Strahl 1 in the tenrec (Ccntctc* ecttinffifufi). A large effusion of maternal 

 blood destroys the centre of the allantoic placenta, and. leaves only 

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



; Strahl, " Beitrage zur vergleichenden Anatomie der Placenta," AM. Senct- 

 /'/'-/-/. .\'< ft a ,-f.-f >>*., 1905. See also Rolleston, "On the Placental Structures of 

 the Tenrec (Centete* ecaudatnx), etc., : " 7'/v///x. /,<,,!. .sv,-., London, vol. v., 1863. 



