482 



THE PHYSIOLOGY OF REPRODUCTION 



cell-outlines in the epithelium of the cushions are lost, and a 

 symplasma is fomied. At the saiue time the tropholilast becomes 

 syncytial, is hised to the uterine symplasma, and absorljs part of it. 

 Some of the intercellular channels are opened, and the maternal 

 blood thus l^egins to circulate in the syncytial lacunar At tlie same 

 time a deeper cell layer, cori'esponding to the cytoblast of the Ijat, 

 appears in the trophoblast, but it is never so well marked as in the 

 allantoidean region. In this way the avillous yolk-sac placenta is 

 formed (see also p. 424), and it functions for a time. Soon retrogres- 

 sive changes appear, resulting in the absorption <:)f the omphaloidean 

 syncytium and epithelium thickenings ( Fig. 14-!). The disappearance 







^■iCc 









Fig. 142. — Part of the anti-mesoinetrial wall of the uterus of Surer (Hubreoht). 

 The pioliferated epitheliura is arranged in a radial fashion, and later it 

 forms a secondary c-rypt ((V.), when the uterine ejjithelium (U.PJ.) gives 

 way over it. 



is apparenth' )jrought about l)y a ne\vly formed annular proliferation 

 of the trophol)last al)o-\-e the nf)n-placental part, and tlie degenerated 

 products of the thickened uterine epithelium and of a blood 

 extravasate, which constauth" exists between the annulus and the 

 epithelium, are absorbed and transmitted througli the hypoblast to 

 the yolk-sac. From it tlie ^"esseLs of the area vasculosa, which at 

 this time reach their fidl de^'elo2lment, cairy the niitriment to the 

 developing eml.iryo. 



The allantoidean trophoblast is applied against the Ijell-shaped 

 proliferation on the anti-mesometiial side of the uterus, and is fixed 

 by projections which sink into the newly tVirmed crypts. After 

 destroying their epithelial lining, the projections erode capillaiies, 

 and the maternal blood circulates in the syncytial lacuuEc as in the 

 omphaloidean tropholilast. Tlie cytolilast follows the plasmodial 

 projections, and later the troplioblastie villi are vascularised by the 



