520 A. A. W. HUBRECHT. 



blastio tissue that surrounds the allantoidean villi (and through 

 the intervention of which the latter are bathed by maternal 

 blood) are thus identical with the further phenomena of growth 

 and development of the placenta. The latter does not contain 

 maternal elements other than blood. Trophoblastic tissue is 

 the material out of which the placenta is built up. 



It is soaked with maternal blood, and allantoidean vessels 

 with their ramifications have been carried into it by the villi. 

 In the ripe placenta the villi are no longer recognisable as 

 suchj more or less in the same way as I have formerly described 

 and figured this for the hedgehog (1. c, figs. 56 and 57). 



And whereas in the growing placenta of Sorex a maternal 

 (red) and an embryonic (black) part can be distinguished, in 

 the full-grown one the maternal portion is no longer present 

 as such, but forms an insignificant and interrupted sheet of 

 nuclear remains between the main mass of the placenta and 

 the muscularis with the aflferent and efi'ereut vessels. It is in 

 the plane of this sheet that the severing of the placenta at birth 

 is eflfected. In the shrew it is actually shed as in the hedgehog, 

 and not resorbed in loco as in the mole. 



We must now attend to a few details connected with this 

 process of development. Starting from the figs. 81, 89, 93, 

 82, and 94, which have already been referred to, we must 

 repeat that in these stages the fusion of trophoblast and 

 maternal epithelial proliferation has become most intimate, 

 that it would be difficult to draw any sharp line of demarcation 

 in figs. 81, 82, and 89, but that, all the same, the difference 

 between plasmodiblast and maternal crypt tissue and between 

 trophoblastic blood-spaces and maternal capillaries is unmis- 

 takable. The thickness of the maternal layer in comparison 

 to the embryonic one is about 1 : 1 in figs. 12 and 89. The 

 active increase of the maternal crypt tissue by further pro- 

 liferation has now nearly come to a standstill, and henceforth 

 the growth of the placenta means the increase of the tropho- 

 blastic strands and of the villi between them. The increase of 

 the plasmodiblast at the cost of new layers of cytoblast was 

 already noticed above. But as pregnancy advances certain 



