424 THE PHYSIOLOGY OF REPRODUCTION 



necting membrane. In origin it is a non-vascular somatopleur or 

 diplo-trophoblast, but in the mature foetal sac there are intrusive 

 capillaries proceeding into it from the vascular area. Willey suggests 

 that the reason for the existence of the connecting membrane supple- 

 menting the placental implantation in the attachment of the foetal 

 sac is to be sought in the semi-aquatic habits of the beaver, which 

 retains its activity, swimming under the ice to procure food from 

 the submerged stock of winter provender throughout the period of 

 gestation. Were it not for the additional support of the connecting 

 membrane the narrow deciduous root of the massive placenta might 

 be torn asunder. 



Insectivora. — In the hedgehog the yolk-sac forms a placenta 

 which nourishes the embryo until the mesoblast splits into two 

 layers and the allantoic placenta is formed. At a very early stage 

 the epiblastic wall of the blastocyst has spaces in which maternal 

 blood appears. As the mesoblast spreads out in a single layer, the 

 area vasculosa develops, and its branches, contained in mesoblastic 

 warts and ridges, interlock with the adjacent trophoblast to form 

 yolk-villi (Fig. 110). The yolk-sac or om/pJialoidean placenta reaches 

 its full development at the time when the allantois comes in contact 

 with the trophoblast (see p. 480). Then the yolk-sac is gradually 

 separated from the wall, more and more of its villi being peeled out 

 from the trophoblast as the separation increases. The vitelline 

 circulation at the same time diminishes, though it never ceases 

 entirely (Hubrecht ^). 



In the shrew the yolk-sac adheres by a zonary strip to lateral 

 cushions of proliferated mucosa, but the resulting yolk-sac placenta 

 is avillous (Hubrecht ^). The trophoblast is again thickened, and in 

 its spaces maternal blood appears, but at a later date than in the 

 hedgehog. The maternal blood is bodily absorbed, and at the same 

 time the yolk-sac contains a characteristic yellowish-green, glassy 

 coagulum with granules in it. Later the mucosal cushions disappear 

 and the adjacent trophoblast thins (see p. 483). 



In the mole a simple yolk-sac placenta persists throughout 

 pregnancy (Eobinson^). Unlike the hedgehog and the shrew, in 

 which the gland lumina are plugged by the trophoblastic syncytium, 

 there is in the mole a copious glandular secretion containing 

 degenerated cells, which is absorbed by the trophoblast (see p. 484). 



Tupaia javanica differs from the other Insectivora in having a 

 temporary yolk-sac placenta formed in the same situation as the 



' Hubrecht, " The Placentation of Erinaceus ewopcem," Qtiar. Jow. Micr. 

 Science, vol. xxx., 1889. 



2 Hubrecht, " The Placentation of the Shrew," Qiia/r. Jour. Micr. Science, vol. 

 XXXV., 1894. 



' Robinson, Hunterian Lectures, loc. cit. 



