390 THE PHYSIOLOGY OF REPRODUCTION 



area the trophoblast is thickened and maternal blood circulates 

 in its spaces. But the allantois has not yet come in contact 

 with it, and the blood " must serve only for the nutriment of 

 the trophoblast itself." At the eleventh day the trophoblast is 

 vascularised by the allantoic vessels, by which the nutriment is 

 now transmitted as well as by the vitelline vessels in the yolk- 

 viLi. Then the yolk-sac becomes less important. The circula- 

 tion in the decidua reflexa, which surrounds it, decreases and 

 ceases altogether on the sixteenth day, and the wall of the 

 yolk-sac becomes thin and bloodless. " At the same time 

 numerous diverticula grow out from the entodermal sinus into 

 the hilum of the allantoic placenta, and these may still absorb 

 nutriment though they are more probably excretory." Later 

 the outer wall of the invaginated yolk-sac undergoes atrophy 

 and completely disappears. The remains of the yolk-sac cavity 

 are in this way bathed in the uterine fluids. At the same time 

 the villi of the inner wall increase in size and complexity, but 

 whether they absorb the fluids or are entirely excretory is 

 uncertain. 



In the spiny mouse {Acomys cahwrinus), Assheton ^ found in 

 a well-advanced pregnancy that the yolk-sac was stiU extremely 

 vascular, and covered with a colunmar-ceUed epithelium which 

 was much folded. The blood-vessels lay in the folds, and so 

 approached closely to the placenta. The yolk-sac was firmly 

 attached to the placenta over the peripheral area, but not so 

 closely as described above for the rat and common mouse. In 

 the spiny mouse the folds do not become involved in the 

 placental tissues. 



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

 placenta which nourishes the embryo until the mesoblast sphts 

 into two layers and the aUantoic 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 yoIk-viUi (Fig. 85). The yolk- 



' Assheton, " On the Foetus and Placenta of the Spljiy Mouse," Proc. Zool. 

 Soc, London, 1905, vol. ii. 



