474 THE PHYSIOLOGY OF REPRODUCTION 



into the lumen of the uterus, and in time obliterates it by coming in 

 mi it act and fusing, at the tenth day, with the mesometrial mucosa 

 (Fig. 136). Here also the cellular tissue has developed at the expense 

 of the glands, and the surface epithelium disappears. At the fifteenth 

 dav the lumen reappears anti-mesometrially (Fig. 137). Thus a 

 secondary decidua reflexa arises which rapidly thins and becomes 

 vacuolated in its inner half by a loss of tissue. The cause of the 

 tissue excavation is uncertain ; it may be brought about by the large 

 cells which, according to von Spee, are foetal and form a third layer 

 of the trophoblast outside the plasmodiblast, and the disintegrated 

 products are probably absorbed by the ovum. At the same time the 



mes 



fcifi*~~- -' 



Kit;. 130. Implantation cavity of the guinea-pig. (Duval.) 

 mes, Mesometrial border ; /, uterine lumen. 



vessels which penetrate the necrotic zone are opened, and blood is 

 effused into the implantation cavity. 



The placenta develops, as in the mouse, mesometrially. The 

 allantois consists of a tubular passage in the body wall and a solid 

 extra-embryonic stalk of mesoderm. It projects into the ccelom and 

 gradually extends, and becomes applied to the mesoblast underlying 

 the thickened part of the trophoblast, in the spaces of which a 

 circulation of maternal blood is established. The trophoblast continues 

 to attack and absorb maternal tissue and blood, and to advance more 

 deeply into the decidua, while at the same time it is penetrated on 

 the embryonic side by outgrowths of mesoblast containing branches 

 of the allantoic vessels. The tissues intervening between the maternal 

 and fo3tal blood-streams are entirely fo?tal ; they gradually thin with 

 the progress of gestation and the continued branching of the mcso- 

 dermal villi. 



