474 THE PHYSIOLOGY OF REPRODUCTION 



into the lumen of the uterus, and in time ubhterates it b}' coming in 

 contact and fusing, at tlie tenth day, with the mesometrial mucosa 

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

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

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

 secondary decidua retlexa arises which rapidly tliius and Ijecomes 

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

 tissue excavation is uncertain ; it nra}^ be Ijrought about by the large 

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

 of the tropliolilast outside the plasmodililast, and the disintegrated 

 products ai-e jjrobably altsorlied by tlie (»vum. At tlie same time the 



mes 



"Fig. 136. — Implantation cavity of the guinea-pig. (Duval.) 

 mes, Mesometrial boi'der ; J, uterine lumen. 



■^'essels whicli peneti-ate the necrotic zone are opened, and blood is 

 effused into the implantation cavity. 



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

 allantois consists of a tulailar passage in the Ijody wall and a solid 

 exti'a-embryonic stalk of mesoderm. It projects into the cffilom and 

 gradually extends, and becomes applied t" tlie mesolilast underlying 

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

 circulation of maternal Idood is established. The trophoblast continues 

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

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

 the emV)ryonic side liy (jutgrowths of mesoblast containing branches 

 of the allantoic vessels. The tissues intervening between the maternal 

 and fojtal blood-streams are entirely tVctal ; they gradually' tliin with 

 the progress of gestation and the continued l>ranching of the meso- 

 dermal villi. 



