FCETAL NUTRITION: THE PLACENTA 445 



as in the mouse, an inversion of the germinal layers, but in the 

 guinea-pig the amnio-embryonic vesicle is closed and separates 

 the thickened trophoblast from the embryonic ectoderm 

 (Fig. 108). With the growth of the blastodermic vesicle, the 

 roof of the implantation cavity projects into the lumen of the 

 uterus, and in time obliterates it by coming in contact and 

 fusing, at the tenth day, with the mesometrial mucosa (Fig. 109). 

 Here also the cellular tissue has developed at the expense of the 

 glands, and the surface epithelium disappears. At the fifteenth 



mes 



FIG. 109. Implantation cavity of the guinea-pig. (Duval.) 

 mes, mesometrial border ; I, uterine lumen. 



day the lumen reappears anti-mesometrially (Fig. 110). Thus a 

 secondary decidua reflexa arises which rapidly thins and be- 

 comes 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 Spec, 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 vessels which penetrate the 

 necrotic zone are opened, and blood is effused into the implanta- 

 tion cavity. 



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



