482 



THE PHYSIOLOGY OF REPRODUCTION 



cell-outlines in the epithelium of the cushions are lost, and a 

 syinplasma is formed. At the same time the trophoblast becomes 

 syncytial, is fused to the uterine symplasma, and absorbs part of it. 

 Some of the intercellular channels are opened, and the maternal 

 blood thus begins to circulate in the syncytial lacuna 1 . At the same 

 time a deeper cell layer, corresponding to the cytoblast of the bat, 

 appears in the trophoblast, but it is never so well marked as in the 

 allantoidean region. In this way the avillous yolk-sac placenta is 

 formed (see also p. 424), and it functions for a time. Soon retrogres- 

 sive changes appear, resulting in the absorption of the omphaloidean 

 syncytium and epithelium thickenings (Fig. 143). The disappearance 





U.S. 



FIG. 142. Part of the anti-mesometrial wall of the uterus of Sore.? (Hubrecht). 

 The proliferated epithelium is arranged in a radial fashion, and later it 

 forms a secondary crypt (Or.), when the uterine epithelium (U.E.) gives 

 way over it. 



is apparently brought about by a newly formed annular proliferation 

 of the trophoblast above the non-placental part, and the degenerated 

 products of the thickened uterine epithelium and of a blood 

 extravasate, which constantly exists between the annulus and the 

 epithelium, are absorbed and transmitted through the hypoblast to 

 the yolk-sac. From it the vessels of the area vasculosa, which at 

 this time reach their full development, carry the nutriment to the 

 developing embryo. 



The allantoidean trophoblast is applied against the bell-shaped 

 proliferation on the anti-mesometrial side of the uterus, and is fixed 

 by projections which sink into the newly formed crypts. After 

 destroying their epithelial lining, the projections erode capillaries, 

 and the maternal blood circulates in the syncytial lacunse as in the 

 omphaloidean trophoblast. The cytoblast follows the plasmodial 

 projections, and later the trophoblastic villi are vascularised by the 



