454 THE PHYSIOLOGY OF REPRODUCTION 



arranged radially like a fan, and later the internal parts of the 

 cells break away and leave a crypt. No crypts are formed in the 

 lateral cushions (Fig. 115). 



Over the special areas of the mucosa the trophoblast thickens. 

 It comes in contact first with the lateral cushions by a zonary 

 strip against which the vessels of the area vasculosa spread out. 

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

 a symplasma is formed. At the same time the trophoblast be- 

 comes 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 

 lacunae. 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. 391), 

 and it functions for a time. Soon retrogressive changes appear, 

 resulting in the absorption of the omphaloidean syncytium and 

 epithelial thickenings (Fig. 116). The disappearance is ap- 

 parently brought about by a newly formed annular proliferation 

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

 generated 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 lacunae as in the omphaloidean trophoblast. The 

 cytoblast follows the plasmodial projections, and later the 

 trophoblastic villi are vascularised by the allantoic vessels. 

 Subsequently the plasmodiblast thickens to a considerable 

 extent, and in it the mesoblastic villi continue to branch and 

 form secondary and tertiary villi. There is no penetration on 

 their part into the decidual tissue between the crypts, but the 

 maternal part of the placenta as a whole is gradually absorbed 



