FCETAL NUTRITION: THE PLACENTA 483 



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 



Or. 



Fig. 143. — Uterus and embryo of Sorex (Hubrecht). 

 a.T., AUantoidean trophoblast with knobs entering the epithelial crypts {Cr.) ; 

 am., amnion ; all., free knob projecting into the extra-embryonic coelom 

 {E.ec.) ; a.v., area vasculosa ; an', embryonic cells which grow downwards 

 from the upper rim of the trophoblastic annulus (tr.an.), and adhere 

 against the maternal tissue ; np.T., non-placental trophoblast ; GL, 

 glands ; M., mesometrium. 



maternal part of the placenta as a whole is gradually absorbed by 

 the plasmodiblast, and is replaced by foetal elements. In the ripe 

 placenta the only maternal constituent is blood, except a thin discoid 

 sheet of nuclear remnants next the muscularis. The glands are not 

 penetrated by vascular villi. In the early stages they are plugged 

 by syncytium and later disappear. 



