FCETAL NUTRITION:. THE PLACENTA 481 



permanent. The lacunar spaces in it are more complicated than in 

 the omphaloidean trophoblast, and their walls bulge towards the 

 embryo and interlock with vascular projections of the allantois. 

 Hence the villi have a complete trophoblastic covering. The 

 extremity of each villus is attached to the maternal decidua by 

 sti'ands of trophobl&,stic cells. The allantoidean trophospongia 

 develops like the omphaloidean, but it retains its thickness later 

 in pregnancy. The deciduofracts remain distinct to the end, though 

 they partly degenerate. Hence it is probable that during the whole, 

 or nearly the whole, of pregnancy they exercise a phagocytic action 

 on the maternal tissues, and store nutriment which they give up to 

 the embryo in a way as yet unknown. 



Shreiu. — In the shrew (Hubrecht i) the method of embedding is 

 centric, and no decidua reflexa is formed. The yolk-sac placenta 

 is not so well developed as in the hedgehog. 



The attachment of the blastocyst is modified, as in Euminants, 

 by special characteristics of the uterine mucosa. They differ from 

 the cotyledonary burrs, however, in being prohferations of the surface 

 epithelium. Before the fertilised ova reach the uterus, there are 

 variations in thickness in the mucosa. It is thin at the mesometrial 

 and anti-mesometrial sections, but thickened over the sides to form 

 two cushions, in which the blood-vessels are more numerous. No 

 glands are present near the mesometrium. They are collected on 

 the opposite surface and open into a longitudinal anti-mesometrial 

 groove (Fig. 141). 



When the blastocysts reach the uterus, further changes take 

 place. Both the lateral regions increase in thickness by the pro- 

 liferation of connective tissue cells and the formation of new vessels,, 

 while the anti-mesometrial part is widened out into a concave 

 bell-shaped surface into which the glands open. Then the epithelium 

 proliferates, first in the lateral cushions and later in the concave 

 area. In the former the proliferation reaches a thickness of twelve 

 to eighteen cells, and the new elements pass in among the cells and 

 vessels of the deeper layers. In the allantoidean region, the bell- 

 shaped area, the proliferation also leads to a thick epithelial layer 

 with vascular channels between the cells. At intervals, however, 

 the cells are 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. 142). 



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 



1 Hubrecht, " The Placentation of the Shrew," Quar. Jour. 2[icr. Science, vol. 

 XXXV., 1894. 



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