f)10 THE HUMAN EMBRYO. 



is nearly half an inch thick. The surface layer, about one-fourth 

 of the entire thickness, is spoken of as the stratum compactum : 

 in it the gland tubes remain comparatively straight and narrow, 

 while in the interglandular tissue numbers of large, epithelium- 

 like^ decidual cells appear, apparently formed by modification 

 of connective-tissue corpuscles. The deeper three-fourths of 

 the thickness of the mucous membrane, or stratum spongiosum, 

 has a different structure, the gland tubes being greatly dilated 

 and very irregular in shape, and their lining epithelium consist- 

 ing of flattened or cubical, in place of columnar cells. 



After the fifth month, by which time the chorion has met 

 with the decidua vera so as to obliterate the cavity of the uterus, 

 the decidua vera gradually becomes thinner and less vascular, 

 and undergoes degenerative changes, leading ultimately to the 

 almost complete disappearance of the glands, with the exception 

 of their deepest or outermost ends. 



The decidua reflexa goes through changes of a very similar 

 kind : the glands first become dilated and lengthened, and then, 

 as the decidua reflexa becomes more and more distended through 

 the enlargement of the chorionic vesicle, the glands gradually 

 atrophy, and the entire layer degenerates, and ultimately com- 

 pletely disappears. 



In the decidua serotina, from which the maternal portion ot 

 the placenta is formed, the changes have been followed in more 

 detail, but are still only imperfectly known. There is the same 

 division into two layers as in the decidua vera : (i) an inner or 

 superficial layer, from which the surface epithelium and all traces 

 of the uterine glands disappear entirely, and in which decidual 

 cells are developed in large numbers ; and (ii) an outer or deeper 

 layer, in which the gland cavities remain as irregular clefts, 

 from which the epithelium has disappeared ; except in the very 

 outermost layer, in immediate contact with the muscular wall 

 of the uterus, where the outer or blind ends of the glands 

 persist, greatly compressed, but retaining their epithelium. 

 It is from these outer ends of the glands that the epithelial 

 lining of the uterus is regenerated, after the separation of the 

 placenta. 



The further changes that occur in the decidua serotina, and 

 more especially the relations of the blood-vessels, are described 

 in the next section. 



