THE PLACENTA. 437 



lias already been described (p. 860), lies quite freely within the 

 uterus, and the structure of the uterine walls is as follows 

 (cf. Fig. 168). 



The muscular walls of the uterus are well marked, consisting 

 of outer longitudinal, mi, and inner circular layers, mk. "Within 

 the layer of circular muscles come the submucous and glandular 

 layers. Of the six longitudinal folds of the uterus, the two 

 placental folds, PK, form large and prominent ridges, separated 

 by a deep median cleft, PR. The periplacental folds, pm, are 

 similar, but much smaller ; while the obplacental folds are no 

 longer recognisable, having become flattened out and obliterated 

 by the stretching, which this part of the wall of the uterus has 

 undergone, to make room for the embryo. 



The submucous layer, which is very thick in the placental 

 folds, pk, but comparatively scanty elsewhere, consists of loose 

 connective tissue, with very numerous, branched connective- 

 tissue cells, and is very vascular. The blood-vessels, which are 

 derived from the mesometrium, perforate the muscular walls of 

 the uterus as small arteries and veins, and then dilate, within 

 the submucous layer, into large but very thin-walled capillaries 

 (Fig. 168, uc), which are especially numerous in the subglandular 

 layer of connective tissue, immediately below the surface epi- 

 thelium. 



The epithelium lining the uterus is pitted to form the 

 uterine glands, GU, which are very deep and freely branched 

 in the placental and periplacental folds ; while in the obplacental 

 area, owing to the stretching which this part of the uterus has 

 undergone, the mouths of the glands are greatly dilated, and 

 the glands themselves widened out. 



Early on the eighth day the attachment of the blastodermic 

 vesicle to the wall of the uterus commences, and by the ninth 

 day it is completed. The attachment is effected, as already 

 noticed, by thickening and proliferation of the epiblast cells of 

 the blastodermic vesicle over a horse-shoe shaped patch, the 

 placental area, which surrounds the sides and hinder end of the 

 embryo (Fig. 145, e'). The epiblast cells of this placental area 

 become more numerous, by repeated divisions, and grow out into 

 irregular processes which fuse firmly with the surface of the 

 placental lobes of the uterus (Fig. 169, e). By this time, accord- 



