368 



Changes in the Uterine Wall. 

 The uteri increase enormously in size owing mainly to the hyper- 

 trophy of the mucosa. The uterine glands increase in transverse 

 diameter and in length : the intergiandular connective tissue becomes 

 markedly attenuated and the whole corium becomes very vascular. 

 In particular, numbers of small capillaries appear beneath and in 

 close contact with the uterine epithelium. These become very much 

 larger and more numerous as pregnancy advances. The uterine epi- 

 thelium in my earliest foetal stage is a comparatively thin layer of 

 low columnar cells with close-set deeply staining nuclei. 



Yolk-sac Placenta. 

 In correlation with the degenerate and functionless condition of 

 the allantois, the embryo is nourished exclusively through the agency 

 of the yolk-sac and a definite yolk-sac placenta of somewhat complex 

 character is present. This yolk-sac placenta is constituted of two 

 portions, associated respectively with the vascular and non-vascular 

 portions of the omphalopleure, of which the latter is by far the most 

 complicated. 



a) Relations of the vascular omphalopleure to the uterine sur- 

 face. This consists in an exceedingly close adhesion of the vascular 

 area to the uterine epithelium. The latter is irregularly ridged and 

 in close contact with its deep surface are numbers of maternal capil- 

 laries. The vascular omphalopleure accurately follows the contour 

 of the ridged uterine surface and its thin ectoderm appears in intimate 

 and actual adhesion to the epithelium. The two blood streams, foetal 

 and maternal, are thus separated not only by the thickness of the 

 thin endothelial walls of the capillaries but also by that of two cell 

 layers viz. a thin layer of foetal ectoderm and a thicker layer of 

 uterine epithelium. A point worthy of remark is the comparative 

 poorness as regards the degree of development of the capillary system 

 both foetal and maternal, which suggests that the second portion of 

 the placenta associated with the non-vascular portion of the omphalo- 

 pleure must play a not unimportant part in the sustenance of the 

 embryo. 



b) Relation of the bilaminar omphalopleure to the uterine surface. 



1) Outside the sinus terminalis, an annular zone of the omphalo- 

 pleure enters into intimate union with the uterine epithehum through 

 the agency of its enlarged ectodermal cells. 



In the earliest foetal stage at my disposal the ectoderm of the 



