STUDIES IN MAMMALIAN EMBRYOLOGY. 497 



presented in fig. 13, the uterine tissue enveloping the embryo 

 is stretched and thinned out more considerably as far as the 

 non-placental region is concerned. But even in the placental 

 region the considerable local thickening brought about by the 

 formation of the placenta is, as we shall see in subsequent 

 paragraphs, not due to proliferation of maternal uterine, but of 

 embryonic trophoblastic tissue. Here, too, the actual uterine 

 tissue is flattened and stretched, its inner boundary line being 

 recognisable by the presence of deeply stained nuclear remains 

 of the blind ends of the epithelial crypts, of which the origin 

 and further development was noticed above, further blood- 

 vessels, and finally a few hardly recognisable gland remnants. 

 It deserves observation that the maximum of the decrease of 

 the true uterine tissue in the placental region is brought about 

 in stages long before parturition comes about (cf. fig. 14). 



In the later stages of pregnancy the deeper regions again 

 somewhat increase in thickness in comparison to the bulk of 

 the placenta, preparatory to the severing of this organ and to 

 the restoration of the uterine surface after parturition. 



As a detailed discussion of all these changes would fall out- 

 side of the scope of this paper, I will now only complete this 

 rapid sketch by special reference to some of the figures. 



Figs. 18 and 20, when compared to fig. 16, bear testimony 

 to a very considerable increase of the uterine connective tissue 

 between the epithelium and the muscularis, partly preceding, 

 partly simultaneous with the epithelial proliferation. This 

 growth of connective tissue begins in the lateral cushion-shaped 

 regions ; somewhat later it is also noticed in the future placen- 

 tary region. In figs. 21 — 23 this is represented under lower, 

 in figs. 66, 72, and 73 under higher powers. A network of 

 blood-vessels is evidently being spun out in these regions for 

 the purpose of supplying the placenta in the later stages of 

 pregnancy. The comparative thickness of the connective-tissue 

 layer, which reaches its maximum in the stage of uterus No. 

 73, is soon encroached upon by the epithelial proliferation 

 from which it is separated by the fibrillar layer, which is very 

 clearly indicated in fig. 74, and which in the earlier stages (cf. 



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