STUDIES IN MAMMALIAN EMBRYOLOGY. 491 



more numerous to the right and left than opposite to the 

 mesometrium. Their openings are in the same place as 

 before, but instead of the perpendicular groove of figs. 1 

 and 16,. the region where they open out has become the 

 upper part of the concave upper wall of the bell-shaped 

 lumen. 



The blood-vessels are found in corresponding situations as 

 before. The lateral swellings have chiefly increased by con- 

 nective-tissue proliferation, the muscularis having here also 

 become thinner (compare figs. 16 and 18), and the epithelium 

 being as yet only one cell-layer thick. The histological de- 

 tails are indicated in figs. 17, 18, and 66. In the last 

 figure it is seen that the epithelial lining also of the antimeso- 

 metrical concave surface is in this stage not more than one 

 cell thick. 



It is this point which is the first to be modified. And this 

 modification is at the same time the most important change 

 that takes place in the maternal tissues preparatory to the re- 

 ception, fixation, and nutrition of the blastocyst. Moreover 

 it is a process which in other mammals has up to now not been 

 noticed, rather the contrary. The most recent trustworthy 

 observations on the placentation of mammals have brought 

 to light numerous instances amongst Insectivora, K-odentia, 

 Carnivora, and Cheiroptera, where the maternal epithelium 

 of the uterine lumen disappears in early stages of pregnancy. 

 I have myself described and figured this phenomenon in the 

 hedgehog. And thus it is certainly both an unexpected and 

 an important fact that in the shrew proliferation of this same 

 uterine epithelium takes place to a very considerable extent, 

 and that the cell material resulting from this proliferation is 

 of such high importance for the further attachment of the 

 embryo. Still we shall afterwards have to notice that the com- 

 plicated epithelial arrangement resulting from this proliferation 

 is not permanent, but that it disappears and is destroyed either 

 simultaneously or some time after the embryonic trophoblast 

 becomes attached to it, thus bringing about a final stage in 

 which comparison with the other mammals (where the uterine 



