The placentation of Crocidura in its general features resembles that of Sorex, as described by Hubrecht, but it is far more complex and exhibits many differences in detail, judging from Hubrecht's account. The ripe placenta, which is formed on the antimesometrial side, is of the hemochorialis type, in which maternal blood circulates in lacunze bounded by syncytio-trophoblast, the allantoic capillaries lying between the lacunæ.
The late blastocyst of Crocidura has the typical form of a large thin-walled vesicle with a lenticular embryonal knot (Pl. XXI. fig. 8). After the segregation of the endoderm a cleft arises in the embryonal ectodermal mass. The covering layer of the trophoblast and then the roof of this cleft disappear, with the result that the shield ectoderm becomes exposed on the surface (Pl. XXI. fig. 10). Over a localized discoidal area in the equatorial region of the vesicle, the trophoblast becomes thickened. This specialised area is apparently associated with the first attachment of the blastocyst to the uterine wall. The blastocyst lies in a deep pear-shaped enlargement of the antimesometrial uterine lumen, which we have termed the implantation-cavity. The uterine epithelium lining the cavity proliferates rapidly and gives rise to a thick layer of epithelial crypts (Pl. XXII. fig. 11). The blastocyst first becomes attached to the uterine wall by the small discoidal area of enlarged trophoblast-cells in the equatorial plane. The trophoblast-cells of this area penetrate rapidly into the uterine wall and destroy the hypertrophied uterine epithelium, with the result that a gap is produced in the thick layer of epithelial orypts. This gap is a constant and conspicuous feature in all the early stages of attachment (Pl. XXII. figs. 11, 14). The uterine epithelial crypts, the lumina of which are extremely narrow, rapidly increase in depth and form a series of radially arranged columns separated by richly vascularized sheets of maternal connective-tissue, which frequently terminate in conspicuous enlargements containing many capillaries just inside the superficial layer of uterine epithelium (Pl. XXII. fig. 13).