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Show 282 MR. E. ASSHET0N ON THE FCETUS AND [June 6, was a crumpled mass of membranes, consisting of the yolk-sac and amnion, which had been detached from the foetus during the act of preservation. Description o f the Placenta. The proximal wall of the yolk-sac, which lies up against the face of the placenta, is extremely vascular and covered with an epithelium of large columnar cells. Moreover, this epithelium is much folded; and the blood-vessels lie in the folds, and so approach closely the surface of the placenta (text-fig. 42). The yolk-sac wall is firmly attached to the placenta over the peripheral area. Text-fig. 42. Tlie placenta of Acomys with the proximal wall of the yolk-sac attached showing the radiating vessels of yolk-sac circulation which are covered with a thick epithelium. X 3. This attachment of the yolk-sac to the placenta is not so intimate as it is in the common rat, in which animal the yolk-sac forms villi or at least folds which become embedded in the tissues of the allantoic placenta, but remain quite easily distinguishable therefrom (cf. Robinson, A., " The Nutritive Importance of the Yolk-sac," Journ. Anat. & Pliys. vol. xxvi. p. 308 (1892); Duval, M., " Le placenta des Rongeurs," Journ. Anat. et Phys. 1889-1892). In Acomys the yolk-sac is much folded, but the folds do not become involved in the placental tissues (text-fig. 45, HH). The placenta itself, which we may regard as being nearly full term, shows only a small area of actual vascular attachment to the wall of the uterus (text-figs. 41, 42). Here it is rough, and marked by open blood-sinuses and shreds of tissue. Passing outwards and extending nearly to the rim, there is a layer of rather darkly staining material showing no particular structure |