282 MR. R. ASSHETON ON THE F@TUS 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 of the Placenta. 
The proximal wall of the yolk-sac, which les 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. 
The 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. & Phys. vol. xxvi. p. 308 (1892); 
Duval, M., ‘Le placenta des Rongeurs,” Journ. Anat. et Phys. 
1889-1892). In dAcomys 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 
