282 MR. E. ASSHETON ON THE FCETUS AND [June 6, 



was a crumpled mass of membranes, consisting of the yolk-sac 

 and amnion, which had been detached fi'om the foetus during the 

 act of preservation. 



Description of 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. 



The placenta of Acomys with the proximal wall of the yolk-sac attached showing 

 the radiating vessels of j'olk-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 i-emain 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 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 i-egard as being neai-ly 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 



