No. 2.] FCETAL MEMBRANES OF THE MARSUPIALS. T^yg 



amoebiform processes (Fig. 4, arnb), which are closely applied 

 to the lining epithelium of the uterine wall. Fig. 4 represents 

 a section of the rim of one of the yolk-sac placentae which 

 was removed. Fig. 3 represents a more extended section, 

 upon a less enlarged scale, of the edge of one of the yolk-sac 

 placentas which has been partially detached from the epithelium. 

 Over the central depressed region of the vascular disc the cells to 

 some extent lose their amoebiform character. These cells imme- 

 diately overlie a thin layer of small cells, usually two deep, inky; 

 these, probably, represent the mesoblastic elements of the yolk- 

 sac — the hypoblast having been absorbed. This layer is in- 

 terspersed with blood-vessels, fb. As the attaching cells are 

 reflected around the smus terminalis they gradually lose their 

 amoebiform character until, upon the lower surface of the disc, 

 they pass into the simple elongated cells of the subzonal mem- 

 brane. This and the adjoining amcebiform cells are probably 

 derived from the epiblastic layer of the subzonal membrane. 

 This under layer is attached only at scattered points to the 

 yolk-sac placenta, and is reflected to the stalk of the placenta, 

 where it forms part of the investment of the umbilical stalk 

 and allantois. 



So far as I have observed, the upper surface of the yolk-sac 

 placenta is the only portion of the fcetal membranes which at- 

 taches to the wall of the uterus. The condition of the allatttois 

 is difficult to determine at this advanced period. According to 

 Selenka it remains distinct from the subzonal membrane through- 

 out (p. no), and its blood-vessels diminish in size towards the 

 close of foetal life. In transverse sections of the membranes, 

 between the navel and yolk-sac placenta, I find the separation 

 of the allantois from the subzonal membrane is almost com- 

 plete, although at one or two points there is quite an extensive 

 union. The walls of the allantois are much folded, and the in- 

 terior is partially filled with a stroma or network in which are 

 suspended numerous blood corpuscles, which indicate that the 

 allantoic vessels empty into vascular sinuses. The stalk of the 

 yolk-sac is apparently flattened upon itself, with the obliteration 

 of its primitive cavity at this point. The subzonal membrane 

 surrounding this area («/, Fig. 2) consists chiefly of flattened 

 cells, but in some places they are columnar. 



The yolk-sac placentae, as shown in Fig. i, lie nearest the 



