No. 2.] FCETAL MEMBRANES OF THE MARSUPIALS. 375 



limits of the sinus tcnninalis." The subzonal membrane cover- 

 ing these low villi consisted of squamous cells, readily detached 

 and exposing a vascular papillus beneath. The position of 

 these villi with reference to the simis termhialis was wholly dif- 

 ferent from that observed in Didelphys, as they were situated 

 between the smiis terminalis and the umbilical stalk, while in 

 Didelphys they were observed in the distal area circumscribed 

 by the sinus terminalis. This difference was explained subse- 

 quently by Caldwell's investigations, which showed that in 

 Phascolarctos and Halmaturus the vascular and non-vascular 

 areas of the yolk-sac are not applied to each other with the 

 obliteration of the cavity of the sac, as in Didelphys (see cut). 

 In the light of present evidence the inference drawn at the time 

 from these facts, that we had here a rudimentary yolk-sac pla- 

 centa covered with villi physiologically similar to those of the 

 true allantoic placenta, appears to have been insufficiently 

 founded. 



Shortly after this, Caldwell,^ not having seen my paper, inde- 

 pendently described the foetal membranes of Phascolarctos and 

 Halmaturus from his fine series of embryos procured in 

 Australia. 



In these the allantois and yolk-sac were found to be exten- 

 sively applied to the subzonal membrane; but, unlike Didelphys, 

 the cavity of the yolk-sac was widely open and the non-vascular 

 portion of the subzonal membrane, overlying the hypoblast, 

 i.e., beyond the area of the mesoblast and sinus terminalis, was 

 composed of amoeboid cells which were attached to the uterine 

 epithelium. " This attachment is caused by the growth of the 

 cells of the subzonal membrane immediately outside the sinus 

 terminalis; the cells of the subzonal membrane begin to enlarge 

 and become amoeboid. They throw out pseudopodia-like 

 processes, which fit in between the cells of the uterine epithe- 

 lium, and serve to attach the blastodermic vesicle to the uterus ; 

 this attachment is entirely non-vascular, and is the sole means 

 by which the vesicle is attached to the uterus" (p. 657). No 

 villi were observed at any period, and " there is no vascular 

 connection developed in any stages of the development between 

 the embryo and the uterine walls." The embryo is nourished 



' Quarterly Journal of Microscopical Science, Vol. XXIV., October, 1884. "On 

 the Arrangement of Embryonic Membranes in Marsupial Animals." 



