410 JAS. F. HILL. 



That the yolk-sac placenta is of high functional importance 

 in the nutrition of the embryo at this stage is borne out not 

 only by the fact that the vitelline vein is nearly three times as 

 large as the allantoic, but also by the further fact that the 

 larger proportion of the purified and food-laden blood coming 

 from the vascular area passes directly to the heart. So far as 

 oue can judge from structure alone, this veritable yolk-sac 

 placenta of Perameles appears to be more efficiently adapted 

 for respiratory and nutritive functions than the arrangement 

 found in other described Marsupials ; e. g. in Macropods the 

 ectoderm of the vascular omphalopleure is a comparatively 

 thick layer, the uterine epithelium persists, though in a some- 

 what modified form, and the maternal capillaries existing 

 below it are not very numerous, and nowhere directly project 

 on the free surface. 



(e) Bilaminar Omphalopleure. — Beyond the sinus 

 terminalis (fig. 6, s. t.) mesoderm is absent, the wall consisting 

 here solely of ectoderm and entoderm, for the most part in 

 close contact with each other. 



The entoderm is, on the whole, slightly thicker than that of 

 the vascular omphalopleure. It consists of a layer of cells of 

 varying size and shape, so tiiat its inner contour is somewhat 

 irregular (fig. 6, hil. omph., and fig. 12, etit.). In places the 

 entoderm cells present a vacuolated appearance. 



The ectoderm differs markedly from that of the vascular 

 omphalopleure. It is a very much thicker layer, the cells 

 are large, rich in protoplasm, and vary greatly in form and 

 size. Their outer ends project more or less freely in a quite 

 irregular manner, so that the free surface of the layer presents 

 a roughened irregular appearance (fig. 6, hil. 07nph., and 

 fig. 12, ect.). 



Like Seraon, I see no evidence in sections of the existence 

 at this stage of *^^ pseudopodia-like" processes of these ecto- 

 derm cells, such as Caldwell (12) describes as serving to attach 

 the blastodermic vesicle to the uterus. 



The significance of the persistence of this bilaminar portion 

 of the omphalopleure for a longer or shorter period in dif- 



