EARLY ONTOGENETIC PHENOMENA IN MAMMALS. 99 
1. Didelphia Nonplacentalia. 
An important case is that of the opossum, in which we 
notice in Selenka’s figure here copied (Fig. 134) how a con- 
siderable proliferation occurs in the trophoblast at a yet very 
early age, and how in this proliferation cavities or sinuses 
appear in which the surrounding nutritive fluids contained 
in the uterus lumen and partly enclosing the ege as a sort of 
albumen layer can penetrate. There is no doubt that this 
nutritive matter, when once it is surrounded by trophoblast 
cells, many of which undergo special proliferation, can be 
absorbed in an accelerated and intensified fashion, and is then 
utilised for the benefit of the developing embryo, most 
probably by its passing in some form or other inside the 
cavity of the umbilical vesicle. 
We have not to go higher than these same Didelphia to 
find that also properties of adhesiveness are characteristic for 
the trophoblast cells. In the genus Perameles it has been 
made known by J. P. Hill (’97) that far from being aplacental 
—as was the current opinion concerning all Didelphia—the 
blastocyst of this genus possesses a very well developed 
adhesive surface, by which it fuses with the maternal uterine 
mucosa, and against which after a time allantoidean blood- 
vessels become applied, thus forming a full-fledged allan- 
toidean placentation. 
The trophoblast at these points of adhesion between blasto- 
cyst and maternal epithelium undergoes marked changes, as 
can be concluded from Hill’s Figs. 1388 and 159. About 
the nature of the fusion between maternal epithelium and 
trophoblast we will have something to say further on (p. 
115)!; here it may suffice to state that the changes are only 
brought about in those trophoblast cells which partake in the 
placentation process, not in those which are present over the 
1 T may here add that I differ from Hill in the interpretation of the later 
stages of the Perameles placenta, and that I am inclined to ascribe a much 
more considerable part to the proliferating trophoblast than he does. 
