113 A. A. W. HUBRECHT. 
most distinctly brought out in such mammals as Pteropus, 
Cavia, Tupaja, and others. Take, for instance, Selenka’s figure 
of the guinea-pig’s blastocyst,! Gohring’s of that of Pteropus.? 
In these latter figures, we see the epiblastic knob which is 
enclosed between the trophoblast and the hypoblast of the 
didermic blastocyst, hollowing out into a cell-mass, of which 
the upper surface thins out and becomes the epiblastic clothing 
of the amnion cavity, whereas the lower surface thickens and 
becomes the epiblast of the blastodermic surface out of which 
the embryo will be modelled. 
I have no doubt that in the cases of Erinaceus and Sorex a 
similar sharp line of demarcation may be drawn between the 
epiblast that will develop into the lining of the amnion and 
between the trophoblast, although here this distinction is not 
so self-evident as in the preceding cases. And I suspect that 
even such cases as that of the rabbit will some day admit of a 
sharper delimitation of these two. 
But even where such a sharp delimitation is not as yet 
always possible in the later stages of the blastocyst, the earlier 
stages are all the more evident. 
The Ornithodelphia are not as yet affected by the causes 
which determine the differentiation of a special trophoblast in 
the higher placental Mammalia. In the Didelphia we may 
hope to find certain transitory stages. Thus the early stages 
of Phascolarctos, the ovum of which has been described by 
Caldwell, may be expected to be especially instructive. 
Already in the opossum Selenka has described the very 
peculiar proliferation in the outer layer of the early blasto- 
cyst (l.c., Heft 4, pl. 20, figs. 2,5, and ), which is no doubt 
precursory to the ever so much more important proliferations 
of the trophoblast which occur in most of the higher orders of 
mammals. 
In this paper it has already been noticed how both in 
Tupaja and in Tarsius, portions of the trophoblast undergo 
1 «Studien zur Entwickelungsgesch. der Thiere,’ Heft 3, pl. 12, figs. 13— 
15, 73. 
2 Tbid., Heft 5, pl. 41, figs. A—C, 1; 2, 4, and 6. 
