68 CARL G. HARTMAN 
that the entoderm is present in a watch-crystal-shaped mass at 
one pole of the egg in vesicles of 0.30 to 0.85 mm. The mass is 
thicker in the middle, being even three to four cells deep. Only 
the outer superficial layer is ectodermal, the massed cells 
beneath being all entodermal. 
The opossum blastocyst differs, then, both from the corre- 
sponding stage of Dasyurus, on the one hand, and of the higher 
mammals, on the other. In Dasyurus the entodermal cells 
flatten out and spread singly as they are formed and never pile 
up in a mass as in the opossum. ‘There would seem to be in the 
opossum a hearer approach to the Eutherian ovum in its pos- 
session of a. kind of ‘inner cell mass’ (fig. 15, pl. 17; fig. 1, pl. 18). 
But there are fundamental differences. For in the Eutheria 
the entoderm seems to arise only from the cells on the inner 
surface of the inner cell mass, presumably from a single layer. 
The outer or superficial layer of the blastocyst is Rauber’s 
layer; between the two is the embryonic ectoderm, a layer of 
cells variable in thickness and at first irregularly dispersed. 
Thus, if figure 1, plate 18, represented an Eutherian egg, the 
superficial layer would constitute Rauber’s layer; beneath would 
be the irregularly disposed ectoderm (cells marked ‘EHNT*”’), 
and only the innermost layer would be the entoderm. In the 
opossum there are only two layers: 1) embryonic ectoderm, a 
superficial layer, one cell deep, and 2) all the remainder which 
is entodermal. If there seem to be more than two layers, as in 
the figure just referred to, the outer layer is the ectoderm, the 
inner the differentiated entoderm, and between the two a mass 
of cells which. are still undifferentiated or primitive entoderm 
which are yet to spread and form entoderm. The opossum is, 
therefore, fundamentally like Dasyurus; but the resemb!ance is 
obscured by the temporary massing of entodermal cells, the re- 
sulting picture superficially resembling an Eutherian vesicle with 
spreading inner cell mass. 
l. The end of entoderm formation and the spreading of the entoderm 
In my previous publication I described some vesicles from 0.3 
to 0.5 mm. in diameter, of the type shown in figures 1 to 4, 
