66 CARL G. HARTMAN 
volume of shell membrane (text fig. 2; fig. 18, pl. 18; fig. 5, 
ple): . 
The development of the trophoblastic ectoderm is seen to 
have continued in the direction indicated above, so that in this 
litter of eggs it now occupies about three-fourths of the circum- 
ference of the blastocyst. Little more need be said of this 
layer. It becomes progressively more attenuated until it may 
have the appearance of endothelium, and even at high mag- ~* 
nifications appear as a sharp narrow line with here and there a 
swelling which marks the location of a nucleus (fig. 1, pl. 18). 
The region may come to occupy from four-fifths to five-sixths 
of the entire circumference of the blastocyst; and this again 
constitutes a point of contrast with the egg of Dasyurus, in 
which the formative area occupies, in section, from one-third 
to one-half of the blastocyst wall. 
In general, the marsupial trophoblast does not differ markedly 
in structure from that of Eutherian vesicles, but more interesting 
and important changes take place in the embryonic area of the 
opossum blastocyst. For a short period, which includes the 
stage represented by litter No. 194’ (figs. 13 to 15, pl. 17), these 
changes now appear to be chiefly of two kinds: 1) further 
proliferation of entoderm mother cells from the peripheral 
layer, and 2) multiplication of all types of cells. 
The former process gives every evidence of having slowed 
down considerably since the preceding stage, the cells which 
can be identified as migrating inward from the superficial layer 
being of comparatively rare occurrence. Such cells are shown 
at HNT”, figs. 14 and 15; they stand with their long axes at 
right angles to the surface of the area and project inward among 
the primitive entodermal cells now everywhere closely applied 
to the ectoderm. It is clear that entodermal proliferation from 
the superficial entectoderm is approaching the end. 
As a result of the cell multiplication, the embryonic area has 
become crowded, so that in places it is three and occasionally 
four cells deep; and it may be stated parenthetically that this is 
the only stage before the formation of the mesoderm that the 
blastocyst wall is anywhere more than two cells deep, as the 
