64 CARL G. HARTMAN 
section nearer the middle of the series, shows entodermal cells 
only at the margin. 
At the stage just described there is still a considerable quantity 
of yolk and coagulum in the egg. This is usually collected near 
the inner surface of the embryonic area as well as among or 
within the cells of the area, more rarely also in the trophoblastic 
cells. Occasionally the yolk is collected in a large spherule, 
as in egg No. 356 (11); this spherule measures 0.04 mm. in 
diameter and a portion of it, cut tangentially, is shown at Y, 
figure 4, plate 17. 
1. The embryonic area superficial in position 
The question may arise whether there appears at any time 
over the embryonic area a transitory layer that may at all be 
compared with Rauber’s layer in the Eutherian egg. Professor 
Hill has homologized the embryonic area of the Eutherian egg 
with the inner cell mass and the non-embryonic area with 
Rauber’s layer, and hence he uses the term ‘trophoblastic’ to 
designate the latter. According to this view, the embryonic 
cells lie upon the surface of the ovum from the beginning and are 
potentially ectoderm, entoderm, and mesoderm; in other words, 
the embryonic cells are never covered with trophoblastic cells. 
I believe this to be the true interpretation of the facts. Since 
my collection includes an unbroken series of critical stages on 
this point, if there were such a layer, it could not escape de- 
tection. Nowhere is there the slightest suggestion of a transitory 
layer of cells. Mitoses are always present in the superficial 
layer (pl. 17), disintegrating cells never. The very method of 
blastocyst formation, as described in these pages, precludes the 
probability of a trophoblastic cover over the embryonic area, 
which is differentiated very soon after the establishment of the 
blastocyst. For, if the upper half of the unilaminar opossum 
egg is not embryonic, it is trophoblastic and there can be no 
embryonic area; in which case we should be forced to derive the 
embryo from the trophoblast, a manifest absurdity. The pre- 
ceding and the succeeding stages all show that in the blasto- 
