58 CARL G. HARTMAN 
non-embryonic areas, for there is no longer any doubt as to 
their identity: the embryonic area is marked by the position of 
the entoderm mother cells and the polarity of the ovum is 
definitely reestablished. 
A study of the young blastocysts just considered, as well as 
immediately succeeding stages, seems to show, moreover, that 
the first proliferation of entoderm takes place more actively on 
the margin of the future embryonic area, for one often finds 
them most numerous on opposite sides, as shown, for example, 
in figures 11, 18, and 22, plate 16, and figures 8, 10, and 11, 
plate 17. 
The blastocyst now contains two types of cells: 1) those: 
clearly entodermal in destiny, as just described, and 2) the 
peripheral or enveloping layer, which, of course, gives rise to 
all of the ectoderm, embryonic and trophoblastic. All of the 
cells at the lower pole are ectodermal, being trophoblastic. But 
the epithelial cells at the embryonic pole, since they will for 
some time still continue to: proliferate entoderm mother cells, 
are potentially both ectodermal and entodermal and should 
better be called entectoderm until the entoderm is fully formed. 
They are, however, also potentially mesoderm, as my next paper 
will clearly show. 
The ovum of the opossum has not grown much in volume since 
its discharge from the ovary, being still less than 0.15 mm. in 
diameter (pls. 12 and 13), which is in striking contrast with the 
blastocyst of Dasyurus, where, at the appearance of the first 
entoderm mother cells, the vesicle is nearly 4 mm. in diameter. 
In the opossum the period of growth follows the formation of 
the entoderm, but in Dasyurus this is preceded by a long period 
of growth. The process of entoderm formation in Dasyurus 
may, therefore, be studied from surface mounts of pieces easily 
cut from the blastocyst wall, as well as from serial sections; but» 
the opossum egg at this stage may be studied in section only, 
for it is small, covered with a thick layer of albumen, and is 
densely packed with more or less opaque yolk. The vesicular 
structure can well be made out from in toto preparations, but 
for detailed study such preparations are w6rthless. 
