94 CARL G. HARTMAN 
cubical to columnar and more densely granular, whereas the 
trophoblastic ectoderm remains flat and its cytoplasm reticular 
(pl. 21). 
43. The entoderm in the early stages is everywhere the same, 
consisting of the typical squamous cells with swellings at the 
nuclei. In surface view the flatter entodermal nuclei, as a rule, 
appear larger than the ectodermal. The entoderm nowhere 
comes to the surface of the blastocyst (pls. 10 and 19 to 22). 
44. In blastocysts over 1 mm. in diameter the entoderm is 
often modified at one side of the embryonic area. These cells 
increase in number, thickness of nuclei, and density of the 
cytoplasm, and I believe them to mark the future posterior, not 
the future anterior margin of the embryonic area. The primitive 
streak will be laid down here (ENT, fig. 9A, pl. 22). 
45. These eggs also exhibit a clear field a little to one side of 
the center in the embryonic area (figs. 1 and 2, pl. 10; fig. 9, 
pl. 22). This is due to a thinning out of the embryonic ectoderm. 
Where the light field comes nearest to the margin is the posterior 
margin of the area, for here the modified entoderm is also found 
(pl. 22). 
46. Yolk spherules occur in the bilaminar stage even in the 
largest specimens. They are remnants of the extrusions of 
cleavage stages. They are found often within the cells, usually 
of the embryonic area only, both ectodermal and entodermal, 
and the nuclei frequently surround the masses as if to engulf 
them (figs. 9B, 10 to 12, pl. 22). 
47. The egg of the opossum is like that of Dasyurus in its 
possession of a large amount of yolk, in the absence of the morula 
stage and in the formation of entoderm from entoderm mother 
cells coming from the wall of the unilaminar blastocyst. But it 
differs in many regards: in the absence of polarity, in the 
uniform distribution of the yolk, and in the consequent manner 
of deutoplasmolysis; in the indeterminate type of cleavage; in 
the crossed arrangement of the 4-celled egg; in the early period 
in which the blastocyst is formed; in the very early formation 
of the entoderm; in the simulation of an inner cell mass due to 
the crowding of the primitive entoderm cells,—in these respects 
