48 CARL G. HARTMAN 
The later cleavage is represented in the collection by eggs 
with every number: of: blastomeres from the 4-celled stage in 
which two blastomeres only are in mitosis (figs. 5 and 6, pl. 15) 
to the fully formed blastocysts of about 32 to 36 cells. Cleavage 
proceeds very irregularly after the 4-celled stage, which explains 
the fact that the 8-celled and the 16-celled eggs are only slightly 
in the plurality (compare litter No. 85). There is a retardation 
in division of cells at one pole of the egg, presumably among the 
lineal descendants of one of the first two blastomeres. In 
models of 10- and 12-celled eggs the larger cells are grouped at 
one pole, but, aside from this fact, there is nothing that would 
point to a polar differentiation, and in the 16-celled stage even 
this criterion is lost. 
After the 4-celled stage is passed, the ovum of the opossum 
behaves no longer as a typical Eutherian, but as a marsupial 
ovum. In the former the blastomeres of the successive divisions 
cling together to form a solid mass or ‘morula,’ which is soon 
overgrown by a layer of cells, Rauber’s layer or the trophoblast. 
The mass within is the ‘inner cell mass’ which gives rise to the 
embryo and its envelopes. The blastocyst is formed by the 
appearance of a cavity between the trophoblast and the inner 
cell mass at the lower pole of the egg. | 
In the marsupials the morula stage is absent. Already in 
the 2- and 4-celled opossum eggs the space between the blasto- 
meres represents, potentially, the blastocyst cavity, for at the 
16-celled stage, or even earlier, the blastocyst cavity is clearly 
indicated. As early as the 6-celled stage the blastomeres mani- 
fest a tendency to migrate to the zona pellucida and to apply 
themselves to the wall of the ovum (fig. 15, pl. 15). In 12- 
and 15-celled eggs the blastomeres are usually well flattened out 
at the periphery, as seen in figure 8, plate 3; figure 9, plate 13, 
and figure 16, plate 15. At the 16-celled stage it is exceptional 
to find rounded cells, and models of such eggs show the outer 
surface of the blastomeres molded against the curvature of the 
surrounding albumen (compare figs. 17 and 18, pl. 15). 
It thus happens that the eliminated yolk comes to lie within 
the cavity of the blastocyst, for the blastomeres migrate to their 
