POLYEMBRYONIC DEVELOPMENT IN TATUSIA 575: 
There are, however, two distinct types of cells. The bulk of the mass 
consists of rather large, ill-defined cells, the cytoplasmic and nuclear por- 
tions of which do not stain well. Scattered among these are smaller 
cells, which are found much more abundantly in that part of the mass 
lying towards the cavity of the vesicle. The smaller cells are character- 
ized by their sharply defined outlines and by the ease with which they 
take the stain. There is considerable evidence to indicate that the 
smaller cells are gradually undergoing a segregation from their larger 
fellows to form the hypoblastic layer of the vesicle. At any rate the 
hypoblast upon its completion possesses cellular elements that very 
closely simulate the smaller cells of the earlier stages.* 
This statement was made before I had had an opportunity of 
examining Hill’s (’10) excellent paper on the early development of 
the ‘native cat,’ Dasyurus viverrinus. In this paper Hill shows 
in the clearest possible way that the entoderm arises in a manner 
quite similar to that in the blastocyst of Tatusia, and I shall there- 
fore briefly state his general results on the point under discussion. 
It will be recalled that in the 16-cell stage the blastomeres are ar- 
ranged in two superimposed rings of eight cells each. The eight 
upper cells, which are smaller than the eight lower, are destined 
to produce the formative or embryonal region of the blastocyst 
wall, while the eight lower cells will give rise to the non-formative 
or extra-embryonal region. In accordance with the characteristic 
mode of development in the marsupial, no morula stage is formed 
in the egg of Dasyurus, but the blastomeres proceed directly to 
form the wall of the blastocyst. This is brought about through 
the division of the blastomeres of each ring and their gradual 
spreading toward the opposite poles, on contact with the inner 
surface of the sphere formed by the zona and the shell-membrane. 
The daughter blastomeres continue to divide and eventually pro- 
duce a complete cellular lining to the zona sphere, constituting 
the unilaminar wall of the blastocyst. The wall remains in this 
unilaminar condition until the blastocyst attains a diameter of 
4to 5mm. 
Hill here draws the most fundamental conclusions of his paper, 
pointing out that the formative or embryonal region, which from 
the first possesses no covering of trophoblast (i.e., Rauber’s layer), 
3 Loc. cit., pp. 369-370. 
