580 J. T. PATTERSON 
The distribution of the entodermal cells is interesting and in- 
structive. Except for a single cell (fig. 10 m) the entire upper half 
of the embryonic mass is free from them; and certainly this suggests 
that these cells are gradually passing towards the lower surface. 
Already nine of the remaining twelve cells have reached the lower 
surface. The position of the twelve lower cells indicates that they 
probably came from five different sources (five entodermal mother- 
cells) as follows: cell a is in the act of migrating to the surface; 
cells b-e have had a common origin; likewise cells 7~-k have proba- 
bly come from a single mother-cell; and finally, cell / gives evidence 
of once having occupied a position against the trophoblast between 
the two ectodermal cells situated farthest to the right in the sec- 
tion. It is now clearly in the act of migrating out from between 
these two cells. 
I have already stated that cell m is the only entodermal element © 
situated in the upper half of the mass. There is no way of deter- 
mining whther this cell will also migrate to the lower surface. A 
pseudopodial-like process from its lower border is pushed in 
between two ectodermal cells, and suggests at least that it is about 
to move down. Furthermore, in the later stages, when the ento- 
derm is completed as a distinct layer, no such cell as m is found 
within the ectoderm. 
The cell which lies just above e has caused me considerable 
difficulty in attempting to determine to which of the two catego- 
riesit belongs. Its relatively small, deeply staining nucleus closely 
resembles those of the entoderm, but its faintly tinted cytoplasm, 
together with its square-like outline, are sufficient to place it 
among the ectodermal cells. 
A word here as to the changes occuring in the size and thickness 
of the inner cell-mass may be said. These changes can be seen by 
comparing figures 6 to 11. At first the inner cell-mass is composed 
of a group of spherical cells, but, as development progresses, the 
mass becomes flattened out against the trophoblast, until finally 
it forms a circular plate of cells about two deep. This period is 
then followed by one in which there is a distinct increase in the 
thickness of the mass, due evidently to the multiplication and 
growth of the cells. Growth and multiplication of the cells con- 
sien 
