574. J. T. PATTERSON 
lying about one-third the distance from the right end of the sec- 
tion (d and e). In the first pair one of the cells borders on the 
under surface of the mass, while the other is wedged in between 
two large cells. In the other pair one of the components also 
comes to the under surface and the other is in contact with the 
trophoblastic cells. 
I interpret the condition here to mean that the nuclei of the 
small cells are identical with the smaller nuclei of the preceding 
stages, or else the progeny of such nuclei. 
The size relation of the different cells is not the only evidence 
which indicates that the embryonic mass is gradually differentiat- 
ing into two types of cells; for the small cells have begun to take on 
a slightly deeper tint of stain than do their larger fellows, and this 
difference in the staining capacity of the two types of cells becomes 
more and more evident as development progresses, until finally 
it becomes one of the most striking features of the armadillo 
blastocyst. 
2. The didermic stage 
The changes which we have just observed forshadow the trans- 
formation of the monodermic blastocyst into one of the didermic 
type; that is to say, the differentiation of the inner cell-mass into 
its two primary components, the embryonic ectoderm and ento- 
derm. We have seen that the inner cell-mass gradually differen- 
tiates into two rather distinct types of cells, which differ from each 
other both in size and in staining properties. The bulk of the 
mass is composed of large, faintly staining cells which are more 
numerous in that part of the mass which is situated towards the 
trophoblast. The other type of cell is much smaller than the pre- 
ceding, takes the stain much more readily, and has a sharply de- 
fined outline. These smaller cells are at first evenly distributed 
among the larger (except in a few cases to be noted later), but 
later become collected toward that side of the inner cell-mass 
which borders on the cavity of the vesicle. Subsequently they 
become split off from the lower side of the mass to form the ento- 
derm. In speaking of these cells in my preliminary paper I make 
the following statement. 
