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. I A 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. 



