580 J. T. PATTERSON 



The distribution of the entodermal cells is interesting and in- 

 structive. Except for a single cell (fig. 10m) 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 i-k have proba- 

 bly come from a single mother-cell; and finally, cell I 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- 

 ries it 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 1 1 . 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- 



