578 J. T. PATTERSON 
stituting the embryonic spot (fig. 9). There are six entodermal 
and thirteen ectodermal cells, or rather ectodermal nuclei, for in 
some cases two or more of these nuclei are included within a single 
cytoplasmic mass. No entodermal cells have been found which 
show more than one nucleus. 
The entodermal cells in the section are located in three regions, 
similarly to those of the preceding figure. On the extreme left 
the single cell (fig. 9, a) is easily distinguishable from its adjacent, 
binucleated, ectodermal fellow. It gives evidence of beginning to 
spread out beneath the binucleated cell. To the right of this is a 
group of three entodermal cells (6, c, and d), one of which comes to 
the lower surface, while the other two lie one above the other, well 
within the mass. These cells are darker than the neighboring ec- 
todermal cells, but the difference in the size of the nuclei that was 
so striking a feature of the preceding stage is here not so marked. 
In fact, the nuclei of some of the ectodermal cells are smaller than 
those of the entodermal ones. It must be kept in mind, however, 
that in these thin sections only the tip or at least a small portion 
of a large nucleus may be visible in a given section. A study of the 
preparation shows that on the average the entodermal nuclei are 
smaller than the nuclei of the ectodermal cells. The remaining 
entodermal cells constitute a pair situated about in the middle of 
the right half of the section (fig. 9,e and f). Both of these cells 
are on the surface of the mass. Their cytoplasmic and nuclear 
portions do not stain so deeply as in the other entodermal cells, 
but still more deeply than in the case of the ectodermal cells. 
The affinity of the entodermal cells for the stains evidently lies 
in the nature of the protoplasm itself. In the ectodermal cells, 
both in the cytoplasm and nucleus, the structural configuration of 
the protoplasm is of a more open mesh-like character than that of 
the entodermal cells, in which it may assume a finely granular 
appearance. 
The conditions observed in this and the other sections of the 
series is interpreted to mean that the more deeply staining cells are 
entodermal, all of which will eventually migrate to the lower sur- 
face of the embryonic mass to form the characteristic entodermal 
layer of the mammalian blastocyst. The cells marked c, e, and f 
