78 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
Figures 49, 50, 51, and 64 (Plates G and 8), until the cloud of granules 
is enclosed at the seventh cleavage in the smaller of the two entoderm 
cells (Fig. 65). This whole process shows clearly that other changes 
of astriking character are taking place at the same time as the division 
of the egg into smaller portions; evidently cleavage is not a mere 
separation of the egg into smaller masses, each similar to the other and 
to the original egg. The final destiny of this granular mass is not 
known, but such a peculiar and well characterized phenomenon as it 
exhibits cannot be considered meaningless. 
The differentiation in this instance is of the kind admitted by Driesch 
(see page 9), in that it is cytoplasmic in nature. It is not, however, 
a direct consequence of the original distribution of materials within the 
egg; the migrations of the granules show that processes are taking 
place in the cytoplasm that are only indirectly connected with cell 
division. 
We have in this case a distinctly visible differentiation accompanying 
cleavage. Certain other phenomena give evidence that there are like- 
wise invisible differentiations accompanying the process. 
At the division of the second “layer” of ectodermal cells in the 
sixth cleavage, shown in Figure 55 (Plate 7), the two rows of cells a’ — 
c5, gt oe, Tec”, and aus o are produced. The cells of these 
two rows, as shown in Figure 61, are of the same size and the same 
form, having similar relations to the surrounding cells and to the axis of 
the embryo. Yet, as has been repeatedly stated, all the cells of one row 
divide meridionally and equally with spindles in the long axes, the cells 
of the other row equatorially and unequally with spindles in the short 
axes. What causes this difference 
The difference must, of course, be due either to a different stimu- 
lus from the outside, or to a different structure of the cells. The 
problem may be expressed clearly in this way : If one of the cells of tho 
more dorsal row, as a"? (Fig. 61), could be removed and placed in 
the position now occupied by d, in the more ventral row, would it 
change its method of division? That is, would it cleave equatorially 
and unequally, with its spindle in the short axis, like the other cells of 
the ventral row, instead of meridionally and equally, with its spindle in 
the long axis, as it actually does? 
There is, of course, no way of answering this question directly. It 
scarcely appears probable, however, tbat there is such a difference in the 
influences affecting the two cells as to cause so fundamental a difference 
in the cleavage. And if there is not, the only alternative is, that there 
