CELL SIZE AND NUCLEAR SIZE 79 
Acknowledgedly the position of the spindle conditions the plane 
of the cleavage, the division wall passing through the equator of 
the spindle. When by any means the spindle is displaced from 
its normal position the division plane is displaced. In this way 
giant polar bodies may be formed, as shown in fig. 11, or macro- 
meres may be formed which are small and free from yolk, as 
shown in figs. 16, 21—28, ef al. 
Are the inequalities and differentiations of normal cleavage 
due to similar causes, viz., external or internal pressure? Clearly 
external pressure cannot be involved in the unequal division 
of free cells, such as the maturation divisions of the egg; and 
the fact that isolated blastomeres of the 4-cell stage divide 
in the normal manner into small protoplasmic micromeres and 
large yolk-rich macromeres, shows that these unequal divisions 
during the cleavage period cannot be explained as the result of 
reciprocal pressure among cells. On the other hand, the forma- 
tion of micromeres of normal size and constitution from purely 
protoplasmic macromeres, as shown in figs. 24, 27, 28, 33, 36, 
et al., indicates that this inequality of division cannot be due to 
the crowding of the spindle to one side of the cell by internal 
pressure, such as might come from the presence of a mass of 
yolk—because in the cases cited, little or no yolk is present in 
the macromeres. If internal pressure is involved in the unequal 
division of these protoplasmic cells it must be pressure of a very 
different ‘sort from that involved in the presence of a mass of 
metabolic products at one side of the cell. While the spindle 
may be pressed out of position by external or internal pressure 
this will not serve to explain the eccentric position of the spindle 
in such cases as I have described. 
A satisfactory explanation of unequal and differential cell divi- 
sion must also be able to be applied to equal and non-differential 
cleavage, for the causes of the latter are not simple mechanical 
conditions, such as pressure. In the case of cleavages which 
are normally equal, if the spindle and yolk are moved to eccen- 
tric positions in the cell, they come back, if possible, to their 
normal positions when the pressure is removed; indeed they some- 
times seem to come back against considerable pressure, as when 
