CELL SIZE AND NUCLEAR SIZE 9 
In the case of normal eggs it cannot be demonstrated that the 
inequality may not be due to mutual pressure among the cells, 
but in certain experiments which will be described in another 
paper, this factor may be entirely eliminated, isolated blastomeres 
showing the same inequalities of division as do those in the cell 
complex. In all of these cases of definite types of cleavage, the 
position of the spindle, and consequently the direction of division 
and the relative size of the daughter cells, is determined by some 
structural peculiarity of the protoplasm and not by the presence 
of metabolic substances within the cell or by pressurefrom without. 
3. Significance of the yolk lobe. Under normal conditions 
the line of intersection of the first and second cleavage planes 
marks the chief axis of the egg, its two ends being the animal and 
vegetal poles. In eggs in which the cleavages are unequal, the 
chief axis, thus defined, runs from the animal pole, which is marked 
by the position of the polar bodies, to a point more or less removed 
from the diametrically opposite pole. 
Is this chief axis predetermined in the egg or is it established 
by the positions of the first and second cleavage planes? Observ- 
ation alone affords no positive answer to this question, but the 
fact that the spindle takes a definite and characteristic position 
in the egg indicates that something outside the spindle determines 
its position, and points to the conclusion that the chief axis is 
already present in the egg, as a structural differentiation before 
cleavage begins. This conclusion is well supported by experi- 
ment, as will be shown later. 
In this connection the significance of the so-called ‘yolk lobe’ 
is interesting. As is well known this lobe is found in many eggs, 
especially in those in which the first and second cleavages are 
unequal. It is present however in minute form in such eggs as 
those of Crepidula and Fulgur in which the first two cleavages 
are approximately equal, but in cases in which these cleavages 
are unequal it is much larger, and in general the size of the yolk 
lobe is proportional to the inequality of division. In all cases 
so far as I am aware the yolk lobe lies diametrically opposite the 
animal pole, and if detached from the egg at the time when it is 
fully formed, the egg divides into equal blastomeres, as Wilson 
