CELL SIZE AND NUCLEAR SIZE 83 
always mark out the position of the spindle. These lobes are 
formed only when the egg is subjected to pressure and then only at 
those points on the cell surface which mark the position which 
will be taken by the poles of the spindle. Since the spindle axes 
change in successive cleavages it follows that this point of reduced 
tension also changes in successive cell generations. 
I conclude therefore that the position of the spindle, and all 
the morphogenetic results which follow from this, is dependent 
upon the polarity of the cell; which polarity manifests itself 
not only in the localization of cytoplasmic substances, but also, 
and more fundamentally, in definite movements of the odplasm 
and in reduced tension of the cell membrane at the poles of the 
cell. 
GENERAL SUMMARY AND INDEX 
Part I. Observations 
1. The equality or inequality of cell division in normal cleavage 
is due to internal causes, rather than to the presence of metabolic 
substances, such as yolk, within the cell or to pressure from with- 
out. These internal causes are to be found in the polarity of the 
cell, in movements of the cytoplasm, and in the structure of the 
cellmembrane. Since the position and axes of the spindles change 
regularly in successive divisions this protoplasmic organization 
must also change regularly (pp. 6-9). 
2. The yolk-lobe is a temporary extrusion of yolk or odplasm 
during mitotic pressure, at the former point of attachment to the 
ovarian wall and a little to one side of the vegetative pole. If 
this lobe is large, the resulting cleavage is unequal, although the 
furrow cuts through the chief axis and the center of the egg. The 
degree of inequality of the first and second cleavages is measured 
by the size of the yolk-lobe. The yolk-lobe is the result of an 
unsymmetrical distribution of yolk or egg substance with refer- 
ence to the egg axis (pp. 9-11). 
3. In Crepidula plana the Kernplasma-Relation varies greatly 
in different blastomeres and at different stages, depending chiefly 
upon the length of the resting period (interkinesis). In cases 
