CYTOLOGICAL STUDIES OF CENTRIFUGED EGGS 617 
the polar spindle moves to the pole of the egg irrespective of the 
materials that have been driven into that region. The spindle 
will push through the dense mass of yolk if that lies in its path, or 
push the oil cap to one side should it lie in the polar regions. The 
placing of the segmentation spindle shows again that the spindle 
orients with respect to the poles, however much the nuclei are 
driven out of position. 
These facts lead irresistably to the view that the position as- 
sumed by the spindle is not hap-hazard, but in direct response to 
conditions existing in the egg, that are independent of its visible 
materials. This goes far towards showing that the egg is some- 
thing more than a mixture of fluids, and that a system exists in 
the egg of such a kind that it has a guiding influence on the forma- 
tion and movement of the karyokinetic process. That this sys- 
tem is not rigid, but flexible, is also shown by many experimental 
results. We shall have to include, I believe, in the phj^siology 
of the egg the recognition of directive factors of a physical nature. 
The physiology of form-changes will have to reckon not simply 
with physiological changes in enzymes or stuffs, but with phys- 
ical factors as well. 
The hearing of the results on the phenomenon of cleavage. The 
most important fact brought out by the study of Cumingia is in 
regard to the nature of the cleavage process. It has been shown 
that in the first division, which is very unequal, the smaller cell 
may contain all of the oil, or all of the yolk, or pigment without 
thereby changing the size of the cell or its relation to the pole. 
Sections show that the small cell may be almost entirely filled by 
the yolk or by the oil, plate 1, figs. G, H, I and plate 6, figs. 29, 30. 
This means that the greater part of the cytoplasm in these regions 
has been almost totally displaced, yet the normal size ratio of the 
cells is retained. How can we explain such a result on the ground 
that the form of the cleavage pattern is determined by regional 
differences in the egg. It is true that the yolk and the oil do not 
totally displace the cytoplasm, but are permeated by it. But the 
quantity must be often very small compared to that of the intro- 
duced materials, and if the different regions are characterized by 
differences in the cytoplasm the proportion of the cells would have 
