3} EDWIN G. CONKLIN 
which are at first so small and have so long a resting period, be- 
come much larger than adjoining cells before they divide. R. 
Lillie (10) maintains that ‘‘the primary change in the initiation 
of cell division and development is an increase in the permeability 
of the plasma membrane.” It is well known that the general 
surface tension of the cell increases during mitosis, and I have 
found that the tension of the cell membrane is locally reduced at 
the two poles of the cell before and during division (see Conklin, 
02, p. 94; also this paper, p. 82). It is quite possible that this 
polar reduction in surface tension before mitosis begins may have 
something to do with initiating division. 
On the whole it seems probable that the time of cell division is 
dependent upon the coincidence of several more or less independ- 
ent factors. Boveri has shown that the division phases in nu- 
cleus and centrosome may be more or less independent of each 
other, though complete cell division depends upon the coincidence 
of the two. To these factors may, perhaps, be added the quantity 
of protoplasm, and thus indirectly the ‘Kernplasma-Relation’ 
and perhaps also increased permeability of the cell membrane, 
and a local reduction of surface tension at the poles of the cell. 
Gurwitsch- (’08) maintains that. the blastomeres are ready for 
division at all times, and that only “Kernplasma-Koinzidenz’ 
or ‘Zustands-Koinzidenz,’ is necessary to start division. He sug- 
gests that a coincidence of polarity of nucleus and plasma may be 
necessary, and he concludes from the apparently accidental 
occurrence of divisions in different parts of an egg or embryo, that 
several independently variable factors may be concerned, the 
coincidence of which is necessary to bring on cell division. The 
latter part of this conclusion seems to me to be justified by the 
facts which I have presented. 
V. Growth of protoplasm during cleavage 
It is well known that the egg as a whole does not increase in vol- 
ume until after the cleavage period. Indeed Godlewski (’08) finds 
that there is in Echinus and in Strongylocentrotus, no change in 
the quantity of plasma at the 64-cell stage, as compared with the 
