CELL SIZE AND NUCLEAR SIZE 29 
IV. The inciting causes of cell division 
The relative sizes of cells and of nuclei are dependent, in part, 
upon the rate of cell division. Cells which divide infrequently 
are larger, other things being equal, than those which divide often. 
The turret cells (1a?-1d?) of Crepidula are the smallest cells in 
the entire embryo at the time of their formation (figs. 3, 4); 
however they divide but twice during the whole of the cleavage 
period, and consequently they grow to be very large; whereas each 
of the apical cells from which they were derived, gives rise during 
the cleavage period to twelve cells the combined volume of which 
is not much greater than that of one full-grown turret cell. 
Evidently the factors which bring on or delay cell-division have 
much to do, indirectly, with the sizes of cells and nuclei. 
Strasburger (’93) supposed that cell division occurred when the 
ratio of the cell body to the nucleus increased beyond a certain 
point, which might be regarded as marking the limit of the ‘work- 
ing sphere of the nucleus;’ with the division of the cell the normal 
ratio was once more restored. 
Boveri (’04) sought to find the inciting cause of cell division in 
the chromosomes. He believed that the chromosomes divide 
when they have reached a size double that which they had at the 
close of the preceding division. At the same time he showed 
that the rhythm of the division of the centrosomes may be inde- 
pendent of that of the chromosomes and that division of the cell 
depends upon the centrosomal rhythm rather than upon the chro- 
mosomal rhythm. e 
That there is a rhythm of division for chromosomes and centro- 
somes seems to be well established by Boveri’s work, but this 
rhythm in the case of the chromosomes is not determined by the 
time when they have grown to double their size at the close of the 
preceding division. Marcus (’06) and Erdmann (’08) have shown 
that the chromosome size throughout the cleavage of Strongy- 
locentrotus is a constantly decreasing one. Baltzer (08) admits 
that the chromosomes do not double in size at each cycle of divi- 
sion; he does not find any great diminution in chromosome size 
up to the 16-cell stage, though the chromosomes in the blastula 
