CELL SIZE AND NUCLEAR SIZE 87 
enters the egg near the vegetal pole, lies in a cytoplasmic field, 
the egg nucleus in a yolk field, and the former grows more rapidly 
than the latter, thus reversing the usual size relations of the germ 
nuclei. The relative size of the germ nuclei is dependent upon 
the volume of the cytoplasm in which they lie as well as upon the 
length of time that the sperm nucleus has been in the egg (pp. 
Ove, 0): 
21. If centrifuging occurs during the cleavage almost all the 
yolk present may go into one daughter cell, almost all the cyto- 
plasm into the other (figs. 16, 19). Under these circumstances 
the subsequent growth of the daughter nuclei is proportional to 
the volume of the cytoplasm of the middle zone in which they 
lie. Neither the yolk nor the substances of the lighter zone con- 
tribute directly to the growth of the nucleus (pp. 75-76). 
22. The size of spindle, centrosome, and sphere in any cell is 
not definitely fixed, but may be modified by altering the quantity 
of cytoplasm; the larger the quantity of cytoplasm in a cell, the 
larger are all the structures named (p. 77). 
23. The rhythm of division may be modified, but only to a 
slight extent, by altering the quantity of cytoplasm in a cell. 
In general, cells rich in cytoplasm divide a little earlier than those 
poor in this substance; but though the quantity of cytoplasm in 
a cell and the size of its nucleus may be greatly changed by cen- 
trifuging, the rhythm of cleavage is but slightly changed (p. 77). 
24. When the daughter chromosomes at one pole of a spindle 
are left in a cell composed almost entirely of yolk, they do not 
form a vesicular nucleus until yolk has been dissolved and a 
certain amount of cytoplasm has been formed around the chromo- 
somes. It is evident that something, perhaps an enzyme, is given 
off from the chromosomes or chromatin, which leads to the trans- 
formation of yolk into cytoplasm; this cytoplasm is in turn taken 
up by the chromosomes and ultimately contributes to the growth 
of the chromatin, (pp. 77-78). 
25. The typical size, position and constitution of blastomeres, 
and consequently the type of cleavage, do not depend upon exter- 
nal or internal pressure, but upon a definite polarity, symmetry 
and movement of the cell contents, and upon reduced surface 
