liK)?.] XA'rrUAL SCIKXCES OF IMlILADMLrillA. 335 



The diameter of the misejiineuted ejig is about l.TOO/i; that of each 

 of the macromeres during the formation of the quartets about 970 }x\ 

 while in the stages after the formation of the fourth quartet it is some- 

 wliat less, say aljout 900 [.l Therefore, although the diameter of the 

 cell body in these later stages is but little more than half that of the 

 unscgmented egg, the nucleai- diameter is twice as great as at any 

 pro\'ious stage. 



The ratio of the nucleai' diameter to the cell diameter, the "Kern- 

 plasmarelation" (k/p) of Hertwig (1903), is about 1 : 40 in the unseg- 

 mented egg; about 1 : 24 in the 4-cell to 24-cell stages; about 1 : 20 

 just before the formation of the Secondary macromeres; and about 

 1 : 12 at the close of the long resting period of the macromeres. shown 

 in fig. 24. 



In the micromeres of the 8-24-cell stages the ratio of the nuclear 

 diameter to the cell diameter is about 1:5; and in the later stages, 

 such as fig. IS, it rises to 1 : 3. 



It seems perfectly evident from these figures that the a])solute size 

 of the nucleus is tlependent not only upon the size of the cell, but also 

 \\\)im the length of the resting period; and by the same showing the 

 length of the resting period is not determined primarily by the size of 

 ti\c nucleus. In all the earlier divisions of the macromeres of Fulgur 

 the division comes on when the nucleus is relatively small (k/p = 1 : 

 24) ; in later stages it does not come on until the nucleus has grown to 

 twice this size (k/p =^ 1 : 12). 



In Crepidula I found (1902) that the si2;e of the nucleus, chromo- 

 somes, centrosomes, spindles and asters was, in the last analysis, 

 proportional to the volume of the cytoplasm, and I tried to show that 

 the inmiediate cause of division was the growth of the nucleus to a 

 ])oint where the ratio of nucleus to cytoplasm exceeded a critical limit. 

 This is certainly not the principal factor w^hich brings on division in 

 Fuhiur: and the fact that mitosis may occur in one cleavage when the 

 nucleus is relatively small and in another not imtil it becomes very 

 large, suggests that the moment of division may be dependent upon 

 some intrinsic condition in the nucleus or centrosome, rather than 

 upon the ratio of nuclear size to cell size. Support is lent to this view- 

 by the phenomena of oogenesis ; here we have in the germinal vesicle 

 the largest nucleus in the entire life cycle, following upon one of the 

 longest i-esting periods, while the second matui-ation division follows 

 immediately after the first; here also the moment of di\-ision seems to 

 depend upon intrinsic conditions in the cell. 



