1 86 THE POPULAR SCIENCE MONTHLY 



cells which do not continue to divide throughout life, the size of cells 

 varies directly with the body size and with the infrequency of division. 



III. Cell Size and Nucleae Size 



In a series of recent papers Eichard Hertwig^^- ^^ and several of his 

 pupils have maintained that there is a definite ratio between the size of 

 the nucleus and the size of the cell ; this is the " Kernplasmarelation," 

 or the nucleus-plasma ratio. When this ratio is altered by the greater 

 growth of the nucleus, Hertwig thinks that it leads to a "tension," 

 which brings about division, and thus the normal nucleus-plasma ratio 

 is restored. This ratio is supposed to be a constant one under normal 

 conditions, and if at any time it is altered it is capable of self regulation. 



On the other hand, I^* have found that this ratio varies greatly in 

 different cells of an animal, and indeed within the same cell at different 

 stages of the division cycle, that it may be experimentally altered, and 

 that it is a result, rather than a cause, of the frequency of cell division. 



Within the same cell the size of the nucleus varies greatly at differ- 

 ent stages of the division cycle, while the volume of the cell as a whole 

 remains relatively constant. The nucleus is smallest during the 

 anaphase, or later stages of division, when it consists of a compact plate 

 of condensed chromosomes ; it is largest immediately before the nuclear 

 membrane dissolves at the prophase of the next division. In the 

 cleavage of the egg of Crepidula plana the nucleus-plasma ratio in 

 identically the same cell varies from approximately 1 : 6 whto the nucleus 

 is largest, to 1 : 286 when it is smallest ; that is, the volume of the nucleus 

 increases nearly 50 times .during the resting period between the previous 

 anaphase and the subsequent prophase ; during this same time the 

 volume of the cell remains practically unchanged. 



Even when measured at the same phase of the division cycle the 

 nucleus-plasma ratio differs greatly in different cleavage cells; at maxi- 

 mum nuclear size the volume of the nucleus of certain cells of Crepidula 

 {4:A^C) may be 3 times that of the protoplasm, whereas in other cells 

 (lA-lD) the volume of the protoplasm may be 14.5 times that of the 

 nucleus. At minimum nuclear size the nucleus-plasma ratio may vary 

 from 1 : 29 in the cells la--ld-, to 1 : 285.5 in the cells lA-lD. 



The growth of the nucleus between successive divisions is due to 

 the absorption from the cell body of a particular kind of cell substance, 

 which constitutes the achromatin of the nucleus; at the beginning of 

 this growth the nucleus is composed of compact chromosomes, at its 



** Hertwig, E., * ' Ueber Korrelation von Zell- und Kerngrosse, etc., ' ' Biol. 

 Centralb., 22, 1903. 



"Hertwig, E., "Ueber neue Probleme der Zellenlehre, " Archiv Zellfors., 

 1908. 



"Conklin, "Cell Size and Nuclear Size," Jour. Exp. Zool., 12, 1912. 



