THE MITOTIC CYCLE 



However, it is still possible for the growth of the interphase nucleus 

 to be rhythmic as Jacobj suggested, even if his own indirect evidence for 

 a periodic increase in size may be insufficient. Wermel and Portu- 

 GALOW^"^ studied this question directly by taking time-lapse films of 

 living cells in tissue cultures, and measured individual nuclei at intervals 

 over periods of several hours. Their published curves for nuclear volume 

 plotted against time show an irregular periodicity, with a rapid increase 

 in size every fifty minutes or so (Figure 23). Wermel and Portugalow 

 concluded that these periodic increases in volume were much smaller 

 than twofold. They assumed that these nuclei in their cultures were 

 spherical; this is certainly untrue for flattened cells in the outgrowth 

 of a tissue culture. It would be of value to repeat the work of these 

 authors although the significance of such rhythmic changes might still 

 remain obscure. 



g 







wo 



200 



300 min 



Ti, 



Figure 23 Change in volume of the nucleus of a chick cell in tissue 

 culture with time. The nucleus finally divided by amitosis. From 

 Wermel and Portugalow.'"^ Time in minutes; volume in arbitrary 



units. 



ScHREiBER^**® in a review of investigations in the field of 'caryometric 

 analysis' has discussed the possible events in the life of the nucleus 

 which could correspond with the increase in volume which Jacobj 

 refers to as 'innere Teilung'. This process could involve the duplication 

 of either the individual chromosomes or of chromatic material at a 

 lower order of size within the chromosomes. Where the increase in size 

 of a nucleus at each step is less than twofold, Schreiber suggests that 

 this may be due to the doubling of its maternal and paternal chromatin 

 on separate and possibly alternate occasions. A less unplausible sug- 

 gestion is that of Hertwig (G,)^'^' who claims that it is nuclear surface 

 rather than nuclear volume which doubles at each step; here the 

 increase in volume would be proportional to the square root of 2. 

 This is understandable if the interphase chromonemata are usually 

 distributed over the inner surface of the nuclear membrane as seems 

 true in some nuclei (p 30). 



If discontinuous changes in size of nuclei do occur, the question 



60 



