THE MITOTIC CYCLE 



the same speed. In the mouse, the data of Fell and Hughes^^ do not 

 suggest any differences between nuclei of different sizes in this respect. 



An extreme instance of the prolongation of the mitotic period is 

 found among the Opalinid ciliates which, according to Metcalf,^* are 

 unique among organisms for in many their nuclei are not customarily 

 found in a reticulate condition, but rather come to 'rest' in some other 

 phase of the mitotic cycle, which varies with the species. The remarkable 

 flagellate Holomastigotoides, however, provides another example of this, 

 for the resting period here is in late prophase (Cleveland^^). It is 

 common for meiotic divisions to be interrupted at one point or another 

 in the maturation cycle ; thus the egg of the Annelid Chaetopterus is laid 

 at the stage of the first metaphase, and remains in this condition until 

 fertilization. 



Some comment is required on the difference between the mitotic 

 period of mouse cells in culture and in the epithelium of the intact ear 

 of the same animal. This divergence may be due to the fact that the 

 temperature of the ear lobe is below 37°C. This view is supported by 

 an inspection of Bullough's figures for the proportions of each stage of 

 mitosis in the ear epithelium (Bullough^^). The proportion of meta- 

 phases is very high and generally is more than half the total. It might be 

 expected that metaphase would be prolonged at a temperature below 

 the optimum. Buschke et alii^^ are of the opinion that the duration of 

 mitosis in the corneal epithelium of the rat is 'slightly over one hour' ; 

 they derive this result from a study of the rate at which cells in meta- 

 phase accumulate when the cornea is treated with colchicine. 



Effects of temperature 



The marked effect of temperature on the rate of thfe mitotic process has 

 attracted the attention of a number of workers. As with most biological 

 processes, there is an optimal temperature at which mitosis proceeds 

 most rapidly, above and below which the rate is decreased (Figure 28). 

 Thus de Wildeman,^^ as early as 1891, observed that in Spirogyra, cell 

 division took place in 45 minutes at i2°C. but occupied several hours 

 at both lower and higher temperatures. It would be desirable to see 

 whether this very sharp fall in the speed of mitosis on either side of the 

 optimum could be confirmed. It is well known that the relationships 

 between temperature and the rates of several biological processes have 

 been studied as a possible means of analysing the nature of the 'master 

 reactions' v/hich govern or limit the visible events in cells and tissues. 

 Such data are frequently expressed as the co-efficient Qio, which is the 

 ratio of the speed of a process at one temperature to the corresponding 

 velocity at a temperature io°C, higher. A general account of this field 

 of research is given in Chapter 31 of Heilbrunn's text-book. ^^ In the 

 inorganic world 'many chemical reactions have Qio values between 



86 



