CELLS IN DIVISION 



figures have been converted to the respective proportions of the total 

 duration of mitosis.* 



If cells in a tissue do not enter mitosis at a regular and even rate, then 

 proportionate counting of stages cannot be used to estimate their rela- 

 tive duration, unless it can be assumed that the irregularities can be 

 averaged out by taking a large number of samples. If, however, cells 



16 20 ZV 



Time (Kr GM.T.) 



Of 



Figure 27 Daily variation in the number of mitoses in three 



stratified epitheHa of the mouse. From Bullough*^ (By 



courtesy, Proc. roy. Soc. B). 



enter division in 'waves' which follow a relatively quiescent period, 

 then the time occupied by mitosis can be approximately estimated by 

 following the progress of such a peak through a series of samples of 

 tissue taken at successive intervals. It will first be detected as an 

 increase in the number of prophases; this increase will subsequently be 

 transferred to later stages of mitosis, and finally when the high telo- 

 phase count begins to subside, it can be surmised that those cells which 

 first entered mitosis in increased numbers have now finished their 

 division. An involved statistical analysis was made of mitosis in the root 



* Brown^^* has made similar observations on the roots oiPisum seedlings; he estimates the 

 rate of production of cells in the meristem by a more direct method. 



83 



