THE MITOTIC CYCLE 



that 'un semis de tres fines vesicules' appear in the cytoplasm at telo- 

 phase; these run together and form the contractile vacuoles of the 

 daughter individuals, which at once begin their pulsations. 



TIME SCALE OF THE MITOTIC CYCLE 



The methods by which the duration of mitosis may be measured are of 

 several kinds. Clearly the most direct is to use living cells in prepara- 

 tions where they are optically accessible, and to time the events which 

 occur either by continuous observation, or through the medium of 

 photographic recording. The earliest example of direct measurement of 

 the duration of a phase of cell division dates from 1848, when Mitscher- 

 LiCH^*' observed that cytokinesis in Conferva glomerata [Cladophora sp.) 

 took four to five hours to be completed. Other methods are used when 

 it is not possible to observe the living nucleus, because the cells either 

 are not sufficiently transparent or cannot yet be effectively isolated 

 under physiological conditions. If a number of similar cells can be 

 induced to enter mitosis all at the same time, then batches can be 

 fixed at known intervals, and their stages of division can be investigated 

 subsequently by staining methods. This is the method applied to egg 

 cells which can be fertihzed experimentally. Some may be caused to 

 resume their development at will. Thus, for example, Faure-Fremiet^^ 

 found that fertilized Ascaris eggs remain at the stage with unfused male 

 and female pronuclei until exposed to atmospheric oxygen. 



In a tissue where cells are dividing at random, or in an unpredictable 

 manner, indirect and statistical methods of estimation must be used. 

 If it can be assumed that cells are entering mitosis at an approximately 

 even and constant rate, then in a sample of cells fixed at the same in- 

 stant, the percentages of each stage of mitosis are proportional to the 

 time occupied by each phase of division. If the duration of either of the 

 separate phases can be estimated by another method, then these 

 relative figures may readily be converted into absolute intervals of 

 time. An example of the use of this method is provided by Wright's^^ 

 study of cell division in chick tissue cultures. 



Recently, Gray and Scholes^^ have estimated both the mitotic 

 time and the intermitotic period of dividing cells in the bean root by 

 an elegant method which also involves the rate of elongation of cells 

 which do not again divide, beyond the meristem. Only one of each pair 

 of meristematic daughter cells differentiates in this way and so cells 

 enter division and the subsequent period of elongation at the same rate. 

 This quantity can be calculated; and it is equal to the number of 

 dividing cells in the meristem divided by the average time occupied by 

 mitosis. These authors have also computed the percentages of the 

 meristematic cells in three phases of division and in Table III their 



82 



