CELLS IN DIVISION 



In the early cleavages of fertilized eggs, there may be no period of 

 interphase between successive cycles of nuclear division, as in Echinus 

 [Psammechinus) miliaris (Gray^^) where the duration of mitosis remains 

 constant at approximately 33 minutes for at least the first six cleavages, 

 despite the successive halving in size of the blastomeres. Micromeres 

 and macromeres cleave at the same rate. In the frog Rana pipiens 

 (RuGH^*) cleavages 2-4 follow each other at about hourly intervals; 

 the duration of mitosis in these yolky blastomeres is less than in their 

 fibroblastic descendants. Cleavages in the rabbit Q.gg last for 9-10 

 minutes (Pincus^^) and follow each other at intervals of 8-9 hours 

 (Lewis and Gregory^^), most of which the blastomere nuclei must 

 spend in interphase. 



Within a tissue the relative proportions of cells in mitosis and in 

 interphase can be used as a basis for the estimation of the intermitotic 

 period, provided that some estimate of the duration of mitosis is 

 available. However, the fact that two interphase cells result from one 

 cell in normal mitosis complicates the calculations. In the grasshopper 

 neuroblast this is not so, for, of the two daughter cells, one becomes a 

 ganglion cell, and the other a neuroblast; hence the intermitotic period 

 will be directly proportional to the number of neuroblasts in interphase. 

 An equation relating the intermitotic period to the proportion of cells 

 in mitosis in a tissue was derived by Olivo and Slavitgh.^' Their 

 formula however is not correct, for it involves the addition of the time 

 of mitosis to log 2, the result of which will clearly depend upon the unit 

 of time employed. Crigk in 1948 recalculated this equation, and he is 

 kind enough to allow his formula to be quoted here. It is as follows: 



Time of mitosis i + 2R 



X log^2 = loge 



Total time of mitosis and interphase i -f R 



where R is the fraction of cells in mitosis. 



If the intermitotic period is relatively long and R is thus small, then 

 the right-hand side of the equation approximates to R, and hence : 



Time of mitosis 

 ~ I -44 R 



Total time of cycle 



The relationship between the mitotic index and the duration of 

 interphase is plotted in Figure 30 for three possible values of the mitotic 

 period. As an example, this formula may be applied to the measure- 

 ments of Schultz^^ of the mitotic index in the chick embryo between 

 20 and 70 hours of incubation in which the range of percentage values 

 was found to be between 5 and i . The upper limit of this range corres- 

 ponds to the shortest intermitotic periods which have been observed in 



G 89 



