THE MITOTIC CYCLE 



results. Two authors, Laughlin^* and Barber^^ have estimated Qio 

 v^alues for mitotic phases in the cells of flowering plants and the inspec- 

 tion of their results does not suggest any common features. Barber's 

 paper reviews most of the previous work in this field, and in it he 

 calculates values from the data of Bucciante®^ on the duration of 

 metaphase and anaphase of chick cells in tissue culture. Both in 

 Tradescantia and Gallus these values range from i-o to over 8-o. It is 

 not possible to draw any conclusions concerning the underlying nature 

 of the events in cell division from such data. 



Intermitotic period 



We now pass to the consideration of that portion of the whole mitotic 

 cycle which is complementary to the phase of division, the inter- 

 mitotic period. This also can be measured both directly and indirectly. 



r 



=1 



J III'' 



J_J 



1 2 3 ¥ S e 7 8 9 W 11 12 13 IV IS 16 17 18 19 20 

 Infer-mifofic periods Kt 



Figure 29 Histogram of 1 7 intermitotic periods observed 

 in the growth of a single culture of the mouse spleen. 

 From Fell and Hughes** (By courtesy, Qiiart. J. mkr. Soc). 



though it is usually of such a length that either the duration of survival 

 of isolated tissues or the patience of the observer waiting in the hope 

 that a particular cell will divide may prove inadequate.* Probably the 

 shortest intermitotic period which has been followed directly is that of 

 the Chortophaga neuroblast (Carlson and Hollaender'^) which is 

 only 27 minutes, just over a quarter of the duration of prophase in these 

 cells. In tissue cultures of chick or mammalian cells growing under 

 optimal conditions, the intermitotic period is of the order of twelve 

 hours though there are wide variations in individual instances. Fell 

 and Hughes®^ found that of seventeen intermitotic periods measured 

 in a film record of the growth of one mouse spleen culture, the range 

 was between eight and eighteen hours (Figure 29). Longer intervals 

 may have been overlooked in the analysis of this record. 



* In a notebook of Strangeways is the entry: '15th Feb. 1921, Cell watched 49 hrs.— 

 did not divide.' 



88 



