EXPERIMENTAL ANALYSIS 



liable to be blocked, because the effect on the later stages is never 

 seen unless the earlier sensitive periods are passed without hindrance. 

 In Table VII, the effects are given of a number of inhibitory sub- 

 stances on the several phases of mitosis in chick cells in culture. A 

 rather miscellaneous group of agents have the property of preventing 

 cells from entering division, and have either no effect on the course of 

 mitosis in a cell in which prophase has begun, or block the later stages 

 to a much smaller extent. This information by itself is hardly sufficient 

 to suggest any features of the metabolism of the cell which may be 

 affected by these substances, and which could thus be regarded as specially 

 important for the entry of cells into mitosis. However, evidence from 

 other sources may relate to this question. 



10 11 



12 13 IV 

 Time 



15 16 17 18 19 20 21 22 



Figure 64 Uptake of K** by culture of embryo chick tissue, and reversible loss of 



isotope on cooling the culture. At 10° C. {%) and at 5° C. (O). Ordinate: relative 



specific activity of culture. From Wesson et aliP°- {By courtesy, J. gen. Physiol.). 



A number of agents both inhibit cells in tissue culture from entering 

 mitosis, and also allow potassium to escape from the erythrocytes of 

 those mammals which have a high content of this ion (Wilbrandt;1^^ 

 Davson;!^^ 1^8 Maizels^^^). Thus both effects are exerted by sodium 

 fluoride, by iodoacetate, by hypotonic saline, and by isotonic non- 

 electrolytes. Neither effect is provoked by cyanide or urethane (Davson 

 and Danielli;2oo Hughes^oi). Moreover, it has been shown by Wesson 

 et aliP^^ that potassium is reversibly lost from cells in tissue culture into 

 the surrounding medium at temperatures below 15° C (Figure 64); 

 in the same way when stored human blood is cooled to 5° C, there is a 

 comparable migration of this ion (Harris ;203 Danowski^o*) . Spear^o^^" 

 in 1926 showed that on re-incubation of tissue cultures after chilling 

 for four hours at 0-5° C, the number of mitoses present decreased 

 during the next two hours; this suggested that 'the initiation of mitosis 



N 185 



