40 DURATION OF THE SEVERAL MITOTIC STAGES 



While the experunents and discussions of this paper are confined 

 to the method of mitotic analysis based upon velocity-responses 

 characteristic of definite temperatures, which method doubtless will 

 continue to yield profitable returns, the study of specific mitotic stage- 

 duration as affected by other physical forces, such as light, electricity, 

 pressure, and gravity, and by chemical agents, and finally by given 

 complexes of these forces and agents, must be resorted to for a better 

 determination of the details of mitotic dynamics. The method of 

 measuring the durations of mitotic stages presented in this paper is 

 applicable equally well to each of these situations. 



Gradually the physiological complex of the cell is being analyzed, 

 each factor measured, and coefficients and indices of reaction of 

 definite living organisms to controlled environmental conditions are 

 being worked out so far as velocity-reactions to temperature are 

 concerned. The fact that mitosis in its complexity does not behave 

 throughout like a uniform and simple chemical reaction is to be ex- 

 pected. In mitosis there exists a microcosm of chemical and physical 

 forces, each with its characteristic response to temperature-increments. 

 Indeed the differential reactions of the several stages of the mitotic 

 process-train present the only possible but nevertheless a most prom- 

 ising key to further analysis of the forces involved in cell division by 

 the method of measuring velocity-response to temperature-changes. 

 Especially valuable will this key be if used under a wide range of con- 

 trolled conditions and applied to mitotic stages of very definite but 

 small differences. Finally, of course, velocity-analysis in its various 

 relations will (like temperature-analysis) reach its Hmits of usefulness, 

 but its possibilities in determining the nature of the dynamics of 

 mitotis are thus far only sampled. 



(c). ISOLATION OF FACTORS. 



Elimination hy comparative experimental evidence. — When a physi- 

 ologist confines his investigations to a definite, localized, relatively 

 homogeneous reaction, he may expect results more closely approxi- 

 mating those of the chemist deahng with homogeneous systems. But 

 even then the varying factors may act upon processes controlling the 

 one sought to measure alone. Riddle^ experimented with four species 

 of cold-blooded vertebrates, with a view to determining the velocity of 

 digestion in relation to temperature. He recognized the difficulty in 

 measuring the effects of temperature upon the digestive process alone. 

 In regard to complicating factors he says: 



"The data indicate that the effects of temperature on the digestive proc- 

 esses must be considered under two heads: First, the accelerating action of 

 increased temperature on the chemical processes involved; and second, the 

 retarding action of very high or very low temperatures due (a) to the pro- 

 duction by the animal of smaller amounts of digestive enzymes under these 



' Riddle, Oscar. "Rate of digestion in cold-blooded vertebrates." Amer. Jour. Physiol. 24: 

 447 et seq., 1909. 



