18 DURATION OF THE SEVERAL MITOTIC STAGES 



PRELIMINARY EXPERIMENTS. 



In the first experiments the samples used were the growing root-tips 

 of a reddish commercial onion about 1.5 inches in diameter. They 

 were sprouted in water at an ordinary room temperature which during 

 their period of growth fluctuated around 18° C, thus preventing the 

 possibiHty of eliminating the temperature factor, but that was not the 

 purpose of the initial study; temperature effects were to be considered 

 in a later investigation. After 5 or 6 days the root-tips had reached a 

 length of 5 to 10 mm. Thirteen samples were taken at lO-minute inter- 

 vals, from 10 a. m. until 12 noon on the same day early in February 

 1916. Each sample was dropped immediately into a numbered vial 

 of Fleming's fluid, and each was duly prepared, sectioned longitudinally 

 (6 microns), mounted and stained with Heidenhain's hematoxyhn. 

 Then, within two root-tip diameters of the extreme tips, that is, in the 

 mitotically most active region, microscopic fields were selected at ran- 

 dom in which the cells were counted and classified as to the stages of 

 their mitotic progress. In each of the 13 successively cut root-tips 

 1,000 cells, including both those mitotically active and resting, were 

 observed and classified. The same 10 active mitotic stages which were 

 used in the subsequent and fuller study constituted the basis of classi- 

 fication. 



The accompanying Summary Chart figures and describes each of these 

 arbitrarily marked sections of the mitotic cycle. Since the mitotic 

 process is a continuous one, there are as many stages in its course as one 

 may care to mark ; nevertheless there are striking transformations which 

 appear to occur with relatively great rapidity, and hence their begin- 

 nings and ends make suitable mile-posts for studying and comparing 

 absolute and differential progress. When less numerous divisions are 

 required, cytologists generally have named the stages of the mitotic 

 cycle as follows: (1) resting, (2) prophase, (3) metaphase, (4) ana- 

 phase, (5) telophase. In these studies ten stages were marked off 

 with arbitrary but definite boundaries in order to provide a more re- 

 fined analysis of the mitotic cycle than the usual fewer and more indefi- 

 nite stages just named imply. 



AVERAGE RELATIVE DURATIONS OF THE SEVERAL MITOTIC STAGES. 



PRELIMINARY EXPERIMENTS. 



Applying the principles demonstrated in the method chart, the stage 

 index chart of the preliminary work gives for the average relative dura- 

 tions of the successive stages the following series: 



0.4473, 0.2218, 0.0933, 0.0266, 0.0077, 0.0096, 0.0089, 0.02S1, 0.0367, 0.1196 



These results are based upon 13,000 individual cell-counts, and if the 



