3. — Principles and formulas for determining the relative and the absolute 

 durations of the several mitotic stages. 



PRINCIPLES. 



1. The duration of a mitotic stage is directly proportional to the summation 

 of its percent-frequencies (^. e., stage indices [S, I.]) observed at successive 

 intervals, in accordance with the principles of sampling, during the mitotic 

 process. 



2. The absolute duration of a succession of mitotic stages is measured by 

 the time intei-val between two points in a recognizable procession through 

 time intervals and mitotic stages of the procession indices [P. L], marking, 

 respectively, the first and last stages in the selected succession. 



FORMULAS. 



1. Mitotic index. 



M I = No. cells dividing. ^ ^^^^ / p. ct. PpXPi 



Total number of cells (both ' a/ ^ 



resting and dividing) ob- 

 served in the same fields. 



2. Stage index. 



g J No. cells in a given mitotic stage. 



Total number of mitotically active cells (i. e., excluding the resting 

 cells) observed in the same fields. 



3. Average relative duration of the active cycle. 



A. R. D. of C. = l when "resting" is not included as a stage; = 1— A. R.D. 

 of R. when "resting" is included as a stage. 



4- Average relative duration of a given mitotic stage. 



S S. I. of the given stage in all S S. I. of the given stage in all 

 A T> Tk ^f a observations. observations. 



A. K. D. 01 b. = ^ r^ ^ ;; — Ti — : : \ — ; — r Or 



2 S. I. of aZZ stages included No. of observation-instants, 

 in the cycle, in all obser- 

 vations. 

 = also the average stage index of the given stage. 



5. Procession index. 



S.I. 



P. I.= 



A. R. D.of S. 



6. Average absolute duration of the entire active mitotic cycle. 



Time periods elapsing between 

 two points in a recognizable 

 procession of P. I. 



A.A.D.ofC.= 



No. of stages covered. 



X No. stages in cycle. 



' No. P. I. waves followed. 



7. Average absolute duration of a given mitotic stage. 



A. A. D. of S. = A. A. D. of C.XA. R. D. of S. 



Note: An observation consists of 1,000 cells from the same root-tip selected by counting 

 all cells withm a sufficient number of microscopic fields selected at random within two root- 

 diameters of the extreme tip. 



