THE MITOTIC CYCLE 



tip of the onion by Laughlin^* along these lines, in which the duration 

 of each phase of mitosis is expressed in minutes to two places of deci- 

 mals, though perhaps of doubtful significance. In animals, mitotic 

 rhythms are being discovered (Cooper and Schiff;^^ Cooper and 

 Franklin ;^^ Blumenfeld;^' Bullough;^^ Mollerberg^^) (Figure 

 27) though these are usually not traceable with sufficient clarity for the 

 purpose of mitotic timing. 



In the course of a series of researches on factors which affect mitotic 

 activity in the mouse, Bullough'^ has observed that the injection of 

 carbohydrate stimulates epidermal mitosis and has been able to use 

 the consequent wave of cell division to estimate the time occupied by 

 these cells in mitosis. To his results we shall again refer. 



The accuracy of all methods of measuring mitotic periods is limited 

 by the difficulty of choosing recognizable events to serve as boundaries 

 between interphase and mitosis, and between each of the several phases 

 of the division process. There is only one sharply defined moment 

 during mitosis, namely the beginning of anaphase ; ^n film records this 

 can be recognized within the limits of a minute. Other events in 

 mitosis demarcate phases sufficiently clearly in some cells, but these 

 criteria may break down when we compare mitosis in different species. 

 For example, the disappearance of the nucleoli which marks the end 

 of prophase in a chick or mammalian cell either passes unnoticed in 

 plant cells with larger chromosomes or may occur at the very beginning 

 of prophase in a Protozoan such as Acanthamoeba. The earliest signs of 

 prophase are usually difficult to recognize. 



Comparisons of the data in each column of the table of mitotic 

 phases (Table III) must be made with these difficulties in mind. It is 

 clear, however, that the range of recorded times for each phase is 

 much beyond possible observational errors. An important consideration 

 is the eflfect of temperature which, as several authors have shown, can 

 vary the length of the mitotic period by a factor of several times within 

 the physiological range. To this subject we shall return in discussing the 

 attempts which have been made to analyse the mitotic process by 

 means of temperature co-eflftcients. 



In so far as any conclusions may be drawn from the data in Table 

 III, it would appear that the range of mitotic times is less among verte- 

 brate cells than in plants or Protozoa. The shortest duration of mitosis 

 so far known would seem to be that of the embryo of Drosophila 

 melanogaster, which according to Huettner^^ is no more than 10 min- 

 utes at 23° C, though no further details are given by this author. 

 Lengthy mitotic times are generally due to extended prophase or 

 telophase ; the apparent variety in the length of telophase in ffowering 

 plants is notable. It seems that the duration of mitosis in different 

 organisms varies with the size of the mitotic figure. That of Drosophila 



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