CELL DIVISION, MORPHOLOGY, VIABILITY 767 



cycle of the zygote can be identified accurately only in fixed material, the 

 procedure is to determine the mitotic stage at any desired time by fixing a 

 sample of the eggs for future examination. Cleavage delay is customarily 

 expressed as the difference in time required by the treated and the control 

 eggs to progress from the stage of treatment to cleavage of 50 per cent of 

 the zygotes. If sperm or unfertilized eggs are treated, cleavage time is 

 measured from insemination. In addition to giving precise data on the 

 relation of dose to cleavage delay, this material is useful in obtaining 

 quantitative data on the degree and rate of recovery from mitotic 

 retardation. 



Mitotically active parts of living plants or animals in vivo or in vitro 

 may be irradiated and fixed at the end of a certain time interval. The 

 effect on mitosis can then be determined by comparing the proportions of 

 cells in different mitotic stages with those of controls. Since the fre- 

 quency of different stages after treatment depends on both the dose and 

 the time after treatment at which the material is fixed, an accurate 

 picture of mitotic events following treatment is secured only by the 

 analysis of a series of preparations fixed at short time intervals after 

 irradiation. Unlike the marine invertebrate egg, this kind of tissue will 

 ordinarily consist of cells in interphase and various stages of mitosis at the 

 time of irradiation. The results are therefore expressed as changes in the 

 proportion of cells in different stages at different times after irradiation. 

 Part of the error inherent in the use of different preparations for different 

 counts can be eliminated by the use of culture preparations in which suc- 

 cessive counts can be made of the same group of cells. 



It cannot be emphasized too strongly, however, that valid conclusions 

 regarding the effect of a given treatment on mitosis cannot be drawn from 

 counts alone of cells in different stages of the mitotic cycle at intervals 

 following treatment. For example, an increase in the number of cells in 

 middle prophase following a certain treatment must be the result of cells 

 entering middle prophase at a faster rate than they leave it. This may 

 be brought about in different ways, none of which could be confirmed or 

 negated by counts alone. It could result from: (1) an increase in the 

 mitotic rate of cells in early prophase, so that a number greater than 

 normal enters middle prophase in a given time; (2) a decrease in the 

 mitotic rate of cells in middle prophase, so that a number smaller than 

 normal leaves middle for late prophase in a given time interval; (3) a 

 reversal of mitotic progress in late prophase, so that late prophase cells 

 regress to become middle prophase cells ; or (4) any combination of these. 

 It may be seen, therefore, that a simple conclusion that treatment has 

 accelerated division, or that it has retarded it, cannot be reached merely 

 by comparing mitotic counts made at intervals after treatment. All too 

 often one comes across the wholly unjustifiable conclusion that, because 



