CELL DIVISION, MORPHOLOGY, VIABILITY 779 



about 2>2 hours to progress from early prophase through telophase (Gray 

 and Scholes, 1951), while mitosis does not fall to zero until 9-12 hours 

 after treatment (Mottram, 1936; Deufel, 1951). Simon-Reuss and Spear 

 (1947) found that as small a dose of y rays as 88 r retarded temporarily or 

 permanently mitotic progression in over one-half the cells treated in 

 metaphase, anaphase, and telophase. Neuroblast cells of the grasshopper 

 require on the average about 26 minutes to progress through very late 

 prophase, prometaphase, metaphase, and anaphase (Carlson and Hol- 

 laender, 1948), but after irradiation the zero level is not reached until 

 about 50 minutes after treatment. Recent studies carried out in our 

 laboratory on neuroblasts of Chortophaga seem to indicate that doses of 

 250 r and greater delay the completion of mitosis by retarding the dis- 

 solution of the nuclear membrane, by producing chromosome stickiness 

 and delayed separation of the chromatids at anaphase, or by retarding the 

 formation of the cleavage furrow (Carlson and Harrington, unpublished). 

 Smaller doses do not affect appreciably the rate at which cells past the 

 critical period complete mitosis (Carlson and Harrington, unpublished; 

 Wolf son, unpublished). After small doses, e.g., 32 r, up to half of the 

 late prophase cells may pass the block and thus contribute to this dis- 

 crepancy, but this does not appear to take place at 128 r. Our present 

 conclusion, reached from studies of the grasshopper neuroblast in vitro, is 

 that among both treated and control cells considerable variation exists 

 in the duration of very late prophase, so that, although most cells require 

 no more than 26 minutes to progress from the beginning of very late 

 prophase to the end of anaphase, enough cells require a longer time to 

 account for the presence of mid-mitoses 44 minutes after treatment. 

 These data cast doubt on the validity of using the time required for 

 mitosis to disappear after treatment as a measure of the average time 

 required for the cells to pass through the stages in question, unless a 

 correction is made for this variation. 



Knowlton and Widner (1950) have recently proposed a method of 

 using X-ray-induced stoppage of mitosis to calculate the duration of the 

 intermitotic period. In any given tissue the ratio of the number of 

 mitotic to intermitotic cells should equal the ratio of the duration of 

 mitotic to intermitotic stages. In the equation, 



No. of mitotic cells duration of mitosis 



No. of intermitotic cells duration of intermitotic period 



the number of mitotic and intermitotic cells is obtained by direct counts 

 and the duration of mitosis is determined from rate of fall of the number 

 of mitotic cells following treatment, The accuracy of this method 

 depends on the validity of certain assumptions that can be determined 

 only with difficulty in many kinds of biological materials; therefore, 

 certain information about the material and the X-ray effect must precede 



