774 RADIATION BIOLOGY 



1940), rat retina cells (Tansley, Spear, and Gliicksmann, 1937), crocus 

 root tips (Stone, 1933), and Tradescantia pollen grains (Roller, 1943). In 

 the Chortophaga neuroblast, on the other hand, middle and late prophase 

 cells show the greatest radiosensitivity, with early prophases and inter- 

 phases exhibiting successively less sensitivity (Carlson, 1941, 1950). 

 Deufel (1951) has also described a slowing down of prophases in the 

 Vicia root tip soon after treatment. Three possible explanations of these 

 apparently contradictory results may be considered. 



First, the method used to determine the stage of greatest sensitivity 

 may not be adequate. Decrease in number or disappearance of prophase 

 cells from an irradiated tissue does not necessarily mean that blockage 

 is at late interphase and that the prophases have continued through 

 mitosis unchecked. As pointed out in an earlier section (p. 709), X 

 irradiation may lead to a disappearance of prophases by causing them to 

 revert to a condition resembling interphase, the real point of blockage 

 being in late prophase. 



Second, the mitotic stage of greatest sensitivity may differ in different 

 kinds of cells, so that in certain cells blockage occurs at late interphase or 

 early prophase while in others it takes place at late prophase. If this 

 were true it would mean that the mitotic radiosensitivity of the cell 

 depends on factors other than those we commonly use in distinguishing 

 interphases and early, middle, and late prophases, namely, chromosome 

 morphology. 



Third, the appearance of the prophase chromosomes of different kinds 

 of cells may be so different, because of differences in size and visibility, 

 that early prophase in one tissue may resemble late prophase in another 

 (Carlson, 1942), especially when examined in the living condition. 

 According to the description of Strangeways (1922) of the chick fibroblast 

 in vitro the beginning of prophase is preceded by spheration of the cell 

 and followed immediately by the disappearance of the nucleoli, the dura- 

 tion of prophase averaging about seven minutes at 39°C. In the Chorto- 

 phaga neuroblast culture, on the other hand, both the rounding-up of the 

 cell and the disappearance of the nucleoli occur within a few minutes of 

 the end of a prophase that extends over an average period of 102 minutes 

 at 38°C (Carlson and Hollaender, 1948; Carlson, 1950). It would seem, 

 therefore, that the long early and middle prophases of the grasshopper 

 neuroblast, which are characterized by slender, highly convoluted intra- 

 nuclear chromosomal threads, are passed over very briefly or even 

 omitted, perhaps because of the invisibility of the fine threads in the 

 smaller fibroblast, and what is termed late prophase in the neuroblast may 

 correspond to what is thought of as the whole prophase in the fibroblast. 

 In their study of the effects of y rays on the rat retina, Tansley et al., 

 (1937) classed as prophase "all the early changes in the nucleus up to the 

 appearance of discrete chromosomes," and as metaphase, the stage from 



