806 RADIATION BIOLOGY 



Feeding one week after irradiation, which occurred on the seventh day 

 of fasting, led to the appearance of degenerate cells, presumably because 

 feeding stimulated the cells to undertake division. 



(2) After the exposure of chick fibroblasts in vitro to such large doses of 

 X radiation that mitosis did not reappear within at least 24 hours of 

 treatment, Lasnitski (1943b) found that there nevertheless occurred an 

 appreciable rise in the number of degenerate cells. This reached a 

 maximum 3 hours after the end of treatment. Doses of 2500, 5000, and 

 10,000 r were followed by 53, 58, and 75 per cent, respectively, of degener- 

 ate cells. She has postulated that the lethal radiosensitivity of these 

 cells may be positively correlated with their proximity to mitosis, so that 

 at successively higher doses, cells successively farther removed from 

 mitosis may be killed, even though they make no attempt at cell division. 



(3) Whether the radiation dose is large or small as judged by its effect 

 on mitotic activity, degenerate cells are generally present at least in 

 small numbers, very soon after treatment, when the mitotic activity is at 

 a minimum or completely absent (Strangeways and Fell, 1927; Tansley 

 et al., 1937; Spear and Glucksmann, 1938; Glucksmann and Spear, 1939; 

 Lasnitski, 1943a, b, 1945). Their interpretation that this is an effect on 

 cells that were in division during treatment is confirmed by the work of 

 Simon-Reuss and Spear (1947), who observed the breaking up of living 

 fibroblasts that were in metaphase, anaphase, or telophase at the time of 

 treatment. Such degenerating cells represent only a small proportion of 

 the total number of cells present, regardless of the dosage, because in the 

 tissues studied (tadpole brain and eye, avian fibroblasts, rat retinal cells, 

 mouse malignant cells) , the number of dividing cells is small in relation to 

 the number of resting cells. It is interesting to note that in contrast to 

 the results obtained with chick fibroblast preparations, in which cells 

 may break up before completing mitosis after as small a dose as 88 r 

 (Simon-Reuss and Spear, 1947), neuroblasts of Chortophaga complete 

 mitosis almost without exception after doses at least as large as 4000 r, 

 yet the latter cells are much more radiosensitive than the former when the 

 criterion is decreased mitotic activity (Fig. 11-6). 



By comparing the lethal effects of X radiation on malignant cells of the 

 mouse in vitro and in vivo Lasnitski (1947) concluded that the immediate 

 effect of the treatment was mainly a direct one, for the amounts of cell 

 degeneration in the two were similar. A much higher percentage of cell 

 degeneration in vivo than in vitro occurred on the second day after 

 treatment, however, which suggested an indirect effect through damaged 

 blood circulation. 



The nuclear changes leading to pyknosis and cell disintegration in the 

 amphibian egg are described by Duryee (1949) as follows: 



(1) Prophase chromosomes normally transparent and invisible become shorter, 

 thicker, darker, and beaded. (2) In young ovocytes the nascent chromomere 



