798 RADIATION BIOLOGY 



(1949). No striking differences in the relation of dose to rate of decrease 

 of mitotic activity after X radiation are evident. Only the adrenal gland 

 shows a typical compensatory effect a few hours after treatment; epi- 

 dermis, jejunum, and lymph nodes give little or no indication of a rise of 

 mitotic activity above normal. A greatly delayed overshooting does 

 occur 3 and 7 days after treatment in the epidermis and lymph nodes, 

 respectively, but, as the authors point out, this may be due to secondary 

 physiological factors. 



Chromosome Number. Marshak and Bradley (1944) found that the 

 inhibition of mitotic activity in root tips of three species of Triticum and 

 two species of Bromus with diploid chromosome numbers of 14, 28, 42, 56, 

 and 84, as determined by mitotic counts made 3 hours after X irradiation, 

 was neither directly nor inversely proportional to chromosome length but 

 did vary inversely as the chromosome number. Evidence from other 

 studies on plant root tips suggests that mitotic activity has not reached a 

 minimum value at 3 hours. If that is true in these species these results 

 might be a measure of the rate of fall of mitotic activity rather than of 

 minimum activity, and rate of fall is mainly determined by the normal 

 mitotic rate (see p. 775). The inverse relation to chromosome number 

 could be due, therefore, as well to a lower mitotic rate in cells with more 

 chromosomes as to increased resistance to radiation-induced mitotic 

 delay in cells with the larger chromosome number. 



MORPHOLOGICAL EFFECTS 



Nuclear Components. Effects of ionizing radiations on chromosomes 

 fall into two main classes: those referred to as "primary," "physio- 

 logical, " or "stickiness" effects, and those termed "secondary" or "aber- 

 ration" effects. Primary effects may be described as changes of a 

 general, or not highly localized, character in the physical, and possibly 

 also the chemical, nature of the chromosome, which are evidenced during 

 the ensuing metaphase and anaphase in a tendency of sister chromatids 

 or of different chromosomes to adhere. Sister chromatid fusion results in 

 bridge formation at anaphase. Secondary effects, or chromosome abera- 

 tions, are highly localized alterations of the chromatid or chromosome that 

 are evident as breaks, exchanges, or inversions in metaphase and anaphase 

 cells subsequent to irradiation. For obvious reasons the terms "pri- 

 mary," "physiological," "stickiness," and "secondary," as applied to 

 effects on the chromosome are objectionable. The terms " nonlocalized " 

 and "localized" are used here as substitutes for "primary" and "second- 

 ary," respectively. 



Localized effects are of interest in this chapter only insofar as the time 

 of their appearance is related to that of nonlocalized effects and the 

 period of mitotic inhibition. It seems apparent from evidence now avail- 

 able that nonlocalized effects are limited to cells that complete mitosis 



