INDUCED CHROMOSOMAL ABERRATIONS IN ANIMALS 1171 



X-rays (Mavor, 61, 62; Anderson, 3, 4, 5, 7). Exceptional females are 

 less frequent in the offspring of treated flies than are the exceptional 

 males, suggesting that both nondisjunction and elimination of chromo- 

 somes are increased by X-rays. Mavor (64) has studied the effects of 

 exposing different parts of pupae to X-rays. In some cases the anterior 

 half of the body was exposed to radiation, the posterior half being shielded 

 by a silver plate. In other cases only the posterior half of the body 

 (containing the gonads) was exposed. Mavor found that flies coming 

 from pupae whose anterior half was irradiated produce no more excep- 

 tional eggs than control flies. On the other hand, irradiation of the 

 posterior half produces as much effect as irradiation of the whole body. 

 These data show that the action of X-rays on the disjunction of chromo- 

 somes is probably direct, rather than indirect, through an upset of the 

 general physiological condition of the organism. 



Demerec and Farrow (22, 23) found that X-ray treatment increases 

 the frequency of primary nondisjunction also in Drosophila virilis. 

 Most of the exceptional individuals produced in that species are males 

 (no-X eggs), the exceptional females {XX eggs) being very rare. It 

 appears therefore that in Drosophila virilis elimination of X-chromosomes 

 rather than nondisjunction in the strict sense is produced by X-rays. 

 The frequency of nondisjunction and elimination increases proportionally 

 to the amount of treatment up to the dosage of 2000 r-units. Higher 

 dosages produce a relatively small increase of the frequency of exceptional 

 individuals. 



Gynandromorphism in Drosophila was also induced by X-rays 

 (Mavor, 63; Patterson, 100). Since the appearance of gynandromorphs 

 is mostly due to elimination of one of the X-chromosomes in cleavage 

 divisions this result seems to indicate that X-rays provoke elimination 

 in both meiotic and mitotic divisions. This conclusion is justified in 

 spite of the fact that the more recent studies of Patterson (101, 102, 103, 

 105, 106) have proved that a part of the gynandromorphs produced by 

 radiation have not a whole X but only a part of it lost, and consequently 

 gynandromorphism in this case is partly due to breakages of chromosomes 

 and subsequent losses of some of the fragments, a phenomenon not so far 

 observed in gynandromorphs appearing spontaneously. 



Spontaneous nondisjunction of the fourth chromosomes in Drosophila 

 was first studied by Bridges (15). Individuals monosomic for the fourth 

 chromosome are easily distinguishable from normal flies in appearance, 

 having a complex of the so-called "Haplo-IV" characters. Mohr (68) 

 described mosaic individuals, the origin of which must have been due to 

 nondisjunction or elimination of the fourth chromosomes in cleavage 

 divisions. Muller (72, 75) and other authors found among mutations 

 induced by X-rays many so-called "Minutes," some of which were 

 undoubtedly "Haplo-IV." Dobzhansky (26) observed in the offspring 



