252 RADIATION mOLOGY 



parallel to the axis of the egK fj;reatly increases the j;;enetic elTectivencss of 

 a fiiveii ultraviolet dose while lessening the physiological damage to the 

 embryo (Altenhurg et al., I'JoO; Meyer et al., 1950). Dechorionation of 

 the eggs by immersion in a 5 per cent solution of sodium hypochlorite also 

 facilitates penetration of the radiation (Clark, 1948). 



At the time of irradiation, the pole cells number about 20. Their later 

 incorporation into the germ tract is accompanied by an increase in the 

 number of cells as the sex organs and the reproductive cells are formed. 

 Therefore, if a mutation is induced in one gene at the polar cap stage, it 

 theoretically should appear replicated in about 5 per cent or more of the 

 sex cells from that particular individual. Should the mutation occur 

 after the process of multiplication has begun, a smaller proportion of the 

 germ cells would receive it. 



Two assumptions are involved if the 5 per cent level of mutation repli- 

 cation among the Fi offspring provides the distinction between induced 

 and spontaneous changes. In the first place, the pole cells at the time of 

 exposure to radiation must be 20 or less in number, and second, an ecjual 

 rate of multiplication of mutated and normal pole cells must be assumed 

 to take place up to the time of formation of the reproductive cells. 

 Unequal exposure of the pole cells through unavoidable shielding as well 

 as through differences in the degree of penetration of radiation would sug- 

 gest that variations in cellular injury are to be expected. These varia- 

 tions would be expressed in unequal rates of cell multiplication, with the 

 most heavily injured cells having the slowest rate of division. The latter 

 cells would also be most likely to possess mutated genes. As a means for 

 demonstrating that ultraviolet is effective in inducing mutations, the 

 technique is entirely satisfactory, but it is unsuitable for accurate deter- 

 minations of the frequency of mutation as a function of dosage. 



Recessive lethals induced in the X chromosome by the pole cap method 

 of exposure may be determined by testing, through the CIB technique, 

 the Fi daughters of males arising from irradiated eggs. Replicated 

 lethals from any single male must be further tested for identity because 

 of the possibility of two or more coincident lethals. Visible mutations 

 in the X chromosome may be detected in Fi males by breeding the 



treated Pi males to XX females. 



Reuss (1935) developed an effective method for the exposure of mature 

 spermatozoa to ultraviolet radiation. The abdomens of adult male 

 Drosophila are compressed gently between (luartz plates, the radiation 

 being applied ventrally. Since the testes are superficially located, the 

 amount of overlying tissue is at a minimum and consists of the chitinous 

 exoskeleton, the wall of the testis, and the intervening connective tissues. 

 Clear areas of chitin, which has chemical and absorptive properties char- 

 acteristic of polysaccharides, are in general highly penetrable by wave 

 lengths from 250-400 m^, with the degree of absorption increasing rapidly 



