508 RADIATION BIOLOGY 



mutagenic hits must be initiated within or even near the genes or chromo- 

 some threads which are thereby caused to undergo the given genetic 

 changes. It is a priori entirely conceivable that a hit which takes place 

 far from a given gene or chromosome can give rise to an altered atom, 

 radical, molecule, or large particle which, by migration or by an elongated 

 chain of reactions, finally turns out to be mutagenic at a point very distant 

 from its origin. Empirical evidence is therefore required for determining, 

 both in space and in time, the length of the mutagenic pathway leading 

 from the effective hit to the permanently altered genetic material. 



The first experiments of this kind dealing with spatial limitation 

 (Mavor, 1929) were not concerned with mutational changes but with the 

 influence of X rays on the frequencies of crossing over and of nondisjunc- 

 tion. Mavor found that, when only the posterior, gonad-containing 

 halves of Drosophila pupae were irradiated, the same amount of effect 

 was produced on these genetic processes as when the entire pupae were 

 given the same dose, whereas, when only the anterior halves were irradi- 

 ated, no perceptible influence of this type was exerted, although the 

 gonads were only about 3^ mm from the treated part of the body. Since 

 the pupa is largely filled with a lymph, containing numerous separated 

 cells, it is evident that neither the lymphatic medium nor its cellular ele- 

 ments transmit to an appreciable extent those effects of the radiation on 

 which the frequencies of either crossing over or nondisjunction depend. 



A similar technique which involved the shielding of the anterior and 

 posterior parts of the body, respectively, during irradiation was used by 

 Gulbekian (1934, 1936b) with the adult male Drosophila to determine 

 whether the production of sex-linked lethals by X rays is similarly local- 

 ized. According to Gulbekian's data the mutagenic influence did appear 

 to be transmitted to some extent since there was an appreciable produc- 

 tion of mutations when, supposedly, only the anterior region had been 

 irradiated, and a higher freciuency was obtained from irradiation of the 

 entire body than from irradiation of the posterior half. However, more 

 accurate work, designed to check this point, by Kerkis (1935a), using 

 pupae, showed that there are no such effects when the shielding is 

 properly arranged. 



Evidence that even closer proximity than that achieved in the fore- 

 going experiments does not allow transmission of a mutagenic effect of 

 ionizing radiation in Drosophila was provided by tests conducted by 

 Timofeeff-Ressovsky and his co-workers (Timofeeff-Ressovsky and Zim- 

 mer, 1935b; Wilhelmy et al, 1936; Timofeeff-Ressovsky, 1937b) with X 

 rays of different wave lengths. It was found that apparently the same 

 frequency of sex-linked lethals was produced by irradiating spermatozoa 

 (in mature males) with X rays of 0.5- or 1.25-A wave length as by irradia- 

 tion with ordinary X rays (about 0.18 A) or with 7 rays of radium, pro- 

 vided that the dose received within the gonads was the same. With the 



