MANNER OF PRODUCTION OF MUTATIONS 569 



procedure which by itself was not mutagenic) reduced the frequency of 

 chromosomal aberrations produced by X rays by 30-50 per cent and that 

 the pretreatment, at least, reduced the injury to growth significantly also. 

 On the other hand, the frequency of visible mutations was influenced in 

 the opposite direction by either the previous or subsequent heat shock, 

 being increased by 30-50 per cent. This oppositeness in effects on 

 aberrations and visible mutations had previously been noted by Kaplan 

 when certain chemical treatments were used with X rays on barley seeds 

 (see pp. 575-576). 



Visible evidence of a semipermanent change in the properties of chromo- 

 somes caused by heat shock has recently been reported by Gaulden 

 (1951). She found that the chromosomes in neuroblast cells of grass- 

 hopper embryos exposed to 47°C for 6 minutes underwent an increase in 

 their refractive index, accompanied by the development of birefringence 

 in polarized light, and that this visible alteration persisted through 

 several cell generations after treatment. It would be strange if such a 

 change were unaccompanied by differences in the behavior of the chromo- 

 somes or of their pieces. 



As for the promoting effect of heat shocks on point mutations in 

 barley, there is too much room for different hypotheses to make specula- 

 tion seem very profitable. Possibly certain metabolic processes are 

 interfered with (as by enzyme inactivation) which normally tend to pre- 

 vent the completion of gene mutations. In this connection attention 

 may again be called to the findings of Hollaender on E. coli and of Meyer 

 and Muller as well as Edmondson and Meyer on Drosophila, all of which 

 showed that the production of mutations by ultraviolet was in part pre- 

 vented by aftertreatment at a normally warm temperature, as opposed 

 to a cool but not freezing temperature. In these cases, in which the 

 warmth was insufficient to constitute a heat shock, the process involved 

 was opposite in its effect on point mutations to the promoting influence 

 of heat shocks on X-ray-induced mutations. It seems not unlikely 

 that a similar reparative process may take place to some extent with 

 ionizing radiation also, just as Watson (1950) showed that, in phage, 

 " photoreactivation " works to a certain extent even after ionizing radia- 

 tion, although not as powerfully as after ultraviolet. If this is the case, 

 then heat shocks might well act on point mutations in the opposite direc- 

 tion from mere warmth, by hindering the occurrence of the reparative 

 metabolic reactions which are promoted by a degree of warmth within the 

 normal range. In any event, temperature change is most complicated 

 in its possibilities. 



The influence of infrared in promoting the production of chromosome 

 aberrations by X rays was discovered by Kaufmann, Hollaender, and 

 Gay (1946), working with cytologically detected changes induced by 

 irradiation of Drosophila spermatozoa, and by Swanson and Hollaender 



