PROTOZOA AND INVERTEBRATE EGGS 313 



macroniiclei and the micromicloi of tlie exautogamous clone are derived. 

 Sonnehorn also has given evidence that the macronucleus is a compound 

 structure in which each gene and chromosome are represented many 

 times. The evidence is reviewed by Soinieborn (1947). Under the cir- 

 cumstances, mutations or chromosomal aberrations would not be expected 

 to express themselves immediately following irradiation but only after 

 homozygosity of both the macro- and micronuclei had been brought about 



by autogamy. 



On this basis and from the results of several breeding experiments, 

 Kimball (1949b) came to the conclusion that most of the death and low 

 rate of multiplication in the exautogamous progeny of irradiated animals 

 were the results of gene mutations or chromosomal aberrations. Kimball 

 (1949a) showed that a given total dose of particles divided into several 

 small daily fractions, with the animals undergoing several cell divisions 

 between each fraction, was as effective as the same dose given in a single 

 exposure of a half-hour's duration or less. This was taken to mean that 

 the mutational changes must have been gene mutations or one-break 

 chromosomal aberrations rather than two-break aberrations. However, 

 the dosage curve was more nearly typical of a "multiple hit" than a "one- 

 hit" curve. For this reason, Kimball (1949a) suggested that most of the 

 non-normal exautogamous clones were the result of the combined action 

 of a number of mutant genes with individual effects too small to be 

 detected. 



Powers and Shefner (1948) using X rays and Geckler (1950) using 

 nitrogen mustard have both presented further e^'idence from breeding 

 experiments for the genie or at least the micronuclear basis of the post- 

 autogamous effect. However, Geckler (1950) reported on a number of 

 findings which can be explained in terms of micronuclear inheritance only 

 with great difficulty if at all. Kimball (1949b) reported one case of 

 inheritance which did not conform to expectations. 



Another phenomenon, not at present explained, is that reported by 

 Powers and Shefner (1950) for very high doses of X rays and by Powers 

 and Raper (1950) for nitrogen mustard. In both cases, they found that 

 death after autogamy rose to a maximum as the dose increased, then 

 declined somewhat. With X rays, there was a secondary rise at very 

 high doses. 



The reviewer believes that there is strong evidence that radiations and 

 nitrogen mustard induce mutations in the micronuclei of P. aurclia and 

 that these mutations express themselves in death and low rate of multi- 

 plication of exautogamous clones. Ho\yever, there is rather convincing 

 evidence that this is not the whole story and that other phenomena may 

 also play a significant role. Further experiments to define this situation 

 more thoroughly are needed. 



Most of the work with Paramecium has concerned itself more with the 



