1312 BIOLOGICAL EFFECTS OF RADIATION 



Genetic evidence on the occurrence of natural mutation seems to 

 show that the frequency with which genes change by mutation into 

 others which are lethal is considerably greater than the frequency with 

 which they mutate to those having visible nonlethal effects. Data on 

 mutation ratios of normal genes exposed to X-ray likewise bear out 

 this same conclusion (12, 25, 27 to 39, 59 to 69, 70, 71, 81). 



Coincident with the genetic research but having different per- 

 sonnel and objectives, investigators have used X-rays and radium in 

 medicine, as in the treatment of neoplasms, and in biology, as in study- 

 ing abnormal growths. In both cases the end result rather than the 

 process by which this result was attained was the important considera- 

 tion. The descriptive facts, while at the time seeming to give little 

 insight into the real nature of the changes or their cause, may have more 

 significance. The series of experiments of Hertwig and his students 

 (41, 42) showed that the beta and gamma rays of radium can injure 

 the sperm or egg nucleus without destroying either cell's activity in the 

 fertilization process. An irradiated sperm is motile and can initiate 

 development in the normal egg. If a slight irradiation is given, it may 

 cause abnormal mitosis with the ultimate death of the egg it fertilizes. 

 If more heavily irradiated, it may cause development to start but play 

 no more part in it, although the development is often quite normal. 

 The nuclei in such embryos are haploid. A like situation where the egg 

 nucleus is functionally destroyed with the cytoplasm remaining normal 

 is noted in irradiated eggs of Chaetopterus (72). The sperm nucleus 

 under these conditions continues normal development. The nuclear 

 material, chromatin, is most sensitive to the irradiation, while the rest 

 of the cell protoplasm either takes a much larger amount of energy 

 to affect it or the cytoplasmic changes seen in the later stages of cell 

 degeneration of irradiated cells really arise in previous nuclear mutations. 



The interpretation of the cause of death of an organism irradiated 

 by X-ray or radium as due to the absorption of the energy in discrete 

 quanta within a relatively small vital spot within the cell seems to have 

 occurred independently to several investigators (4, 8, 9, 10, 11, 14, 15, 

 17 to 21, 44 to 48, 93 to 97). The theoretical grounds on which this 

 reasoning was based arose largely in physical considerations — the sig- 

 nificance of the biological facts of mutation and lethal factors not then 

 having been appreciated. The X-rays were regarded as being absorbed 

 in tissue as discrete units. If a is the probability that an electron will 

 hit the object under experimentation and n the number of electrons, 

 then the probability that the object will be hit just once is 



na(\ — a)""* 



The probability that the object will absorb r electrons out of the n 

 possible is 



