NEOPLASTIC ABNORMAL GROWTH 225 



can be regarded as minute rearrangements of a size too small to be seen 

 cytologically has been reviewed by Muller (19). 



Glucksmann reports that under certain conditions the process of dif- 

 ferentiation of mammalian embryonic cells is promoted by irradiation 

 (31), and that their sensitivity to injury decreases as differentiation 

 proceeds. Can it be that the differentiated cell is no longer sensitive to 

 certain dosage of x-rays because many of the points on the chromosome 

 which are particularly vulnerable to this agent have previously altered 

 spontaneously? Adequate data are at present not at hand to prove that 

 x-ray effects are not additive and that they cause variations which are 

 resistant to the agent. Work of this type is in progress and the results 

 are said to indicate that resistance does develop. 



The experiments of Beadle (25) prove completely that defects in cer- 

 tain biosyntheses can be produced in Neurospora by irradiation with 

 x-rays with resultant mutations of specific genes. The growth factor re- 

 quirements of the mutated organism result from the induced inability 

 to manufacture essential substances from simpler constituents. These 

 requirements are characteristic of a given strain and are transmitted to 

 each descendant cell, unless back mutation occurs. Gray and Tatum 

 (32), furthermore, proved that x-ray treatment of two species of bac- 

 teria also produced strains characterized by their inability to carry out 

 specific biochemical reactions and suggested that biosyntheses in bac- 

 teria, like those of Neurospora, are controlled by specific genes. 



The evidence indicates that a single molecular change affects a single 

 gene of sexual cells. A similar effect in vegetative cells is suggested by 

 the fact that in the case of both viruses and bacteria a single atom 

 ionized can kill. Lea (33) has shown that this atom cannot be one lo- 

 cated just anywhere; it must be in a sensitive component, just as in cells 

 which have demonstrable genes. Gowen (34) compared the mutation 

 rates induced by x-rays in Drosophila, tobacco mosaic virus and the 

 bacteria causing corn wilt disease. The frequency of variation was 

 increased at the same rate in all three, a fact which again suggests a 

 similar structure controlling mutation in both sexual and vegetative 

 cells. These observations provide examples of the similarity between the 

 variations induced by the same means in sexual (mold) and in vegetative 

 (bacterial) cells. Since genes are involved in the former, it is reasonable 

 to assume the participation of genes or gene-like substances in the latter. 

 This conclusion is, of course, not susceptible to absolute proof, a point 

 conceded by the authors. Furthermore, it is not possible to prove whether 



