60 



THE CELL AND PROTOPLASM 



some material. How a hit produces a break 

 and how the two breaks necessary for a re- 

 arrangement are produced are much mooted 

 questions of great theoretical importance. 

 There are also additional details and some 

 discrepancies regarding the relation of 

 dosage to breaks. But for the point under 

 discussion the all-important fact is that 

 X-raying seems, as some authors put it, a 

 specific for production of chromosome 

 breaks. I should go even as far as to say 

 that a discovery of these facts prior to the 

 discovery of mutation would have strangled 

 the particulate gene concept before it was 

 born. There is no question about the im- 

 mense importance of the facts revealed by 

 X-ray work. But if we are asked what in- 

 formation they have furnished regarding 

 the nature of the gene (in the classical 

 sense of a discrete body of at least molecular 

 constitution as well as in the sense of a 

 specific sidechain), the only objective an- 

 swer which can be given is: nothing what- 

 soever. Nay, it may be said even that the 

 X-ray work has furnished some of the most 

 powerful arguments against the actual ex- 

 istence of the gene as a discrete body. 



Mutations may be also induced by ultra- 

 violet rays and by chemicals (Altenburg, 

 Stadler and Uber, Noethling and Stubbe, 

 Muller and Mackenzie, Sakharoff, et al.). 

 Authors who studied these mutants cyto- 

 logically (or genetically) found that here 

 chromatin-rearrangements are very rare 

 and that the mutants must be classified 

 mainly as point mutations (point mutations 

 meaning that no rearrangements are visible 

 with present-day methods, wherefore they 

 are assumed to be the real gene mutations). 

 This fact has now received a significance 

 which points to important future develop- 

 ments. Knapp, Reuss, Risse, and Schreiber, 

 working with monochromatic ultraviolet 

 rays of different wave-lengths, made use of 

 Warburg's famous method of concluding 

 upon the chemical nature of a substance 

 from its spectrum of action. It turned out 

 that in the liverwort Sphaerocarpus the rate 

 of mutations produced by ultraviolet light 

 is different for different wave-lengths, the 

 maximum being at 265 mp. But this is the 



point of maximal absorption for thymo- 

 nucleic acid, which substance again is sup- 

 posed to be contained in the chromomeres. 

 What conclusions may be drawn from this 

 important fact will depend upon further 

 work. But I am sure that decisive insight 

 will be gained from it. 



Still more important is Stadler 's recent 

 work, which includes also the same discov- 

 eries for maize as just mentioned for Sphae- 

 rocarpus. He found a surprisingly high 

 frequency of terminal deficiencies in addi- 

 tion to the point mutations. This means 

 production of single breaks by the less 

 crude action of ultra-violet radiation as 

 compared with X-rays. One might use 

 this result in favor of Stadler 's and Sere- 

 brovsky's former view that all point-mu- 

 tants are small deficiencies. But the facts 

 might also suggest that point mutations 

 are in fact breaks which result in a pattern 

 change, in this case a subdivision of a 

 longer chain into two smaller ones. We 

 shall mention below facts which point in the 

 same direction. It is certainly too early for 

 definite conclusions, but such facts and 

 their possible meaning have to be kept in 

 mind. 



When it was found that most of the 

 X-ray induced lethals are chromatin rear- 

 rangements (deficiencies, inversions, trans- 

 locations), it was stated simultaneously by 

 the same authors that most of the natural 

 (spontaneous) lethal mutations, as well as 

 visible ones, did not show any detectable 

 chromosomal rearrangement. This, then, 

 turns our attention to the much-neglected 

 topic of spontaneous mutation in its bearing 

 upon our problem, chromosome and gene. 

 To be sure, there is among the natural mu- 

 tants of Drosophila quite a number which 

 turned out to be the effect of chromatin re- 

 arrangements, and with improved technique 

 their number is constantly increasing. But 

 it is safe to say that the majority of re- 

 cessive mutants do not exhibit recognizable 

 chromosome rearrangements, though it must 

 be kept in mind that some types of very 

 small rearrangements may be invisible in 

 the salivary chromosomes of Drosophila or 

 in the chromosomes of maize on account of 



