MANNER OF PRODUCTION OF MUTATIONS 531 



they did give evidence of a relatively small amount of influence of this 

 kind. It is probable that the extent to which such transmission from 

 outside may occur differs much with different types of cells and of organ- 

 isms, and perhaps also under different conditions. This matter will be 

 discussed later. Meanwhile, it may be noted that any such influences 

 would tend to cause the calculated sensitive volume to be greater than 

 that of the gene itself. 



10. PRODUCTION OF POINT MUTATIONS AND CHROMOSOME CHANGES 



BY ULTRAVIOLET 



Following Altenburg's (1930, 1931) finding that sex-linked lethals can 

 be induced, although at a rather low frequency, by ultraviolet irradiation 

 of spermatozoa through the body w^all of Drosophila, Promptov (1932) 

 reported that he obtained visible mutations (although rather doubtful 

 ones) in this way, and Geigy (1931) gave rather inconclusive evidence of 

 having obtained visible mutations by ultraviolet irradiation of the germ 

 cells contained in the superficially lying polar cap of the blastula stage. 

 The latter technique was improved on its genetic side, with a considerable 

 increase in the mutation frequency, by Altenburg (1933, 1934, 1936a, b). 

 Meanwhile during the same decade, mutations were obtained by ultra- 

 violet treatment of the following plant material: ascospores of the fun- 

 gus Chaetomiuni (Dickson, 1932), pollen of the snapdragon Antirrhinum 

 (Noethhng and Stubbe, 1934), maize pollen (Stadler and Sprague, 1936, 

 1937; Stadler and Uber, 1938, 1942; Stadler, 1939), spores of the fungus 

 Triclwphyton (Hollaender, 1939; Emmons and Hollaender, 1939), gametes 

 of the liverwort Sphaerocarpus (Knapp, 1938; Knapp et al., 1939; Knapp 

 and Schreiber, 1939). Since 1940, ultraviolet mutagenesis has been 

 extended to various other material. 



Most of the mutations obtained in the pioneer investigations with 

 ultra^dolet invoh^ed lethal or visible effects unconnected with the semi- 

 sterility indicative of chromosome changes. In the work of Reuss (1935, 

 1938) on Drosophila spermatozoa, in which a higher frequency of sex- 

 linked lethals (up to about 5 per cent) was obtained than previously by 

 passing filtered ultraviolet through the flattened ventral wall of the abdo- 

 men of males while they were still highly flexible and relatively trans- 

 parent, it was shown that the ratio of lethals to visibles was similar to 

 that among spontaneous mutations (this ratio being slightly lower than 

 among those produced by X rays). In general, ultraviolet mutations of 

 Drosophila appeared similar in type to spontaneous ones (see, however, 

 Chap. 7, for differences between ultraviolet and X-ray mutations at cer- 

 tain loci in maize). 



There was evidence almost from the beginning, however, that ultra- 

 violet does produce some chromosome changes in Drosophila. Altenburg 



