THE POSSIBLE ORIGIN OF MUTATIONS IN 

 SOMATIC CELLS 



That mutations are accompanied by some change in the 

 germ-plasm is, I take it, indisputable. Have we, however, any 

 reason to suppose that the change takes place within the germ 

 cells ? I am not sure, as a matter of fact, that genetists in gen- 

 eral regard the gametes as the place of origin of mutations. It 

 is true, however, that experiments in the artificial production 

 of mutations in plants 1 have been limited largely to treatments 

 of the ovaries from about the time of the reduction division to 

 about the time of fertilization. This suggests a belief on the 

 part of investigators that mutations are most likely to be induced 

 in the gametes or in the stages of the plant closely associated 

 with gamete formation. MacDougal (loc. tit.) considered it 

 most probable that mutations take place just prior to the reduc- 

 tion division. 



The very uniqueness of the reduction division has perhaps 

 suggested the likelihood of the occurrence of chance irregulari- 

 ties in it resulting in the production of mutations. Davis 2 has 

 interpreted the occurrence of 21 chromosomes in scmi-gigas 

 forms of CEnothera as possibly brought about by a pushing 

 forward of the premature fission of the chromosomes from the 

 anaphase to the metaphase of a heterotypic mitosis followed by 

 another fission before the metaphase of the following homotypic 

 mitosis, resulting in the production of gametes with 14 chromo- 

 somes, which are supposed to unite with normal gametes (with 

 7 chromosomes). The gigas forms of CEnothera, with their 28 

 chromosomes, however, seem more readily explained by the 

 assumption of a double fission of chromosomes in some mitosis 

 after fertilization. Otherwise we must assume that both male 

 and female gametes with 14 chromosomes are produced at about 

 the same time and that two such gametes happen to meet in 

 fertilization— certainly a rare chance. 



The heterozygous condition of the new character in some 

 mutations and the frequent appearance of mutations as seed- 

 sports rather than as bud-sports may, at first thought, make it 

 seem reasonable that they might have their origin in the gametes 

 or at least at about the time of gametogenesis. Neither of these 

 occurrences, however, affords any real evidence for placing any 

 such limit upon the time of origin of a mutation. The reason 

 for this statement will become apparent later. 



East 3 has called attention to the asexual production of varia- 



1 MacDougal, D. T., Pop. Sci. Mm., 69: 207-225, 1906; Carnegie Pub. 

 81: 61-64, 1907. Gager, C. S., Mem. N. Y. Bot. Gard., 4: 22, 1908. 

 Humbert, E. P., Zeit. ind. Abst. Vererb., 4: 161-226, 1911. 



2 Davis, B. M., Aiinals of Botany, 25: 959, 1911. 



•East, E. M., Ann. Ept. Conn. Agr. Expt. Sta, 1910, p. 139. 

 375 



