158 OTTO L. MOHR 



The embryology of Drosophila has in this respect not yet been 

 worked out. In a general sense it must be added, — as also em- 

 phazised by the authors mentioned — that some reservation is ne- 

 cessary when it is a question of deductions from the distribution of 

 characters in mosaics and gynandromorphs to the distribution oi 

 segmentation nuclei, as long as our knowledge as to the destiny of 

 the cells derived from different embryonal anläge (ventral plate, 

 imaginai plates) is still defective. Our lack of exact knowledge as 

 to this point may account for the apparent inconsistency here 

 discussed. 



— x\ttention should finally be called to the fact that cases like 

 the one here described to a certain extent modify the familar concep- 

 tion of the mutation as a hereditary change in the genotype. Muta- 

 tional changes can, of course, only be inherited if they occur in the 

 germ tract or in the common germinal-somatic blastomeres. If en- 

 tirely identical genotypical changes occur in a purely somatic cell 

 there is no possibility of their being inherited. They can here be 

 transmitted only to the somatic daughter cells through ordinary cell 

 division. A mutation is a change in the genotype of a cell due to causes 

 other than ordinary segregation and recombination of genes. Whether 

 this change can be inherited or not is of no importance for the de- 

 finition. 



SUMMARY. 



1. A striking case of somatic mutation has been described. The 

 mutation, singed^, which produces a very typical curling of all the 

 bristles and hairs, appeared in a mosaic found in a Il-chromosome 

 stock bottle. 



2. The mosaic had normal male sex characters. The entire 

 left half of the individual, except for the head, was singed^ in contrast 

 to the right half, including the head, which had normal wild-type 

 bristles and hairs. The border line separating the singed^ and the 

 normal regions could be controlled in every detail. It followed the 

 median plane, except on the dorsal side of the thorax, where the 

 mutant zone proceeded over the median line, including the upper 

 half of the right lateral surface of the thorax. From here the singed^ 

 region continued along the collum to the head, where a narrow, 

 ventro-lateral plate (the bucca) had singed hairs and bristles on the 

 right side. 



3. The mosaic was fertile, and tests proved that the mutant 



