THE MANNER OF OCCURRENCE OF MUTATION 453 
The mutant fly was mated to wild-type females, and proved to 
be fertile, but all the flies in Fi, F:, and subsequent generations 
had the normal red eye, although forked appeared in the expected 
proportions. It is almost certain that the eye color variation 
had been genetic, for environmental variations of anything like 
such magnitude never occur in flies with the factors for normal 
eye color. It is also probable that the variation had taken 
place in the W locus, for, of all the twenty or more different 
mutations for eye color in other loci, none have ever produced so 
marked a dilution of the eye color, whereas seven of the nine 
different mutant allelomorphs of W have resulted in eye color 
at least as light and of the same general ‘quality.’ (For that 
matter, the chances preponderate that any new and different 
sex-linked mutation in eye color chosen at random should le at 
this locus, since more different mutations have occurred here 
than in all other eye-color loci of the X chromosome combined.) 
In this case the mutation. in W—if such it were—must have 
occurred early in development in one of the purely somatic 
cells. The time of occurrence must have been later than the 
separation of at least one epidermal nucleus from the common 
epidermis-germ tract nucleus, but before the division of this 
epidermal nucleus had proceeded far enough to form separate 
nuclei destined for the two sides of the head.’ Assuming that 
the mutation did occur in the W locus, it must have involved 
a change of the normal allelomorph, W, not of the white gene, 
w, since the breeding tests showed that the male was a non- 
crossover which had received the gene for red. 
6 Development in insects is proved by such observations on mosaics of 
Drosophila to be indeterminate to the extent that a nucleus of given lineage 
may enter into different portions of the epidermis. For example, in the last 
case, the two eyes were both derived from a single somatic cell after the latter 
had separated from the common epidermis-germ tract, whereas in the case of 
the new white the right-eye anlage separated from the common tract after the 
anlage for the left eye had already separated from it. The circumstance that 
nuclei of somewhat different lineage may thus, in different cases, enter into the 
formation of the same part of the body is probably to be explained by the lack of 
cell walls and the migration of nuclei during early development in the insect egg. 
It offers another illustration of the ‘equivalence of nuclei.’ 
