THE MANNER OF OCCURRENCE OF MUTATION 471 
b. Ivory, causing a light yellow eye color only slightly darker 
than écru. 
c. White, producing a color identical with the original ‘white.’ 
All three when crossed to white or eosin give females inter- 
mediate in eye color between those of the two parent races. 
The males of the écru and of the white race are like the homo- 
zygous females, but ivory males are slightly lighter than homo- 
zygous ivory females. . 
2. Keru first appeared in a single male, so that it is likely 
that the mutation occurred in a late odgonial cell, odcyte, or 
egg of the mother. 
3. Ivory, found by Sturtevant, appeared in 9 out of 141 male 
offspring of a single female; this mutation must have occurred 
in an early stem cell of the ovaries of the mother. 
4, The new white appeared in a single male, which had one 
eye red and one white; this mutation therefore occurred at a 
stage corresponding to early cleavage. 
5. A single male with orange-colored eyes was found; it did 
not transmit its mutant eye color. Indirect evidence makes it 
probable that this was due to another allelomorph of W which 
arose from the normal allelomorph in a somatic cell of the very 
early embryo. 
6. There is no evidence that mutations are more likely to 
occur in gametes or germ cells near the period of maturation 
than in cells at any other stage in the life-cycle. The peculiari- 
ties of cell lineage would, however, provide a greater chance for 
the appearance of single mutant individuals than for cases of 
twin or multiple mutants; since there are more cells existing 
during the later stages of the germ-cycle, there is a correspond- 
ingly greater chance for mutations to occur there in one cell or 
another, even though mutations may occur equally readily in 
cells at any stage. A method is described for estimating the 
effect of any given type of cell lineage upon the relative numbers 
of fractional, single, twin, and multiple mutants produced. 
7. The fact that mosaic. mutants involving recessive sex- 
linked genes are always males indicates that mutations occur in 
only one member of a pair of chromosomes at a time. The 
