462 H. J. MULLER 
mutants. As a matter of fact, the great majority of the gonial 
cells produced at a given time fail to develop, for in the case of 
the sperm it is notorious that only a minute fraction ever achieve 
fertilization, whereas in the case of the eggs of Drosophila, before 
the production of each oocyte the oogonium probably divides 
four times (multiplication period), giving rise to a cyst of six- 
teen cells, only one of which completes the process of matura- 
tion. But of those gametes descended from the mutant cell 
which do succeed in forming zygotes, only half carry the mutant 
factor under consideration, owing to its being sorted into half 
the cells at the segregation division. Of those zygotes, in turn,. 
which come to carry the mutant factor, only the males—in the 
case of recessive sex-linked factors—will be able to show it; thus 
the cases available for the manifestation of the factor are reduced 
by still another 50 per cent. For these reasons, a large propor- 
tion of those mutations in gonial cells which give rise to several 
mutant gonia or gametes would appear to be mutations in 
gametes or cytes, owing to the fact that only one gamete from 
the mutant gonium happened to develop and to form an indi- 
vidual which bore and showed the mutant gene. 
In order to know just how much to allow for these three 
circumstances, it would be necessary to know exactly the whole 
cell lineage of the germ tract, including the length of time that 
each cell remained ‘at rest’ in every cell generation (or the 
length of time that it remained in those parts of the cell genera- 
tion during which mutation may occur), as well as the number 
and distribution of the gametes which subsequently underwent 
fertilization and development into viable males. We may, 
however, gain a much clearer conception of how these features 
of the cell lineage influence the numbers in which mutants may 
appear among a batch of offspring, if we consider a greatly sim- 
plified diagram, roughly representing the cell-lineage of a parent 
individual (P; female) and her progeny (F), during those por- 
tions of their life-cycles in which the occurrence of a mutation 
would lead to a character variation first visible in the offspring 
generation (F,). The diagram (fig. 1) hence starts with what 
might be called the ‘primordial germ cell’ of the parent; cell 
