THE MANNER OF OCCURRENCE OF. MUTATION 467 
oceur than of multiple mutants could not be used as evidence 
for a higher mutability in the later stages of the germ-tract cycle. 
The reader may perhaps have been disturbed, in accepting 
these conclusions, by the consideration that, in the case of any 
one mutant individual, there can be no more a priori likelihood 
of the mutant-factor which it contains having arisen in one stage 
of its ancestry rather than in another (of equal duration). This 
claim is perfectly valid. If we assume that a cell in one period 
of the life-cycle is no more likely to mutate than in another 
period, then the number of mutant individuals resulting from 
a 
Figure 3 
Fig. 2 Scheme of symmetrical cell multiplication. 
Fig. 3 Scheme of egg production from a single stem 
cell 
Figure 2 
mutations in different periods of equal lengths would be exactly 
equal in frequency. In each of those cases, however, in which 
the mutation happened early in the germ tract, many individuals 
would be formed bearing the same mutation, and there would be 
relatively few such cases of mutation, since few cells exist in the 
early stages; in each of those cases in which the mutation hap- 
pened late, only one or a few individuals would bear the par- 
ticular mutant gene in question, but there would be correspond- 
ingly many such cases, owing to the fact that there are so many 
different cells in existence in the later stages which would be 
capable of giving rise to a mutant. Thus the number of mutant 
THE JOURNAL OF EPXPERIMENTAL ZOOLOGY, VOL. 31, No. 4 
