464 H. J. MULLER 
divisions of the parent prior to the separation of its somatic 
(or at least epidermal) tracts from the germinal tract have not 
been shown, inasmuch as mutations occurring then would often 
be visible in the soma of the parent, and so could not count as 
mutations discovered in the offspring. The diagram is con- 
tinued to approximately the same point (primordial germ-cell 
formation) in the offspring, the first three cell cycles of the latter 
being shown, because a mutation occurring then might still be 
visible either as a ‘self’ or as a mosaic mutant in the individual 
of this generation. 
Each separate line in the diagram represents a particular cell, 
in its passage through its individual cell-cycle; a fork in the line 
represents a cell division, and each line is supposed to be taken 
as proportional, in its horizontal projection, to the length of 
time during which that cell remained undivided. Hence, time 
is the abscissa of the diagram, and any vertical section of the 
diagram must show, for a particular moment, just what cells 
exist (in the individual and its offspring) in which mutations 
that would be visible in the offspring might occur. Those lines 
of cells in the gonad which are abortive in reproductive function, 
such as those resulting from the four divisions of the ‘multi- 
plication period,’ which lead to the fifteen nurse cells for each 
oocyte, and those giving rise to the polar bodies, are only indi- 
cated at their inception, by means of very short broken lines. 
Those cell lines leading to offspring that do not carry the chromo- 
some whose process of mutation is under consideration and those 
cell lines leading to offspring in which the mutant gene could 
not be detected (all the females in the case of recessive sex- 
linked mutant genes) are shown as short cross-hatched lines, in 
which no attempt is made to indicate the further course of cell 
divisions. On the other hand, the main lines of the parental 
germ tract and all lines culminating in viable F, individuals 
which would carry and show the mutant factor, provided it 
arose, are shown as continuous uninterrupted lines. The 
moment of fertilization (or rather, nuclear fusion) is indicated as 
a fusion of two lines (making a - figure immediately following 
the long growth period line and between the two small polar 
