616 Validity of the Doctrine of Mutation. 
Before we proceed to a consideration of the facts 
let us first examine the question itself more closely. We 
may ask in which periods in the life of a plant the con- 
ditions for the appearance of mutation may be different. 
The life of a plant may be divided into the vegetative and 
the reproductive period, and there are therefore four 
possibilities to be taken into consideration. First we 
have the two periods themselves, then the two transi- 
tions from the one to the other, viz., the origin of the 
germ cells (the moment of the so-called numerical re- 
duction of the chromosomes of the nucleus) and the 
fertilization, which latter is the beginning of the vege- 
tative life. Confining ourselves first to general considera- 
tions, the processes involved in the origin of the sexual 
cells are obviously of a much more complicated nature 
than those involved in fertilization. On the other hand 
the sexual cells are usually regarded as more susceptible 
than the vegetative organs. For these reasons therefore, 
we might be inclined to assume for the moment of muta- 
tion not that of fertilization but some previous point of 
time. How much earlier, then becomes a further ques- 
tion. 
In bi-sexual or Mendelian crosses segregation occurs 
during the maturation of the egg- and pollen-cells, and 
it is complete when these elements are being formed. 
For the egg- and pollen-cells of the monohybrids are no 
more of a hybrid nature, but entirely assume the one or 
the other of the two parental types. The same thing 
must hapen in mutation, for all the evidence seems to 
indicate that the egg- and pollen-cells have already mu- 
tated before they unite in fertilization. 
Of course, as a rule the mutation will not find external 
expression until the germ develops, and thus the new 
