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 



