Foundations of the Mutation Concept. 17 



Mutations arising at or after Fertilization. 



De Vries assumes that all mutations take place in the formation 

 of the germ cells, that is presumably at the time of the meiotic 

 divisions, and there is a great deal of evidence that many 

 germinal changes arise at this time. During this period the 

 pairs of chromosomes are being redistributed, and the intricate 

 manoeuvres of the chromatin material just previous to this 

 separation of the chromosomes gives an opportunity for a variety 

 of rearrangements and aberrant distributions of particular chromatin 

 particles. That the hereditary behaviour known as crossing-over, 

 by which part of the factors which were in the same chromosome 

 cross over to its mate, has its basis in changes occurring at this time 

 has been shown experimentally for Drosophila by Plough (1917). 

 He found that by subjecting the females to high or low temperature 

 the amount of crossing-over was increased. Further experiment 

 showed that the critical period, during which crossing-over in the 

 offspring was affected, was during the period of conjugation of the 

 chromosomes in the maturation of the eggs. From this it follows 

 that crossing-over takes place at that time and not earlier. 



In the CEnotheras it has been shown that many of the 

 irregularities leading to new chromosome numbers arise during 

 meiosis. The origin of mutations with an extra chromosome and 

 the phenomena of non-disjunction in Drosophila, of the sex- 

 chromosomes (failure to separate in meiosis), are cases in point ; 

 also the origin of the supernumerary chromosomes in Metapodius 

 (Wilson 1909), which arise through irregularities in chromosome 

 distribution during spermatogenesis. In addition, there is evidence 

 both in plants and animals that germ cells with a double number of 

 chromosomes are occasionally produced, through an alteration in 

 the meiotic phenomena. These matters will be discussed in 

 connection with triploidyand tetraploidy. It will be found that the 

 hypothesis of de Vries (1913) that triploid and tetraploid mutants 

 arise only through the union of germ cells one or both of which is 

 diploid is unnecessary, since both these conditions may arise in 

 other ways. 



The period of fertilization is an equally critical time in the life 

 cycle of an organism, and the evidence indicates that a number of 

 germinal readjustments or mutations date from this period. Both 

 triploidy and tetraploidy may perhaps arise at this time or shortly 

 after. It has already been pointed out 1 that in crosses such as 

 1 The Mutation Factor, p. 222. 



