346 INTRODUCTION TO CYTOLOGY 



A further significant observation jon mutants of this type was that of 

 Gregory (1914) on a tetraploid Primula. He showed by breeding experi- 

 ments that twice the normal number of Mendelian factors are present: 

 thus when the chromosome number is tetraploid the number of factors is 

 also tetraploid; each allelomorphic pair is represented twice. 



It should be stated that not all cases of gigantism are accompanied 

 in this manner by an increase in the chromosome number. In Phragmites 

 communis, for example, Tischler (1918) finds abnormally large size asso- 

 ciated with an increase in the size of the chromosomes, but not in their 

 number. Stomps (1919) points out that among gigas mutants of (Eno- 

 thera, Narcissus, and Primula there are diploid as well as tetraploid 

 individuals, which must mean that the altered chromosome number is not 

 the sole cause of such mutation but is rather one of the characters of the 

 mutant. 



The 15-chromosome Mutants (lata group). The presence of an extra 

 chromosome in the cells of (Enothera lata and other members of this group 

 is due to the fact that the members of one pair of chromosomes in (Eno- 

 thera Lamarckiana fail to separate at the reduction division, both of them 

 going to one -daughter cell. This phenomenon is known as non-disjunc- 

 tion. As a consequence there are gametes with eight and six chromosomes 

 rather than the normal seven; and a union of an 8-chromosome gamete 

 with a normal 7-chromosome gamete results in an individual with 15 

 chromosomes instead of the normal 14 (Fig. 135, C). 



In her study of 15-chromosome mutants Miss Lutz (1917) found 11 or 

 12 types belonging to this group; only two of them were of the usual lata 

 type. This condition may be accounted for on the hypothesis that it is 

 sometimes one pair of chromosomes and sometimes another which fails 

 to separate at the time of reduction, so that the extra chromosome is not 

 in all cases the corresponding one of the complement. If the various 

 chromosomes of the complement differ in hereditary value, as there is 

 much reason to believe, it is evident that this would allow for a great 

 variety of mutants with the same aberrant chromosome number. In 

 (Enothera scintillans Hance (1918) has shown by careful measurements 

 that the extra chromosome can be distinguished from the regular 14. 

 Two classes of gametes are formed, some with seven chromosomes and 

 some with eight (Fig. 135, D) The union of two 7-chromosome gametes 

 gives (Enothera Lamarckiana, the form from which (E. scintillans sprang 

 as a mutant; whereas a union of a 7-chromosome gamete with an 8- 

 chromosome gamete gives (E. scintillans. Hance therefore points out 

 that the scintillans characters are plainly associated with the extra 

 chromosome. (E. scintillans was further observed to give rise to a type 

 resembling (E. oblonga. It is possible that this was due to the union of 

 two 8-chromosome gametes. 



