VARIATION OF CHROMOSOMES 157 



sorts would result, vi^., 7- and 8-chromosome cells.« Such 

 a plant if crossed to a normal plant should give half nor- 

 mal (14), half 15-chromosome types. Such plants have 

 been shown, in fact, to be produced (Lutz). Other com- 

 binations that would give 22, 23, 27, 29 chromosomes have 

 been reported. 



A variation in the number of the chromosomes of a 

 somewhat different kind has been described by Hance for 

 CEnothera scmtillans, one of the 15-chromosome types of 

 0, Lamarchiana. No variation in number was found in 

 the germ-tract of the same individuals that consistently 

 gave two types of pollen grains, one with 7 and the other 

 with 8 chromosomes. The number of chromosomes in the 

 somatic cells was found to vary from 15 to 21. Some of 

 the groups are shown in Fig. 62. When the 15 chromo- 

 somes of the type-group are measured, it is found that 

 they can be arranged in respect to length in 7 pairs, with 

 one odd one (marked a in the figures). There is also 

 found a constant length difference between the pairs. In 

 those cases where there are more than 15 chromosomes in 

 a cell, measurements show that the pieces can be assigned 

 to particular chromosomes. When this is done. Fig. 63, the 

 lengths of the chromosomes come out as in the typical 

 cells. There can be no doubt that the extra chromosomes 

 in these cases represent pieces that have broken off from 

 typical chromosomes. This process of fragmentation 

 does not destroy the ^'individuality of the chromosomes'' 

 since the increase in this way of the number of chromo- 

 somes would not lead to any immediate change in the 

 number of the genes. The peculiarity of the mutant 0. 

 scintilluns is not connected with the increase in the number 

 of its chromosome bodies, but rather to the presence of a 

 15th chromosome. 



Bridges has called attention to a peculiar case in 

 Drosophila (1917) in which an individual behaves as 



' No pollen is produced by most of the lata plants. 



