V NUMBER OF CHROMOSOMES 147 



Number of Chromosomes 15 21 22 23 29 30 

 Number of Plants .. 2 16 25 3 2 4 



This variation is probably due to variation in the lata gametes, since 

 0. gigas breeds true and therefore presumably always has the expected 

 fourteen chromosomes in its gametes. It will be noticed that the most 

 frequent numbers are 21 and 22, which corresponds with the above- 

 mentioned fact that the gametes of 0. lata most commonly have seven 

 or eight chromosomes. These combining with the fourteen of the micro- 

 gamete produce of course these two diploid numbers. The six plants 

 with 29 and 30 chromosomes are probably examples of a phenomenon 

 to be described in the next section ; namely, they have presumably been 

 produced by fertilization of diploid egg cells. 



(3) Variation in Chromosome Number due to Multiplication or Fusion 



Longitudinal fission of the chromosomes, as in ordinary prophase, 

 but without subsequent mitosis leading to separation of the daughter 

 halves, has frequently been described. By this means giant nuclei are 

 formed containing twice the normal number of chromosomes if the 

 process has taken place once, four times if it has happened twice, and 

 so on. 



One of the most striking cases described is that of Culex pipiens 

 (Holt, 1917). Here 2w = 6. During metamorphosis, while the alimentary 

 canal of the pupa is developing, the chromosomes in the cells of the old 

 larval alimentary canal undergo repeated fission, forming nuclei with a 

 large number of chromosomes, but always in multiples of three, and 

 generally in multiples of six. Nuclei were found, for example, with 9, 

 12, 18, 24, 36 and 72 chromosomes. Nuclei with this enormous increase 

 of chromosomes can proceed to apparently normal mitosis. In telophase 

 the chromosomes fuse into three masses (it will be remembered that in 

 the normal somatic mitoses of C. pipiens the pairing of the homologous 

 chromosomes is very pronounced, so that three double chromosomes 

 result, p. 126), from each of which in the next prophase one chromatin 

 filament is formed, which divides up by multiple longitudinal fission so 

 that three groups of chromosomes are produced. The chromosomes 

 within each group are of closely similar lengths, though the length type 

 is different in the different groups. 



These cells, being in the larval ahmentary canal which is being re- 

 placed by that of the pupa, are destined to perish. Nuclei with a multiple 

 supply of chromosomes seem often to be produced in this way, however, 

 and the process as such does not seem to have a necessarily deleterious 

 effect. 



