SOMATIC MITOSIS AND CHROMOSOME INDIVIDUALITY 165 



has been recently thrown upon such apparent variations in number by 

 McClung (1917) and Miss Holt (1917) in their researches on multiple 

 chromosomes and chromosome complexes. 



McClung finds in his analysis of the chromosome groups of the 

 orthopterans Hesperotettix and Mermiria that temporary associations 

 often occur between various members of a group, with the resulting forma- 

 tion of "multiple chromosomes" and a consequent decrease in the appar- 

 ent number. In Hesperotettix, for instance, the cells normally have 12 

 pairs of chromosomes, but because of the formation of such multiple 

 chromosomes individuals with apparently 11, 10, or 9 pairs are frequently 

 found (Fig. 57). For a given individual the number so formed is exactly 

 constant, since the members of a multiple remain together in all the cells 

 of the body; but for the species it is variable within certain limits, owing 

 to the varying numbers of chromosomes which may become involved in 

 such multiple combinations. In all cases the full number of chromosome 

 pairs is present, but some of them are so combined that there is an appar- 

 ent, though not actual, variation in the number. A similar condition is 

 found in other forms by Robertson (1916). 



In Culex there are three pairs of chromosomes in the somatic cells. 

 During a certain stage in the insect's metamorphosis it has been shown 

 by Miss Holt (1917) that the chromosomes may split repeatedly, giving 

 cells with much larger numbers up to 72 in some cases. These larger 

 numbers, however, are nearly always multiples of three, indicating that 

 the subdivision of the chromosomes is an orderly process. The daughter 

 chromosomes, moreover, that are formed by the subdivision of each of 

 the original six, remain more or less closely associated as a "multiple 

 complex," which behaves as a single individual in mitosis. It therefore 

 appears that the three pairs of chromosomes "are made up of quite 

 distinct individuals differing from each other to such a degree that 

 chromatin split from one cannot associate itself with that from another 

 pair. . . . Chromosome individuality, alone, can account for these 

 conditions." 



Somewhat similar evidence has been brought forward by Hance (1917, 

 1918a6). Hance finds that the chromosome number in the spermatogo- 

 nia of the pig is regularly 40, whereas in the somatic cells it varies from 

 40 to 57. Similarly in (Enothera scintillans, which has 15 chromosomes 

 in its microsporocytes, there may be from 15 to 21 chromosomes in the 

 somatic cells. Measurements of the members of the various chromosome 

 groups show that the larger numbers are due to a fragmentation, prob- 

 ably of the larger chromosomes, in the somatic cells. Such fragments 

 divide normally, and it appears probable that the fragments of a single 

 original chromosome are held together by colorless portions and behave 

 . as a unit, much as do the multiple complexes of Culex. 



Sakamura (1920) believes that the chief reason for frequently reported 



