200 INDIVIDUALITY OF CHROMOSOMES [CH. 



species with relatively few chromosomes, such as grass- 

 hoppers and a number of Diptera, each chromosome is 

 easily recognisable in the equatorial plate of any division, 

 and MEEK has shown that in the spermatogonial and sperma- 

 tocyte divisions each chromosome has constant dimensions. 

 In division-figures of nuclei which have the diploid number 

 there are of course two of each kind (excluding the sex- 

 chromosomes, which may be unpaired), and in the Diptera 

 the two homologous chromosomes commonly lie side by side 

 on the spindle in all somatic mitoses. When the maturation 

 period of the germ-cells approaches, these pairs of homo- 

 logous chromosomes unite to form bivalents (gemini), and 

 separate to opposite poles in the reducing division, with the 

 result that each germ-cell contains a single complement in- 

 cluding one of each kind. The individuality of the sex- 

 chromosomes (heterotropic or idio-chromosomes) is of course 

 a still more striking example of the same thing the con- 

 stant appearance of a definite chromosome of characteristic 

 behaviour in every cell of the individual. As their behaviour 

 has been described in Chapter XI, no further reference to 

 them is needed here. 



The argument for chromosome individuality from the 

 definiteness and permanence of appearance and behaviour 

 of particular chromosomes in ordinary individuals of many 

 species is strongly reinforced by the examination of hybrids. 

 It has been observed in a number of cases that when a 

 hybrid is made between two species in which the chromo- 

 somes are recognisably different in size, both kinds of 

 chromosomes retain their characters in the hybrid. The 

 example already referred to in Chapter VI will suffice to 

 make this clear. The moth Biston hirtarius has in its diploid 

 cells 28 chromosomes, of which 24 are moderately large and 

 four quite small. The nearly related moth Nyssia zonaria 



