THE MORPHOLOGY OF THE CHROMOSOMES 123 



natural groups, especially the insects. ^^ In Drosophila melanog aster, so 

 extensively used in genetical studies, the set is made up of four unlike 

 members: one medium-sized chromosome with terminal spindle-attach- 

 ment region (chromosome I), one large chromosome with median attach- 

 ment (II), one slightly larger chromosome of the same type (III), and 

 one very small chromosome (IV) (Fig. 71). In somatic cells and imma- 

 ture germ cells there are two such sets ; as is characteristic of the Diptera, 

 the corresponding chromosomes (the "homologues") actually lie near 

 each other in pairs. ^^ The functional characteristics of these chromo- 

 somes will be discussed in a later chapter. 



The morphology of the chromosome complement has been described in 

 a large number of plants, particularly among the angiosperms. Several 



Fig. 66. — Photographs of Zea chromosomes in late prophase of mitosis in microspore. 



a, monoploid group; note spindle-attachment regions in chromosomes in good focus. 



b, chromosome I. c, chromosome IV, showing "knobs" near lower end. d, chromosome 

 X. Compare Figs. 152 and 170. {Photographs by McClintock.) 



well-known illustrative examples are the following. In Zea Mays^* the 

 monoploid group comprises 10 chromosomes. These can be distinguished 

 from one another on the basis of absolute length, the relative length of 

 the two arms (each has a submedian spindle-attachment region), and 

 other structural characteristics visible at certain stages (Figs. 66, 152, 170). 

 One of them, number VI, has a conspicuous achromatic region setting 

 off a satellite. In some races certain members of the set possess definitely 

 located chromatic "knobs" which differ in character from large chro- 

 momeres; these aid further in identifying the various members. It is 

 known that the chromosome sets of different races of maize vary in 

 certain minor structural details, some such variations presumably being 

 correlated with genie differences. Moreover, certain variations should 

 also be expected in view of the observed fact of occasional translocations, 



12 McClung (1905 et seq.), Harman (1915, 1920), W. R. B. Robertson (1916 et 

 seq.), Carothers (1917, 1921), Metz (1914 et seq.), and many others. 



" Bridges, Morgan, Muller, Metz, Dobzhansky, Painter. 



1^ Kuwada (1919), Fisk (1925, 1927), Longley (1927c), Randolph (1928 et seq.), 

 McClintock (19296 et seq.). 



