96 FUNDAMENTALS OF CYTOLOGY 



No less striking is the visible structure of the salivaiy chromosomes, 

 and it is largely this feature which is responsible for their great value in 

 cytogenetics. Even in the living and unstained nucleus it can be 

 seen that they have conspicuous transverse bands (Fig. 69) . The longest 

 chromosome in Drosophila melanogaster has more than 2000 of thesc^ 

 bands. In fixed matei'ial prepared with the acetocarmine and Feulgen 

 techniques the bands stain vigorously, leaving the interband regions 

 weakly stained or colorless. The differentiation with the latter technique 

 is particularly sharp, showing that the thymonucleic acid is restricted 

 almost entirel}^ to the bands. Ultraviolet-absorption studies lead to the 

 same conclusion. 



Each band appears to be composed of one or more discs of chromatic 

 granules extending across the cylindrical body of the chromosome. 

 These granules have been called chromomeres, although their \'ariation 

 in size and number, even in a given band, indicates that here this term 

 does not designate units all of the same rank. One general interpretation 

 placed upon the whole chromosome is that it consists of a large number — 

 dozens or even many hundreds — of chromonemata that have multiplied 

 as the nucleus grew without any mitoses to separate them, their chromo- 

 meres remaining closely associated or united laterally as the discs. 

 Studies of the earlier stages in the development of salivary-gland nuclei 

 lend some support to this view. If, however, the chromonemata in the 

 fully developed salivary chromosome, which may have a thousand or 

 more times the volume it had before enlargement, are comparable to the 

 original chromonema, they must be present in very great numl^ers ; more- 

 over, if an ordinary chromonema were extended to the length of the 

 salivary chromosome without the addition of new material, it would be of 

 submicroscopic thickness. The longitudinal fibrils seen connecting the 

 discs in stretched chromosomes probably do not, therefore, represent indi- 

 vidual chromonemata. Some workers regard them as large bundles of 

 chromonemata, wiiile others interpret them as distortions of an alveolar 

 structure pervading the chromosome (Fig. 70) . The finer structure of the 

 salivary chromosome is at present a ver}^ controversial subject, although 

 it is agreed that the transverse bands are natural features having the 

 cytogenetic usefulness indicated below-. 



Of the greatest importance is the fact that the bands form a pattern 

 that is constant for a given chromosome. On the basis of differences in 

 size, spacing, and other characteristics of the bands it is possible for the 

 investigator to distinguish particular regions of the various chromosomes. 

 Data of this kind have been recorded in pictorial "chromosome maps." 

 The usefulness of band patterns to the cytogeneticist should be obvious. 

 In later chapters it will be shown how variations in genetical characters 

 and even differences betwx^en races and species may be correlated with 



