The Chromocenter 77 



bands appear not to contain so much nucleic acid. The many 

 chromonemata that make up the salivary gland chromosome are 

 composed of bundles of fibers of complex organic chemical sub- 

 stances known as polypeptids. Part of these bundles of poly- 

 peptid fibers attract nucleic acid, and the remaining parts do not. 

 The parts that contain the nucleic acid stain deeply and form 

 the chromomeres. The fusion of such adjacent deeply staining 

 regions produces a band. If all the chromomeres do not fuse 

 laterally, the band appears as broken. The exact relation be- 

 tween the bands and genes cannot easily be determined. By 

 means of X-rays, certain flies can be produced which have a 

 marked notch in the wing. When the salivary glands of larvae 

 from such flies are examined, one of the bands is usually seen 

 to be missing from one of the chromosomes, although not from 

 its homologue. The many examples of such notch-winged flies 

 that have been found point to the conclusion that every locus 

 corresponds to at least one band. The bands are therefore in 

 some way correlated to genes although it cannot be said 

 definitely that a band is a gene. 



The Chromocenter 



Salivary gland chromosomes exhibit one peculiarity not found 

 in other chromosomes. It has been shown that the Y chromosome 

 is made up largely of heterochromatin and that the X chromo- 

 somes and autosomes have heterochromatic material around 

 their centromeres although they consist mostly of euchromatin. 

 This condition has little effect upon their behavior in meiotic or 

 in ordinary somatic cells, but in the cells of the salivary gland 

 of Drosophila it has a striking effect. In these cells, all the 

 heterochromatic material of all the chromosomes is fused into a 

 mass from which the euchromatic material extends like tentacles. 

 The entire chromatic material appears like an octopus, with a 

 heterochromatic body and five long and one short euchromatic 

 arms. The long arms are the right arm and left arm of both 

 the V-shaped second and third chromosomes and the rod-shaped 

 X chromosome, and the very small fourth chromosome makes 

 up the sixth projection. In the female, all six arms are of 

 uniform thickness for each consists of two paired homologues. 

 In the male, the X chromosome, since it is an unpaired structure, 



