76 Special Chromosomes and Sex Inheritance 



but contain a large number of deeply staining chromomeres, 

 some large, some small, arranged so that the threads appear like 

 strings of loosely strung beads of different sizes. Since all the 

 chromonemata within a common matrix arose from one original 

 chromonema by successive divisions, they should be alike. 

 Apparently they are, for the chromomeres on one chromonema 

 are identical in size and position with the chromomeres on the 

 sister chromonemata. All the chromonemata have a chromomere 

 at exactly the same place on the thread. Since the chromonemata 

 are very close to one another, the chromosomes are in contact 

 laterally. An aggregation of identical chromomeres on the 

 numerous chromonemata appears as a cross-band or disc at 

 right angles to the long axis of the chromosome. One of the 

 outstanding features of these giant chromosomes is the presence 

 of these numerous cross-bands which differ in thickness and in 

 staining capacity and are at various distances apart. Their 

 properties depend upon the size and nature of the chromomeres 

 of which they are composed. 



One other feature of the bands is of great genetic significance. 

 In size, position, and sequence the bands in one chromosome are 

 identical with those in the homologous chromosome. Therefore, 

 when the two homologues are paired, as they always are in the 

 salivary glands, the bands of one must lie exactly alongside the 

 corresponding bands of the other. This is strictly true. Pairing 

 of two homologous salivary gland chromosomes is very precise, 

 just as it is in zygotene of meiotic chromosomes. This is so 

 universal a rule, that if a piece of one of the paired homologues 

 involving several bands is deleted by X-rays, the bands of the 

 other homologue corresponding to those deleted have no bands 

 with which to pair and form a loop to one side. The missing 

 bands in no way alter the pairing of the bands which are pres- 

 ent in both homologues. One of the valuable features of these 

 giant chromosomes is the evidence they give of the nature of 

 chromosome pairing. 



Bands and Genes 



The bands appear to contain a large amount of nucleic acid, 

 whereas the nonstaining or lightly staining regions between the 



