396 AN INTRODUCTION TO MODERN GENETICS 



The differential staining behaviour of the heterochromatic regions 

 presumably depends on a chemical composition or physical state dif- 

 ferent to that of the euchromatin; but in general very Httle is known 

 about this. In salivary glands of Drosophila, the inert^ regions show a 

 structure of longitudinal striations and transverse bands which is some- 

 what similar to that of the euchromatic regions, except that the bands are 

 more feebly staining and the whole structure less clear-cut. Although 

 the inert region of the X chromosome, for instance, is fairly short in 

 the salivary gland chromosomes and has only a small number of bands, 

 it occupies a large proportion of the whole chromosome at metaphase 

 of mitosis. This may perhaps be partly due to a lesser degree of 



3^^ '>.;.:«r.-.-..^.-X',:--. 



LI 



-^•'•'.,,,^.:..».VvW; 



Fig. 159. The Structure of the Chromo- 

 centre (Inert Region) in Salivary Gland 

 Nuclei. — The right arm of the third 

 chromosome is missing. The figure is from 

 a male and the V chromosome can be seen 

 as a small lump pairing with the inert 

 region of the X. 



(From Prokofleva-Belgovskaya.) 



spiralization in mitosis, although, since the region at that time is 

 definitely shorter than it is in salivary chromosomes, some spiralization 

 must occur. It is probable that the large relative volume of the inert 

 region in mitosis is at least partly due to an abnormally large concentra- 

 tion of nucleic acid on to it at this stage. Muller^ claims that the greater 

 bulk of the mitotic region is produced under the influence of only two 

 loci in the region, and it is conceivable that the region contains loci 

 specially concerned with the synthesis of nuclei acid. 



The physical and chemical basis of this structure is unknown, bat 

 it is remarkable to find that the conditions underlying it appear to be 

 transmissible; when inert regions are brought, by translocation, into 

 contact with euchromatic parts of the chromosomes, there is a tendency 

 for the latter to be modified in their appearance in salivary glands, so 

 as to assume more nearly the inert structure.^ This suggests that the 



^ Bridges 1935, 1938, Prokofieva 1935. 



^ Muller 1938, Muller and Gershenson 1935. 



' Prokofieva 1935, Schultz and Caspersson 1938, 



