SPECIAL PROBLEMS OF MITOSIS 36 



that in many other organisms some chromosomes or 

 regions of chromosomes may be genetically inert ; 

 for this reason we must not expect the number of 

 linkage groups and the haploid number of chromo- 

 somes to correspond in all cases. 



Salivary Gland Chromosomes in Diptera 



In the nuclei of tha salivary glands in the Diptera 

 (and also in the nuclei of the rectal epithelium and 

 the Malpighian tubules, which show an essentially 

 identical structure) the chromosomes appear as 

 enormously enlarged threads whose structure could 

 not for a long time be interpreted in terms of ordinary 

 mitotic chromosomes. The first clue to the analysis 

 came when it was realized that in the salivary gland 

 nuclei the phenomenon of somatic pairing is developed 

 to the point where the homologous chromosomes are 

 completely in contact throughout their length. ^^ As 

 a matter of fact it is often possible by means of 

 crushing under a cover -glass to separate slightly the 

 homologous chromosomes ; when this is done it will 

 be seen that they are spirally wound round one 

 another as in a piece of two-strand rope.^^ Xhe 

 number of these threads thus corresponds to the 

 haploid, and not to the diploid number of chromo- 

 somes, but the two arms of V-shaped chromosomes 

 are represented by separate threads ; thus in the 

 female Drosophila melanogaster with eight chromo- 

 somes in the diploid set, four of which are V-shaped 

 the salivary gland nuclei contain six threads (i.e. 

 one representing the two X- chromosomes, one for 

 each of the arms of the Ilnd and Ilird chromosomes 

 and a short one representing the two IVth chromo- 

 somes). In Drosophila all these six threads are 

 attached at one end to a body called the chromocentre, 

 to which the nucleolus is also attached by a thread, 

 but in many other Diptera (e.g. Bibio and Chironomus) 

 there is no chromocentre, the threads being separate 

 and unconnected. 2. ^5 Apparently the chromocentre 



