THE MITOTIC CYCLE 



a clear example of chromomeric structure. This question has further 

 been extensively discussed in relation both to the giant chromosomes of 

 the Diptera, and the 'lamp-brush' chromosomes of the vertebrate 

 oocyte. 



Salivary chromosomes 



The nuclei of the salivary glands of larval Diptera are an extreme 

 instance of a general mode of growth during this stage of cells and tissues 

 of these insects. During embryonic development, the larval organs soon 

 attain their full number of cells ; henceforth all further growth is by 

 increase in cell-size. Painter^^^ suggests that salivary chromosomes 

 repeatedly undergo *endomitotic' growth cycles, and believes that the 

 variability of their reactions towards alkalis is indicative of such changes. 

 Each salivary chromosome at first consists of the chromatids of the 

 diploid pairs. They subsequently increase in length to about one 

 hundred times that of the corresponding normal chromosome, and the 

 whole nucleus increases over two hundredfold in volume (Figure 15).. 

 The characteristic bands of the salivary chromosomes are established 

 early in larval life, long before the chromosomes have completed their 

 growth in length and width (Hinton^^^). It is well known that Pain- 

 ter133 showed that the sequence of the bands in the sahvary chromo- 

 somes of Drosophila was related to that of the genes deduced from geneti- 

 cal evidence. It has been tacitly assumed that the banding of giant 

 chromosomes in other larval tissues would correspond; Berger^^* 

 showed that this was true for salivary and mid-gut chromosomes in the 

 larvae of Sciara, but later Kosswao and Shengun^^^ demonstrated that 

 in Chironomus larvae, the bands were not constant in corresponding 

 chromosomes of the sahvary glands, the Malpighian tubules, and the 

 rectal epithelium. 



It has been argued by Metz and his colleagues (Metz and 

 LawhrencEjI^^ Metz^^'^) that if the salivary chromosomes are made up 

 of chromonemata, then the unitary threads must be much larger than 

 those of normal nuclei, which would be sub-microscopic in diameter 

 if stretched to the length of the salivary chromosomes. The longitudinal 

 striations which are seen in extended salivary chromosomes on this view 

 cannot separate normal chromonemata. Metz believes that the basic 

 structure of the salivary chromosomes is an alveolar one ; on the other 

 hand Ris and Crouse^^^ argue that they are composed of spiral chro- 

 monemata, and that the bands are formed at particular cross-sections 

 of the chromosomes where the coils of all the chromonemata are dis- 

 posed at right angles to the long axis of the whole structure. Kosswig 

 and Shengun^^^ suggest that the variety of banding in the same 

 chromosome in various tissues of Chironomus is due to differences in the 

 coiling of the constituent chromonemata. Experimental investigations 



96 



