THE ACTIVATION OF GENES BY THE CYTOPLASM 357 



which shows a section of chromosome which can be identified with 

 certainty by the presence of a small inverted section in which no pairing 

 occurs. It can be seen, for example, that bands i and 2 appear swollen and 

 'puffy' in the rectum, but compact in the other tissues, while band 3 is 

 puffy in the salivary glands and Malpighians, band 4 again in the rectum, 

 and so on. Comparable differences may be observed when one compares 

 the chromosomes of the same tissue at different stages of development, 

 for instance at the larval and pupal periods. This is very convincing 

 evidence that the state of activity of a band varies according to the tissue. 

 Beerman suggests that the swollen and puffy appearance indicates a high 

 metabolic activity, and he showed in fact that if larvae are brought out of 

 the cold, in which their metabohsm has been reduced, into a higher 

 temperature, there is a rapid formation of droplets, visible by phase- 

 contrast, in the neighbourhood of the most highly developed 'puffs'. 



Mechelke's work (1953) was done on another chironomid, Acricotopus 

 lucidus in which the salivary gland is subdivided into three lobes, a fore- 

 lobe, mainlobe and sidelobe. All of these contain well-developed polytene 

 chromosomes. In general the banding is rather similar in all three lobes, 

 but there are one or two very clear-cut cases of differential activity, one 

 of which is illustrated in Fig. 16.5. In the mid-larval stages, region no. 33 

 of Chromosome i in the forelobe is enormously swollen into a fan-shaped 



^'^'ip^i^^lU^ 



32 33 





h 



SJ 32 I * 



Figure 16.5 



Figure a, the normal structure of part of Chromosome I of Acricotopus 

 lucidus from the main lobe of the salivary gland; h, the 'Balbiani ring' 

 developed from region 3 3 in the forelobe of the larva ; c, a regressing Bal- 

 biani ring from the forelobe of a prepupa; d, a fully regressed Balbiani ring 

 from a later stage. (From Mechelke 1953.) 



