to the state of refinement necessary to exclude the possibUity that genetic 

 material may also lie in the interband regions. 



Not all features of the developmental sequence of the salivary chro- 

 mosomes are known with certainty. It is generally considered that the 

 following events occur: (1) intimate synapsis of homologous chromo- 

 somes, each of which is double and longitudinally differentiated into a 

 series of chromomeres, and (2) increase in length and in diameter. The 

 mechanism of length increase is not known. It may involve some form of 

 "molecular stretching" or actual growth by intussusception. Increase in 

 diameter is quite clearly the result of replication of the component 

 strands to a degree variably estimated to be from 500 to 16.000. The 

 bands on this view would consist of side-by-side association of the 

 replicated chromomeres. Not all workers agree with this multistranded 

 or polytene hypothesis. The most common alternative hypothesis, which 

 has several variants, proposes that there are in reality only four strands 

 — swollen especially in the interband regions but not otherwise consisting 

 of any more basic units than more typical chromosomes. 



The salivary chromosomes of many species show local variations in 

 the degree of condensation of the bands, some bands being discrete and 

 sharply defined, while others appear as diffuse "puffs." Such modifica- 

 tions in chromosomal structure occur in different cell types and at spe- 

 cific times during larval growth. The formation of puffs or diffuse 

 swellings generally occurs at single bands or interbands. Puffing is appar- 

 ently not confined to a single band in the chromosome since adjacent 

 loci may form puffs simultaneous with, or independent of, other loci 

 (Pavan, 1958). In the salivary gland cell the chromosomes are also 

 known to develop puffs called Balbiani rings which are larger than the 

 more typical puffs formed by the giant chromosomes in nuclei of both 

 the salivary gland and other dipteran larval tissues (Figure 4-19). 

 According to Beermann (1956) the chromonemata running through a 

 specific band of the salivary chromosome are spun out laterally to form 

 a series of small loops which together form the Balbiani ring (Figure 

 4-20 (a) and (b)). Pavan (1958) favors the view that most puffs are 

 the result of interaction of a specific locus of the chromosome and the 

 nuclear environment. According to this view, the production of chromo- 

 somal material increases the diameter of the chromosome at a specific 

 point or band and tends to spread adjacent bands apart. The band in- 

 volved in puff formation reacts actively with the nuclear sap to produce 

 the main body of the puff. The formation of puffs, particularly Balbiani 

 rings, has been interpreted as indicating differential gene activity taking 



STRUCTURE AND FUNCTION OF THE NUCLEUS / 97 



