Striations and Bands 75 



possibilities as a tool for studying genetics were not realized 

 for a long time, and during the end of the last century and the 

 early part of the present century they were merely regarded as 

 a puzzling curiosity of no known significance or importance. A 

 little over fifty years after discovery these large chromosomes 

 were correctly interpreted by Heitz and Bauer in Bibio hortu- 

 lanus and by Painter in Drosophila, and since then they have 

 been studied very intensively. 



In addition to being much larger than ordinary chromosomes, 

 the chromosomes in the salivary gland nuclei are atypical in 

 several other respects. Although the nuclei in which these 

 chromosomes are found are not dividing and will not divide 

 again, the chromosomes are not in a typical resting stage but 

 appear to be in a permanent prophase stage with the two homo- 

 logues of each pair of chromosomes closely paired throughout 

 their length. The pairing of homologous somatic chromosomes 

 is certainly not general but is common in the Diptera, where it 

 occurs in other somatic cells as well as in salivary gland cells. 

 The salivary gland chromosomes are therefore mitotic prophase 

 chromosomes which have uncoiled and lost the "relic coils" of the 

 previous division and which show a marked somatic pairing. 



Striations and Bands 



Ordinary somatic chromosomes consist of one or two thin 

 chromonemata or gene strings, but the number of chromonemata 

 in these giant chromosomes is considerably greater. In typical 

 somatic chromosomes, the division of the chromonemata is 

 shortly followed by the division of the chromosome itself, so that 

 the number of chromonemata in a chromosome is never large. 

 In the salivary gland chromosomes of the Diptera, the chro- 

 monemata divide a number of times, but these divisions are not 

 accompanied by division of the chromosome as a whole. The 

 result is that a number of these fine threads will be embedded 

 in a common matrix. The number varies with different mem- 

 bers of the group. In Drosophila it appears to be about 64, but 

 in other members of the Diptera the number is larger. In 

 Chironomus it is about 400. These chromonemata are completely 

 uncoiled and lie parallel and close to one another throughout 

 the length of the chromosome. They are not uniformly thin, 



