Bauer (1933) demonstrated their cytogenetic importance. Since that 

 time they have received an enormous amount of attention, much of it, 

 however, having to do with cytogenetic analyses rather than with the 

 question as to their origin, detailed morphology, and physiological sig- 

 nificance. 



On examination, the salivary gland type of chromosome can be seen 

 to be composed of two homologous chromosomes which are more or 

 less closely paired and loosely twisted around each other (Alfert, 1954). 

 Since these chromosomes are really paired units or bivalents, they are 



■5)U- 



Figure 4-17. Drawing of Salivary Chromosome 4 (below) of Drosophila 

 melatwgaster and, on the Same Scale, the Entire Group of Eight Chromo- 

 somes at Metaphase in Gonial Cells of the Same Organism (upper right). In 

 the gonial group, the fourth chromosomes (paired and highly multistranded 

 in the salivary gland nucleus) are indicated by the arrow. Comparison of the 

 salivary chromosomes with those seen in the gonial nucleus reveals the 

 greater wealth of detail visible in the former. (From Bridges, C. B., 1935. 

 "Salivary Chromosome Maps," /. Heredity, 26, Fig. 4, p. 62.) 



present in the diploid number in the tissues in which they are found 

 (Figure 4-16). The salivary type chromosomes are often of enormous 

 size, having lengths which may be more than 100 times those of the 

 corresponding somatic chromosomes at metaphase (Figure 4-17). Indi- 

 vidual chromosomes are differentiated along their length into a series of 

 alternating chromatic and achromatic regions of variable width which 

 are referred to as the "bands" and "interbands," respectively (Figure 

 4-18). The pattern of banding, which constitutes a morphological mark- 

 ing system of profound cytogenetic utility, is a constant feature of any 

 particular segment of any particular chromosome. Since certain bands 

 can be associated with specific genes, it has been thought that the bands 

 represent genetic loci but, so far, no cytogenetic analysis can be brought 



96 / CHAPTER 4 



