(c) 



Figure 4-18. (a) Photomicrograph of Giant Polytene Chromosomes in 

 Salivary Gland Cell of Drosophila. (b) Drawing of Drosophila Salivary 

 Gland Giant Chromosomes and Those of the Gonial Cells on the Same Scale 

 (upper right). Note longitudinal differentiation of the salivary gland chromo- 

 somes into a series of transverse chromatic and achromatic bands, and the 

 incomplete pairing between homologues in the right arm of chromosome II. 

 (c) Photomicrograph of Part of a Salivary Gland Chromosome in Chirono- 

 mus. The pattern of the bands is particularly well shown. (Fig. (a), courtesy 

 of General Biological Supply House. Inc.. Chicago. Fig. (b), from Painter, 

 T. S., 1934. "Salivary Chromosomes and the Attack on the Gene." J. Hered- 

 ity, 25, Fig. 1, p. 466. Fig. (c). from White, M. J. D.. 1951. "Nucleus, 

 Chromosomes, and Genes," /// "Cytology and Cell Physiology." G. H. 

 Bourne (Ed.), 2nd ed., Oxford University Press, London, England, Fig. 4, 

 Plate 1.) 



place at the chromosomal level (Beermann, 1956; Ficq, et al., 1959; 

 Pavan, 1958). This view is based largely on the finding that pufl: forma- 

 tion takes place at particular bands or interbands in different cell types 

 at specific times during larval growth and is, for the most part, a revers- 

 ible process. Also disproportionate increases in DNA are sometimes 

 detectable in bands previously involved in puff formation (Pavan, 1958). 



LampbnisJi Chromosomes. The nuclei of many vertebrate oocytes, 

 particularly those rich in yolk, possess chromosomes which become enor- 

 mously enlarged in length and assume a brush-like appearance during 

 meiotic prophase. This type of chromosome was first described by Flem- 

 ming in 1882 and given the name "lampbrush" by Riickert in 1892. 



The brush-like appearance is most obvious at the late stage of the first 

 meiotic prophase. These chromosomes are generally described as having 

 a central chromosomal axis from which project a series of lateral loops 



STRUCTURE AND FUNCTION OF THE NUCLEUS / 99 



