as the ultimate chromomere. More recent work involving micromanipu- 

 lation and treatment with uncoiling agents such as KCN have indicated 

 that the chromomeres simply represent regions of tight coiling. 



Special Chromosomes 



"Salivary Gland" Type Chromosomes. In certain tissues of the dip- 

 teran larva such as the salivary gland and some regions of the gut and 

 trachea, the chromosomes are of a size and morphological structure 



Figure 4-16. A Schematic Representation of the Chromosomes of Drosoph- 

 ila melanogaster, as Seen at Metaphase of Mitosis in Dividing Cells (above) 

 and in the Cells of the Salivary Glands of Fully Grown Larvae (below). 

 Each chromosome "limb" is shown with different shading. The hetero- 

 chromatic segments of the metaphase chromosomes, which form the chromo- 

 center in the salivary gland nucleus, are shown in white. The kinetochores 

 of the metaphase chromosomes (invisible in the salivary gland cells) are 

 shown in black. (From Sinnott, E. W., Dunn, L. C, and Dobzhansky, T., 

 1950. "The Principles of Genetics," 4th ed., McGraw-Hill Book Co., Inc., 

 New York, N.Y., Fig. 106, p. 245.) 



which makes them strikingly different from those characteristically found 

 in the more typical somatic cells of the same organism. Such chromo- 

 somes were reported in 1881 by Balbiani but received comparatively 

 little attention until Kostoff (1930), Painter (1933), and Heitz and 



STRUCTURE AND FUNCTION OF THE NUCLEUS / 95 



