74 



Special Chromosomes and Sex Inheritance 



Salivary. Gland Chromosomes 



After fertilization in Drosophila, the egg is laid in moist food 

 and proceeds to hatch out into a small, crawling larva. Inside 

 the larva, extending back from the mouth a distance of one- 

 fourth to one-third the length of the entire body, are two large 



Fig. 25. Chromosomes in a cell of the salivary gland of Drosophila 

 melanogaster. (Courtesy Dr. B. P. Kaufmann.) 



salivary glands. These glands have cells so large that they can 

 easily be seen with the low powers of a dissecting microscope. 

 The nuclei of these cells are much larger than those of ordinary 

 cells, being generally about 25 ju, in diameter, and the chromo- 

 somes in the nuclei are so large that they are 50 to 200 times as 

 large as the chromosomes in the reproductive cells or in the 

 ordinary body cells of this organism (Fig. 25). Such large chro- 

 mosomes are characteristic of the salivary glands, the rectal 

 epithelium, and the Malpighian tubules of the entire group, the 

 Diptera, to which Drosophila belongs. They were first observed 

 in 1881 by Balbiani in the related organism, Chironomus. Their 



