POLYTENE NUCLEI 



175 



Table 23 

 Chiasma Frequency of Hybrids Compared with that of their Parents * 



* C/. Peto, 1934 '» Mather, 1935 

 t Unpaired chromosomes. 



and Ch. X. 



5. PERMANENT PROPHASE : THE SALIVARY GLANDS 



The activity of secretory cells in glandular tissues of animals 

 sometimes depends on mitosis and sometimes excludes it. In 

 the latter case the nucleus loses its ordinar}' property of perpetuating 

 a characteristic complement of chromosomes and may become more 

 or less highly specialised for its immediate physiological functions. 

 The most remarkable and important example of this specialisation 

 is found in certain glandular cells of insects, particularl}^ in the 

 salivary glands of Diptera. 



The nuclei in cells of the salivary glands in the fully developed 

 larvae of Diptera can be seen in life to be enormously enlarged and 

 full of chromosomes which are correspondingly enlarged. In their 

 proportions and marking they look like contorted earthworms. 

 They are about a hundred times as long as metaphase chromosomes. 

 In Drosophila the second and third chromosomes are a quarter of 

 a millimetre long, and they show some hundreds of definite trans- 

 verse striations. This structure was described by Balbiani in 

 Chironomus in 1881, and by Carnoy in Hymenoptera and Neuroptera, 

 as well as in Diptera in 1884. Alverdes, in 1912, considered that 

 the striations were the result of spiralisation, but Kostoff, in 1930, 



