94 FUNDAMENTALS OF CYTOLOGY 



experimental treatments. Thus by treatment with warm water the 

 matrix can be dissolved- away, leaving the chromonemata intact on 

 the slide (Fig. 65, a). In some plants it has been found that the chro- 

 monemata in meiotic chromosomes are doubly coiled, i.e., the}' not only 

 form the large "major" spiral so easily seen but have in addition a 

 minute "minor" spiral running throughout their length (Fig. 65, h). 



Sciara ocellaris 



Fig. 67. — The salivary-gland chromosomes of a fungus gnat {Sciara). Each of the four 

 consists of two homologous members of the diploid somatic complement in intimate lateral 

 union; note the doubleness at end 1 of chromosome A. From an iron-acetocarmine smear 

 preparation. Magnification, 575 X. (Photograph by O. 0. Heard. After C. W. Metz.) 



The same condition has been found in certain somatic chromosomes 

 (Figs. 65, c; 66). 



Salivary-gland Chromosomes. — That the chromosome possesses a 

 definite longitudinal differentiation in structure and functional effect 

 is most convincingly shown by the amazing chromosomes in the salivary 

 glands and certain other larval tissues of the two- winged flies (Diptera). 

 These chromosomes were first observed in 1881, but it is only during 

 the past 10 years that they have become well enough known to be of 

 service to the cytologist and cytogeneticist. It was a stroke of good 

 fortune to find them in the Diptera. Many years of cytogenetical 



