488 



The Determination of Sex 



Chromosome Elimination 



Several peculiar features have been observed in the behavior of 

 chromosomes of the fly Sciara by Metz and his co-workers. In 

 some species, in addition to the usual autosomes and sex chromo- 



FiG. 140. Heteropycnosis of the sex chromosome in the spruce budworm, 

 Archips jmniferana: (a) cells of silk glands of male showing no heteropyc- 

 nosis; (b) similar cells of female showing heteropycnotic chromosome ap- 

 pearing as a large, dark lump. (Courtesy of Dr. S. G. Smith in the Journal 

 of Heredity.) 



somes, which together have been designated in this organism the 

 "ordinary" chromosomes, there is also another type of chromo- 

 some which is larger and often of greater diameter than the 

 ordinary chromosomes. These peculiar chromosomes are the 

 "limited" chromosomes and appear to contain few, if any, genes. 

 In some species each fly must have one complete limited chromo- 

 some if *it is to develop normally, although this chromosome must 

 be necessary only for the proper development of the gonads and 



