CHROMOSOME RELATIONS IN THE SPERMA- 

 TOCYTES OF ONISCUS. 



M. LOUISE NICHOLS. 



Recent work on the germ cells of animals has shown that the 

 chromosomes contained withia one nucleus frequently are not all 

 of the same size. It has been known for some time that in insects 

 and spiders one or more of the chromosomes, the so-called acces- 

 sory, is smaller than the others and also differs in its behavior. 

 \Yith regard to chromosomes other than the accessory, Sutton, 1 

 in his studies on the spermatogenesis of Brachystola discovered a 

 well-marked difference in size. Montgomery's ~ studies of the 

 chromosomes of various Hemipteran species also revealed a 

 variation in size of chromosomes occupying the same nucleus, 

 and McClung, 3 in a recent paper on the spermatocytes of Orthop- 

 tera has described a remarkable structure, strikingly larger than 

 the other chromosomes, which he has named " multiple chromo- 

 some," and which he interprets as being formed by the associa- 

 tion of a number of chromosomes. The still more recent work 

 of Morgan 4 and Stevens 5 on aphids indicates much the same 

 thing. Morgan studied the eggs of Phylloxerans and shows in his 

 figures Q{ Phylloxera caryce-globnli a distinct difference in the size 

 of the chromosomes. Miss Stevens investigated a number of 

 species of plant lice and concludes that each species is character- 

 ized by a definite number of chromosomes. 



Chromosomes of the same nucleus may differ not only in size 

 but also in shape. In a paper published in 1902 fi I showed that 



'W. S. Sutton, "On the Morphology of the Chromosome Group in Brachystola 

 ma^na," BIOL. BULL., 4, No. i, 1902. 



2 T. H. Montgomery, Jr., "A Study of the Germ Cells in Metazoa," Trans. Am. 

 Phil. Soc., 20, 1901. 



3 C. E. McClung, "The Chromosome Complex of Orthopteran Spermatocytes," 

 BIOL. BULL., 9, No. 5, 1905. 



4 T. H. Morgan, " Male and Female Eggs of Phylloxerans of the Hickories,'* 

 BIOL. BULL., 10, No. 5, 1906. 



5 N. M. Stevens, " Germ Cells of Aphids," Carnegie Inst., May, 1906. 



6 M. L. Nichols, "Spermatogenesis of Onistiis asellits, Linn.," Proc. Am. Phil. 

 Soc., 41, No. 1 68, 1902. 



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