A STUDY OF SOMATIC CHROMOSOMES. 



I. THE SOMATIC CHROMOSOMES IN COMPARISON WITH THE 

 CHROMOSOMES IN THE GERM CELLS OF Anasa tristis. 



w. E. HOY, JR., 



PRINCETON UNIVERSITY AND THE UNIVERSITY OF ROCHESTER. 



With the exception of incidental and scattered observations 

 in connection with studies on spermatogenesis there have been 

 only a few papers of a statistical nature on the number, type and 

 combination of the chromosomes in somatic tissues. Usually, 

 and more or less arbitrarily, the diploid or gonial number of 

 chromosomes of the germ cells has been assigned to the cells 

 of the soma. However, the number of chromosomes in the 

 somatic cells is not necessarily the same as that in the germ cells. 

 Cases have been reported, for instance, in Ascaris, some of the 

 Hymenoptera, a Lepidopteran, and in several of the Vertebrata, 

 where the somatic number of chromosomes is higher than the 

 gonial number. On the other hand in Culex, Diaptomus, Phasco- 

 losoma, and some of the Diptera the number of chromosomes 

 may be lower in some or in all of the cells of the body tissues 

 than in the gonia. 



Extensive studies on the development of the germ cells in 

 animals have, in general, offered strong evidence for an "indi- 

 viduality" of the chromosomes. Variations in the number of 

 chromosomes in the germ cells are relatively infrequent. This 

 undoubtedly is of the greatest importance in the modern "chro- 

 mosome theory" of heredity. At first hand the peculiar history 

 of the chromosomes in somatic cells of certain animals may seem 

 to render somewhat untenable this theory of the "individuality" 

 of the chromosomes, and may easily give rise to the idea that 

 they are exceedingly variable structures. With this in mind, a 

 study is being made of the chromosomes in the somatic tissues 

 of various developing embryos for the purpose of determining if 

 chromosomes of the somatic cells generally agree in number and 



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