CHROMOSOMES AND MITO( HOXDRIA 255 



accounted for in any other way (Morgan, 1013, 

 1914). 



The view that the chromosomes are the ])earers of 

 factors in heredity is based upon several liypotheses, 

 of which those of their specificity and genetic con- 

 tinuity will be mentioned here. According to the 

 hypothesis of chromosome specificity each chromo- 

 some possesses certain functions of a specific kind 



A 



Fig. 63. — Twisting of chromosomes according to the chiasmat>T)e 

 theory. A. Two twisted chromosomes each divided longitudinally 

 into two. B. Twisted chromosomes of Batracoseps attenuatus. 

 (From Janssens, 1909.) 



which determine the character of cellular differen- 

 tiation and thus the structural and physiological 

 condition of the embryo, larva, and adult. The hy- 

 pothesis of the genetic continuity was evolved from 

 that of the individuality of the chromosomes. Ac- 

 cording to the latter theory the chromosomes that 

 appear in mitosis do not become scattered during the 

 resting stage of the nucleus (interkinesis), but re- 

 tain their identity throughout this period. Lack 

 of evidence has resulted in the substitution of the 

 hypothesis of genetic continuity, according to 

 which there is a definite relation between the chro- 

 mosomes of successive mitotic divisions. 



Much of the cytological literature of the past dec- 

 ade deals with the history of the X- or sex-chromo- 



