CHROMOSOMES AND MITOCHONDRIA 255 
accounted for in any other way (Morgan, 1913, 
1914). 
The view that the chromosomes are the bearers of 
factors in heredity is based upon several hypotheses, 
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 
B 
A 
Fic. 63.— Twisting of chromosomes according to the chiasmatype 
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- 
