No. 463.] STUDIES ON PLANT CELL .— VI. 479 
When a chromosome divides longitudinally so that each id 
splits in half, the daughter chromosomes are exactly equivalent 
and the division of the chromatin is merely quantitative. But 
should a chromosome divide transversely then two sets of entire 
ids would be separated from one another and the two daughter 
chromosomes would differ in proportion as their component ids 
varied, 7. e. the division of the chromatin would be qualitative. 
These conceptions of the possible structure and mode of division 
of chromosomes outline the basis of Weismann's theory of 
heredity and will serve to illustrate the general attitude of those 
biologists who approach the subject from the standpoint of pre- 
formation, although none have cared to formulate such elaborate 
assumptions as Weismann. However, there is a general agree- 
ment among biologists of this school that elements are present 
in the chromatin which do carry hereditary characters and that 
the chromatin granule and chromosome have a definite architec- 
ture and organic value because of these elements. 
Weismann's theory of heredity rests on an interpretation of 
the complexities of mitosis presented by Roux in 1883. Roux 
assumed that chromatin was not homogeneous in structure 
throughout the nucleus, but differed qualitatively in various 
regions. The elaborate history of mitosis with the formation 
and division of the chromosomes and their distribution through 
the mechanism of the spindle seemed inexplicable to Roux 
except on the theory that portions of the chromatin represented 
specific characteristics which were sorted and distributed accu- 
rately according to some system. There could be no need of 
such a complicated mechanism as mitosis if the distribution of. 
the chromatin was to be merely quantitative for simple direct 
nuclear division could perform that operation as effectively as 
mitosis. Mitosis then became a device for the qualitative dis- 
tribution of chromatin as well as quantitative and the characters 
of the daughter cells were determined chiefly by the specific ele- 
ments which were given to one or the other. : | 
Weismann siezed upon Roux’s suggestion of a possible quali- 
tative distribution of chromatin in mitosis and this assumption 
became a very important feature of his theory of heredity. 
Weismann postulated two methods of mitosis. By the first the 
