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, i. 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 



