104 THE CELL IN RELATION TO 



plants constant differences of size, in some cases 

 also of form, are shown among the chromosomes. 

 Specifically different classes of chromosomes can 

 in some casdte be distinguished, which show con- 

 stant and characteristic peculiarities of behavior 

 in respect to some of the most important opera- 

 I tions of the cell. The probability is increasing 

 [that individual chromosomes possess a particu- 

 llar significance for the development of particular 

 characters. It has become probable that sexual 

 dimorphism in general is determined by a differ- 

 ence of nuclear constitution between the sexes. 

 In some groups of animals the sexes differ in 

 respect to one or more particular chromosomes. 

 In a more general way, Boveri's experiments 

 have proved that abnormal combinations of chro- 

 mosomes lead to falsified forms of development; 

 and these observations give the strongest reason 

 to believe that normal development is dependent 

 upon the normal combination of the chromo- 

 somes. 



All these facts are pointing in the same direc- 

 tion. They render the conclusion almost irre- 

 sistible, not only that the chromatin-substance is 

 involved in heredity, but that the chromosomes 

 are composed of specifically different materials, 

 the ensemble di which is essential to normal devel- 

 opment. It is obvious that the beautiful mechan- 

 ism of karyokinesis is perfectly adapted for the 

 meristic division and equal distribution of these 

 materials. The energies that lie behind the for- 



