CYTOLOGY 39 



and enters the female nucleus simply as a naked nucleus. 

 It is, therefore, supposed that in the nucleus alone all 

 the inherited characters are carried. As the definite 

 rods (the chromosomes) appear always to be so constant 

 in the nucleus they have been suspected of being the 

 actual bearers of the inherited qualities. There is, 

 however, such a small number of them in comparison 

 with the number of characters to be carried that they 

 cannot be the ultimate units, and many theoretical, 

 ultra-microscopic structures have been imagined to 

 do the work. Finality has not yet been reached, 

 though it lies within the province of cytology to discover 

 the nature of the structures that carry the transmitted 

 characters, and that are consequently of such excep- 

 tional interest to us, for in man the problem is ultimately 

 the same as in the plants ; and in the ultimate units 

 composing the chromosomes, it would appear, lies the 

 basis of our mental as well as physical characteristics. 

 Some evidence goes to show that the cytoplasm is also 

 the bearer of inheritable characters, but its importance 

 in this respect has not yet been demonstrated so fully 

 as that of the chromosomes. Plant cytology is of 

 supreme importance in dealing with these questions, 

 because the nature of plants makes them such suitable 

 material for experiment. 



