CHAPTER XIII 



The Theory of the Individuality of the 

 Chromosomes 



IT is now necessary to return to a further consideration 

 of the behaviour and functions of the chromosomes, 

 from which the subject of germ-cell determinants was to 

 some extent a digression, and in particular to discuss a 

 theory which may be regarded as one of the broadest and 

 most fundamental generalisations of modern cytology. This 

 theory, due very largely to the late Prof. BOVERI, is now 

 very widely, but by no means universally, accepted, and is 

 the foundation of all the more recent hypotheses and specu- 

 lations on the function of the chromosomes in relation to the 

 transmission of hereditary characters. Put quite shortly the 

 theory of the individuality of the chromosomes states that 

 the chromosomes behave in a certain sense as individuals, 

 remaining in existence from one nuclear division to the next, 

 during the so-called resting stage of the nucleus, and dividing 

 in mitosis to form two new individuals each exactly like the 

 parent chromosome from which they originated. Stated in 

 an extreme form, the theory maintains that the chromo- 

 somes behave like independent organisms living in a sort of 

 symbiosis with the rest of the cell, deriving their substance 

 from it and secreting substances that are used by it, but 

 remaining as distinct from it as the "Zooxanthellae" in the 

 body of a Radiolarian. Such a statement must of course be 

 regarded as metaphorical, and rather as illustrating the kind 

 of individuality that is meant than as describing the con- 

 dition as it is supposed actually to exist, but even with this 



