BREAKAGE OF CHROMOSOMES 419 



chromosomes are not then ready to divide. And finally, they show 

 that the movements of the spindle and of the chromosomes which 

 are independent in origin and independently controlled, mutually 

 react as soon as they come together. How they come to react we 

 shall consider in detail in relation to the special problems of chromo- 

 some mechanics. 



3. THE EFFECTS OF STRUCTURAL CHANGE 



(i) Conditions of Change. The chromosomes, as we have seen, are 

 subject to changes which may be provisionally classified as intra- 

 genic or molecular and inter-genic or structural. Apparently, as 

 we have seen, all parts of the chromosome are subject to inter-genic, 

 while only the physiologically active parts are subject to intra- 

 genic, changes. Since, however, intra-genic changes are detectable 

 only by their physiological effect we may suppose that inertness 

 merely prohibits change back to an active form. 



The rate at which these changes occur shows that the conditions 

 determining them are various. Temperature, developmental 

 conditions, genotype and the structure of particular genes, affect 

 them. We have previously seen the physiological and structural 

 conditions of chromosome behaviour in antithesis. They are 

 alternative agents of change. We now see on a deeper analysis that 

 they control one another. There is an inter-penetration of opposites. 



Apart from indirect control of structural change certain special 

 agents, short-wave irradiation and direct manipulation, .such as 

 centrifuging (Kostoff, 1935), determine its occurrence. There is no 

 practical means as yet of making a distinction between " natural " 

 changes such as may depend on inherent defects in the reaction of 

 the gene with its substrate, and " induced " changes that depend on 

 violent external interference. It is convenient, however, pro- 

 visionally to separate natural from induced changes according to 

 whether the method of control is internal or external to the organism. 



(ii) Natural Change. Natural changes in the structure of chromo- 

 somes are known chiefly by their results at meiosis and in the 

 ensuing generation. After this interval they can be described in 

 kind. But during the interval we know that many of the nuclei 

 with new chromosomes must have been eliminated and we cannot 



14—2 



