42 MITOSIS: THE CONSTANCY OF THE CHROMOSOMES 



This type of observation has been multiplied in recent years, so 

 that any other hypothesis than the simple one of individuality and 

 permanence would now be preposterous. Abnormal chromosome 

 numbers have been discovered in relation to so many investigations 

 that they must be separately considered in their special categories. 

 All these observations are compatible with the doctrine of 

 permanence, for these reasons : first, such differences as are observed 

 can be understood in every instance as the result of changes that 

 have also been observed, or on other grounds inferred, in the 

 individual chromosome or in the genetic properties of the nucleus 

 as a whole. Secondly, the changes that take place are always 

 irreversible in their effect on the chromosome complement and in 

 the correlated effect on the organism as a whole, except through the 

 reversal of the accident or change which is assumed to be directly 

 responsible for the change in the complement on the hypothesis of 

 permanence (cf. Buxton and D., 1932). 



Now, if the constant structure of the chromosome complement is 

 due to the inherent properties of the chromosomes, it may be 

 supposed to be maintained in one of two ways. Each of the 

 successive particles or chromomeres may have a specific affinity for 

 the particles on either side of it, which causes it to enter into 

 the same combination at successive mitoses. The changes in 

 arrangement that will later be described show that this is not so. 

 Treatment with X-rays will break up the chromosomes and recom- 

 bine their parts more or less at random in new arrangements, most 

 of which are as stable and permanent as the old ones (Ch. X). In 

 fact the changes that take place in chromosomes under X-ray 

 treatment are the strongest evidence of potential permanence 

 without such treatment. Alternatively, the essential linear 

 structure of the chromosome thread seen at prophase may be retained 

 during the resting stage. This idea of the continuous existence of 

 the chromosome we owe in the first instance to Rabl (1885). It 

 was for long a matter of dispute, two opposed views being held 

 with regard to the evidence. 



1 Some consider that the chromosomes cannot be individually 

 represented by any structure in the resting nucleus. Schaede 

 (1925-30) found that the living nucleus, at least when " young." 



