98 THE CHROMOSOMES 



( 1 ) abcdefghijklmnopqrspqrstuvwxyz 



(2 ) abcdefghijklmnopqrssrqptuvwxyz 



(3) abcdefpqrsghijklmnopqrstuvwxyz 



(4) abcdefsrqpghijklmnopqrstuvwxyz 



The ' reversed duplications ' (types 2 and 4) appear 

 to be commoner than the ' direct ' ones (types 1 and 

 3), but the data are not as yet sufficient to form an 

 opinion as to whether there is any special significance 

 in this. In addition to duplications within the same 

 chromosome, duplications of small regions probably 

 also occur in other non-homologous chromosomes of 

 the same set. 



Duplication of the spindle attachment region is 

 obviously a phenomenon of an entirely different 

 nature to duplication of other regions ; if it takes 

 place without the region in question being attached 

 (either terminally or interstitially) to a pre-existing 

 chromosome it will lead directly to the production 

 of a new microchromosome as an addition to the 

 chromosome set. It is this process which may have 

 led to the appearance of microchromosomes in 

 many groups of animals. On the other hand, if it 

 happens in such a way that the duplication is at first 

 attached to one of the original chromosomes (or 

 inserted into it in an interstitial position) a chromo- 

 some with two spindle attachments will have arisen. 

 Such a chromosome will break at about half the 

 mitoses in each cell-generation. There is no reason 

 why it should break at the junction between the 

 duplicated and the non-duplicated region and conse- 

 quently this process, while increasing the number 

 of the chromosomes by one, will not necessarily give 

 rise to a microchromosome, but may equally well 

 give rise to a chromosome of a relatively large size. 



Bridges' discovery (if it applies to animals in 

 general, and not only to Drosophila melanogaskr) 

 means that so-called diploid species are really only 

 partially diploid, a certain number of chromosome 



