CHROMOSOMES AND EVOLUTION 95 



interstitial region ; in this ease the two spindle 

 attachments may be expected to function mechanic- 

 ally as a single unit, as in the case of A scar is megalo- 

 cephala. Breakage of the interstitial region will 

 give two clu'omosomes with quasi-terminal spindle 

 attachments (' rod-shaped ' chromosomes). Con- 

 versely two chromosomes with quasi-terminal spindle 

 attachments may fuse together so as to give a V 

 with two attachments in the middle. Any other 

 kind of fusion will give a chromosome with two 

 widely separated spindle attachments which will 

 break at anaphase (Fig. 6) and any other kind of 

 breakage will give rise to a fragment with no spindle 

 attachment, which consequently cannot form an 

 independent chromosome. Thus aj)art from these 

 two special cases it does not seem possible that 

 * fusion and fragmentation ' have played any part 

 in the evolution of new chromosome numbers. 



In some groups of animals the chromosome number 

 is fairly constant, while in others it varies from species 

 to species. Thus nearly all Urodeles have a diploid 

 number of 24, while in the Crickets (Gryllodea) 

 thirteen different chromosome numbers have been 

 found, ranging from 12 to 30 (in the female diploid 

 set).^"*- In some cases, where it is due to the pro- 

 cesses (hvscHbed above, the origin of new chromosome 

 numbers is clear. Thus most species of Acrididae 

 (Locusts and Grasshoppers) have 11 pairs of rod- 

 shaped autosomes with quasi-terminal spindle attach- 

 ments, but in the genera Chorthlpjms, Stenobothrus 

 and Stauroderus there are only S pairs, 3 of which 

 are V-shaped, with median or submedian spindle 

 attachments. The South American species Alexias 

 vitticolUs has 9 pairs, two of which are V's.^^^ It 

 is clear that here originally separate chromosomes 

 with subterminal spindle attachments have fused 

 together so as to decrease the number of autosomes. 

 Whether the V-shaped autosomes in these genera have 

 two spindle attachments situated very close together 



