96 THE CHROMOSOMES 



is not certain, but appears probable ; the only 

 alternative would be to suppose that one spindle 

 attachment had been lost in each case as a result of 

 a deletion '. In the first case the formation of V's 

 would be a reversible phenomenon, in the second it 

 obviously would not . The only other aberrant chromo- 

 some number in the Acrididae is found in the species 

 Podisma mikado which has only 10 pairs of rod-shaped 

 autosomes. ^^^ In view of the remarkable constancy 

 of the chromosome set in the whole family this is 

 very surprising, particularly as the related species 

 P. sapporoense appears to have the usual number of 

 11 pairs of autosomes. Whether a whole pair of 

 autosomes (possibly genetically inert ?) has been lost 

 in P. mikado J or whether all of it except the spindle 

 attachment has been translocated to another pair of 

 autosomes and the spindle attachments lost cannot 

 be decided at present. 



If one examines a number of groups of animals one 

 finds that there is a correlation between variation in 

 size and variation in number of chromosomes which 

 is sufficiently striking to be significant. Thus in 

 groups such as the genus Drosophila and in the 

 Lacertilia there are nearly always a number of very 

 small chromosomes in the set which may be only 

 just above the limit of resolution of the microscope 

 (like the IVth chromosome in D. melanogaster ^ the 

 Vth in D. pseudo-obscura, the Vlth in D. virilis and 

 the microchromosomes of Lacertilia and Birds). 

 Usually, but not always, there is a sharp difference 

 in size between the microchromosomes and the 

 ordinary ones, with no intermediates (as in the genus 

 Drosophila). A particularly interesting case is that 

 of the Urodela ; the majority of these (all species of 

 Triton J Salamandra, Amphiuma, &c.) have 24 large 

 chromosomes with median or submedian spindle 

 attachments (Fig. 4). Where, as in the genera Crypto- 

 branchus, Megalobatrachus and HynobiuSy ^^^' ^^^' '* 

 there are more than 24 the extra ones are small 



