184 Edmund B. Wilson 



doubt 28 chromosomes, including five smallest ones and three or 

 four next smallest. A few other less clear groups were seen in 

 which appear but 27 chromosomes, the missing one being one of 

 the smallest. In these cases one of the small ones may be hidden 

 among the larger ones; but it is also possible that the 28-group is an 

 abnormality. In this individual there are probably three larger 

 supernumeraries and either two or three small ones. 



C SUMMARY AND CRITIQUE 



1 In the genus Metapodius the number of chromosomes is 

 constant in the individual but varies in different individuals from 

 21 to 27 or 28. The number 21 appears only in the males of M. 

 terminalis (Montgomery's material). 



2 The number is independent of sex and locality, and is not 

 correlated with constant differences of size or visible structure in 

 the adults. 



3 The variation affects only a particular class of chromosomes. 



4 The 22-chromosome forms represent the type from which all 

 the others may readily be derived. These forms possess a pair of 

 unequal idiochromosomes which show the same behavior as in 

 Lygaeus or Euschistus, all the spermatozoa receiving n chromo- 

 somes, and half containingthe large idiochromosome, half the small. 



5 In the 2i-chromosome forms the small idiochromosome has 

 disappeared, leaving the large one as an "odd" or "accessory" 

 chromosome. Half the spermatozoa accordingly receive 1 1 chro- 

 mosomes and half 10. 



6 Numbers above 22 are due to the presence of from one to five 

 or six additional small chromosomes which show in every respect 

 the same behavior as the idiochromosomes, and are probably to be 

 regarded as additional small idiochromosomes. In the growth 

 period they have a condensed form and are typically associated 

 with the idiochromosomes to form a compound chromosome- 

 nucleolus. In the first division they divide as separate univalents. 

 In the second, they are typically (though not invariably) again 

 associated with the idiochromosomes to form a compound element. 

 The components of this element undergo a variable distribution 



