190 Edmund B. Wilson 



chromosome. I still believe that this explanation may be applic- 

 able to many cases; but a different one seems more probable in the 

 case of Metapodius and perhaps may be more widely applicable. 

 This was suggested by the observation (p. 166) that in a very few 

 cases, in 22-chromosome individuals both idiochromosomes were 

 seen passing to the same pole in the second division. The rare- 

 ness of this occurrence shows that it is doubtless to be regarded in 

 one sense as abnormal. But even a single such event in an original 

 22-chromosome male, if the resulting spermatozoa were functional, 

 might give the starting point for the whole series of relations ob- 

 served in the genus, including the establishment of an unpaired idio- 

 chromosome. The result of such a division should be a pair of 

 spermatozoa containing respectively 10 and 12 chromosomes. 

 The former might give rise at once to a race having an unpaired 

 idiochromosome and the somatic number 21 in the male (as in 

 Montgomery's material). The latter might similarly produce an 

 individual having in the first generation a single supernumerary 

 chromosome and in succeeding generations an additional number. 

 This appears from the following considerations : 



1 If a lo-chromosome spermatozoon, arising in the manner 

 indicated, should fertilize an egg of the 22-chromosome class (hav- 

 ing ii chromosomes after reduction) the result should be a male 

 containing 21 chromosomes, the odd one being the large idiochro- 

 mosome derived from the egg. Such an individual would be in 

 no respect distinguishable from those of Montgomery's material, 

 and would similarly form male-producing spermatozoa containing 

 10 chromosomes and female-producing ones containing II (includ- 

 ing the unpaired idiochromosome). A single such male, paired 

 with an ordinary 22-chromosome female, would suffice to establish 

 a stable race identical with the form found by Montgomery at 

 West Chester, Pa., the males having 21 chromosomes, the females 

 having 22, precisely as in Anasa or Leptoglossus. This seems to 

 me the most probable explanation of the conditions found in 

 Montgomery's material; and possibly it may explain the origin 

 of the unpaired idiochromosome in other cases as well. 



2 The result of fertilizing the same type of egg by a spermato- 

 zoon from the 12-chromosome pole would be an individual having 



