536 Edmund B. Wilson. 



premises. My suggestion was that a heterotropic chromosome 

 might arise from a symmetrical bivalent by the gradual reduction 

 and final disappearance of one member of the conjugating pair, 

 conditions corresponding to several of the stages of such a reduc- 

 tion being shown to exist in Nezara, Mineus, Coenus, Euschistus, 

 Murgantia, and Lygaeus. All of the facts seem to me to indicate 

 that this interpretation is the true one. Were the small idio- 

 chromosome to disappear in such forms as Lygaeus or Euschistus, 

 the large idiochromosome would be left as a heterotropic chro- 

 mosome agreeing, point by point, with that of such forms as 

 Alydus, Protenor or Anasa, namely, in its persistence as a chro- 

 mosome-nucleolus during the growth-period; its association with 

 the plasmosome in the earlier part of this period and its subse- 

 quent separation from it; its equal bipartition by an equation- 

 division in the first spermatocyte-mitosis, and the failure of the 

 resulting products to divide in the second mitosis; and in corre- 

 lation with the foregoing the existence of an odd number of 

 spermatogonial chromosomes. The exactness of this corre- 

 spondence is such, I think, as to lend a high degree of probability 

 to the interpretation. 



The only apparent obstacle in its way is the fact that in Banasa 

 a heterotropic chromosome coexists with a typical pair of idio- 

 chromosomes; but this difficulty only exists under the assumption 

 that a heterotropic chromosome has arisen but once in the history 

 of the species, and nothing is known to justify such an assumption. 

 I think, on the contrary, that the facts in Banasa may fairly be 

 taken as evidence that a process is here in progress which if con- 

 tinued would lead to the formation of a second heterotropic 

 chromosome.^ 



3. The formation of a heterotropic chromosome in the manner 

 indicated involves a reduction of the total number of chromo- 

 somes by one; and it is possible that this may represent one process 

 by which changes from a higher to a lower number or chromo- 

 somes have been brought about. But I doubt whether such a 

 process can have gone very far, since, as pointed out beyond, 

 there is reason to believe that it has occurred in only one sex. 



'Should my surmise (stated in the footnote at p. 530) be correct that in the related form Thyanta 

 two pairs of idiochromosomes are present without a heterotropic chromosome, I think additional support 

 will be lent to the above interpretation. 



