158 FERNANDUS PAYNE. 



second step would then be one of differentiation of the two, a 

 becoming different, probably implying also difference of meta- 

 bolic activities. This would account for the lessening affinity 

 of the two as exhibited by the protraction of the time of conjuga- 

 tion. Then would be attained the stage of the second type of 

 diplosomes (idiochromosomes), no longer united but separate 

 in the first maturation spindle. And the last step would be that, 

 instead of a reduction division of them in this spindle there would 

 take place there an equational division of each." This con- 

 ception is rendered untenable by Wilson's discovery that in 

 Mctapodins a typical pair of m-chromosomes and of unequal idio- 

 chromosomes coexist in the same species. In his papers of '01 

 and '05, Montgomery has argued that the accessory chromosome 

 is bivalent and arose by some abnormality in mitosis as by the 

 failure of two spermatogonial chromosomes to separate. Wilson 

 ('05 and '06) suggests that the accessory chromosome may have 

 arisen by the gradual disappearance of the small idiochromosome. 

 Montgomery ('06) realizes this possibility, although he offers 

 objections to it. While Wilson still believes that this interpre- 

 tation may be applicable in many cases, in his work on Mcta- 

 podius f'o9&), he gives another possibility of its origin along 

 with that of the supernumeraries. He says, ' : This was sug- 

 gested by the observation that in a very few cases in 22 chromo- 

 some individuals, both idiochromosomes were seen passing to 

 the same pole in the second division. The rareness of this 

 occurrence shows that it is doubtless to be regarded in one sense 

 as abnormal. But even a single event in an original 22-chromo- 

 some male, if the resulting spermatozoa were functional, might 

 give the starting point for the whole series of relations observed 

 in the genus, including the establishment of an unpaired idiochro- 

 mosome. 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. The 

 latter might similarly produce an individual having in the first 

 generation a single supernumerary chromosome and in succeeding 

 generations an additional number." 



In regard to the possible origin of the supernumeraries in 



