Studies on Chromosomes. 537 



It seems, on the other hand, probable that the m-chromosomes 

 may be of more general significance in this direction, since the 

 facts distinctly suggest that they are diminishing or disappearing, 

 and perhaps in some cases already vestigial, structures in both 

 sexes. Paulmier was the first, as far as I am aware, to suggest 

 that a reduction in the size of particular chromosomes might fore- 

 shadow their total disappearance; that chromosomes might in 

 this way assume a vestigial character; and further, that such 

 chromosomes might represent "somatic characters which belonged 

 to the species in former times, but which characters are disappear- 

 ing" ('99, p. 261). This conception was applied by him to the 

 small m-chromosomes (which he believed to represent the "acces- 

 sory"), but was further supported by his observation of a very 

 small chromatin-body that may divide like a chromosome (Paul- 

 mier, Fig. 28, a) but is only rarely visible.^ Paulmier's suggestion, 

 which I suspect may prove to embody one of the most important 

 results of his paper, has been further developed by Montgomery. 

 This author first suggested that an uneven number ot chro- 

 mosomes "represents a transition stage between a higher number 

 and a lower" ('01, p. 215); and he has more recently assumed 

 that the "unpaired heterochromosomes" ("accessory" or hetero- 

 tropic chromosomes) have arisen from paired heterochromosomes 

 ("chromatin nucleoli") or ordinary chromosomes by fusion of 

 the members of a pair to form a bivalent body ('05, p. 197)- 

 Both the paired and the unpaired heterochromosomes are con- 

 sidered to be chromosomes on the way to disappearance. Though 

 my conclusion regarding the origin of the unpaired or heterotropic 

 chromosome is an entirely different one, it agrees with that of 

 Montgomery in assuming a reduction in the original number of 

 chromosomes; and it is possible that by a subsequent disappear- 

 ance of the heterotropic chromosome a further reduction may take 

 place, though as indicated above there are difficulties in the way 

 of this assumption. My conclusion is, however, distinctly opposed 

 to the view that heterotropic chromosomes have arisen from 

 "paired heterochromosomes" (w-chromosomes), and although 

 they have some features in common the evidence is opposed to 



^It seems quite possible that this body may be the last remnant of a small idiochromosome, of which 

 the corresponding larger one has remained as the heterotropic chromosome; but definite evidence of 

 this is lacking. 



