100 EDMUND B. WILSON 



neous to the eye may nevertheless be compound bodies. An 

 important part of it is derived from the modifications of the X- 

 element reviewed above; but the evidence is now being extended 

 to the 'autosomes' or ordinary chromosomes as well. The double 

 chromosome of Nezara suggests either the initial stages of a sep- 

 aration of one chromosome into two or the reverse process in 

 either case that we have before us one way in which the number, 

 and the composition, of the chromosomes may change from species 

 to species. This is supported by the recent observations of 

 Miss E. N. Browne ('10) on Notonecta. In N. undulata there 

 are always, in addition to a typical unequal XY pair, two small 

 chromosomes that appear in all the divisions as separate elements. 

 In N. irrorata there is always but one such chromosome, the total 

 number in each division being accordingly one less than in N. 

 irrorata. N. insulata presents a condition exactly intermediate, 

 there being sometimes one and sometimes two such small chromo- 

 somes. When, however, but one seems to be present, the second 

 may often be seen closely adherent to one of the larger chromo- 

 somes; and the latter may positively be identified, by its size, as 

 always the same one. It can hardly be doubted, as the author 

 points out, that we here see three stages in a process by which 

 the chromosome-number is changing, either by the fusion of two 

 originally separate chromosomes, or by the separation of one into 

 two. It is of the utmost importance that this process affects 

 a chromosome that can be positively identified as the same in 

 each case; for this proves that the change is not an indefinite 

 fluctation but the expression of a perfectly orderly process. 

 While there is here (as in the case of the d-chromosome of Nezara) 

 no way of knowing in which direction the change is taking 

 place, either alternative involves the conception that the indivi- 

 dual chromosomes may be compound bodies, whether as a re- 

 sult of previous fusion or as possible starting points for a subse- 

 quent segregation. 



The facts now known indicate at least four possible ways in 

 which the chromosome-number (and in three of these also the 

 composition of the individual chromosomes, may change from 

 species to species. 



