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 (710) 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 mtermediate, 
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. 
