74 
EDMUND B. WILSON 
explanation of the facts. It will be evident to readers of my for- 
mer papers that I am fully prepared to accept Boveri's conclusions; 
but there is one very important fact, finally established by the 
present paper, that must be clearly recognized. If we assume that 
different factors of heredity are in some sense unequally distri- 
buted among the chromosomes, we need feel no surprise that the 
duplication of one or more of the ordinary chromosomes should 
produce no perceptible qualitative effect upon development. But 
it is very surprising that no visible effect should be produced by the 
removal of a particular chromosome that has no duplicate to take 
its place. In preceding papers I have called attention to the sing- 
ular fact that Montgomery's material of M. terminalis differs con- 
sistently from my own in the lack of the small idiochromosome or 
Y-element" (see Wilson '09a, for this term) ; but the possibility 
of two distinct species or races having been confused could not be 
absolutely excluded. In the present case, however, no doubt can 
exist, since the two original specimens of M. femoratus from Miami, 
Florida, are in my cabinet, and in perfect condition for identifica- 
tion. One of these, as already stated, contains both the small 
idiochromosome and an additional supernumerary, while both are 
lacking in the other individual ; yet the two individuals seem to be 
otherwise in every essential respect identical. All doubt is thus 
removed that the small idiochromosome or Y-element, which forms 
the synaptic mate of the accessory chromosome or X-element, 
may be present in some individuals and absent in others of the 
same species, without the appearance of any corresponding dif- 
ferences in the sexual or other characters as far as shown in the 
external morphology of the animal. This chromosome, as shown 
^ It will be seen from this how readily discrepancies regarding the number of 
chromosomes might arise between different observers working on the same species. 
It might seem that we have here a simple and plausible explanation of the contra- 
dictions that have arisen in the case of Anasa tristis; for we might assume that the 
diploid number is 21 in some individuals of this species and in others 22; and a simi- 
lar explanation has in fact already been adopted by more than one recent writer 
(cf. Delia Valle: '09, Buchner: '096). 
I am not myself able to take this view of this particular case for several reasons. 
In the first place, if there be individuals of this species that have 22 spermatogonial 
chromosomes, as maintained by Foot and Strobell ('07) we should expect to see 
