148 Edmund B. Wilson 



have been carefully studied. These individuals belong to three 

 well marked species M. terminalis Dall. and M. femoratus Fab. 

 from the Eastern and Southern States, M. granules us Dall. from 

 the Western all of which show a similar numerical variation. 3 

 My first material, including sections of two testes of M. terminalis 

 (Nos. i, 2) from the Paulmier collection, long remained a complete 

 puzzle and led me to the suspicion that the material was patho- 

 logical. This possibility was eliminated by the study of additional 

 material of the same type; but the contradiction with Montgom- 

 ery's results on the same species suggested that his specimens 

 were not correctly identified (Wilson 'o/a). Continued study at 

 length convinced me that this supposition too was probably un- 

 founded. If the identification was correct, as I now believe it 

 was, M. terminalis is a species that varies not only in respect to 

 the individual chromosome number but also in respect to the sex- 

 chromosomes, certain individuals having an unpaired "accessory" 

 chromosome, while others have an unequal pair of idiochromo- 

 somes. The latter condition alone has thus far been found in 

 M. femoratus and M. granulosus. The essential facts, and the 

 general history of the spermatogenesis, are otherwise closely 

 similar in the three species. 



The range of variation in the number of chromosomes is in 

 M. terminalis from 21 to 26, in M. femoratus from 22 to 27 or 28, 

 and in M. granulosus from 22 to 27, the particular number (or 

 its equivalent in the reduced groups) being a characteristic feature 

 of the individual in which it occurs. I do not mean to assert that 

 there is absolutely no fluctuation in the individual. In this genus, 

 as in others, apparent deviations from the typical number fre- 

 quently are seen, and real fluctuations now and then appear; but 

 the latter are so rare that they may practically be disregarded. 

 That the number may be regarded as an individual constant 

 (subject to such deviations as are hereafter explained (p. 185) is 

 abundantly demonstrated, not only by the agreement of large 

 numbers of cells from the same individual but perhaps even more 



3 A complete list of the individuals examined, arranged by localities, is given in the Appendix at 

 p.-2O2, Each individual is there designated by a number by which it is referred to in the text and 

 description of figures. 



