Reprinted from THE JOURNAL OF EXPEIKMENTAL ZOOLOGY VOL. 9, No. 1. 



STUDIES ON CHROMOSOMES 



VI. A NEW TYPE OF CHROMOSOME COMBINATION IN METAPODITJS 



EDMUND B. WILSON 



Professor of Zoology, Columbia University. 



WITH FIVE FIGURES 



Although the peculiar combination of chromosomes here to be 

 described has been seen in only a single individual, it affords new 

 and I think significant evidence regarding some of the most inter- 

 esting of the problems connected with the nuclear organization. 

 As was shown in the fifth of my "Studies on Chromosomes," 1 

 the genus Metapodius is most exceptional and remarkable in 

 that the specific number of chromosomes varies, while that of the 

 individual is on the whole constant. It is true that slight indis- 

 criminate fluctuations in the number of the ordinary chromosomes, 

 or "autosomes, " occur, as they do in many other species; but 

 this is only an inconsiderable source of the specific variation. The 

 evidence shows, beyond a doubt in some individuals, and hence 

 with probability for all, that the numerical differences are pri- 

 marily due to variations in the number of a particular class of 

 chromosomes which I called the "supernumeraries." These 

 may be wholly absent. When present, their number is constant 

 in the individual, but differs in different individuals. They are 

 often recognizable in both sexes by their size, and in the male 

 also by certain very definite peculiarities of behavior in the matu- 

 ration-process. When they are absent, the diploid groups contain 

 22 chromosomes; and this condition is almost certainly the funda- 

 mental type of the genus, of which all the other conditions are 

 variants. Such a group comprises 18 ordinary chromosomes, or 

 "autosomes" + 2 very small microchromosomes, or m-chrom- 

 somes + 2 unequal idiochromosomes = 22 (these respective 



1 Wilson: '09c. 



THE JOURNAL OP EXPERIMENTAL ZOOLOGY VOL. 9, NO. 1. 



