STUDIES ON CHROMOSOMES 
VI. A NEW TYPE OF CHROMOSOME COMBINATION IN METAPODIUS 
EDMUND B. WILSON 
Professor of Zoology, Columbia University. 
WITH FIVE FIGURES 
Although the pecuUar 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,"' 
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 sUght 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" -h 2 very small microchromosomes, or m-chrom- 
somes -h 2 unequal idiochromosomes = 22 (these respective 
1 Wilson: '09c. 
THE JOURNAL OF EXPEBIMENTAL ZOOLOGY VOL. 9, NO. 1. 
