514 WARO NAKAHARA 
always couples with the much larger idiochromosome, but never 
with the m-chromosome, and suggested that the coupling results 
from definite affinities among the chromosomes. He said: 
The possibility no doubt exists that the couplings are produced by 
extrinsic cause (such as the achromatic structure), but the evidence 
seems on the whole opposed to such a conclusion. I consider it more 
probable that they are due to intrinsic qualities of the chromosomes 
and that the differences of behavior shown by different forms may 
probably be ceo ded as due to corresponding physico-chemical differ- 
ences 
Very similar statements were made by Metz (’16), who said: 
Pairing (of chromosomes) is not due to purely mechanical causes, 
but is dependent in some way upon the qualitative nature of the chro- 
mosomes. This conclusion seems evident from the fact that paired 
chromosomes are corresponding or similar chromosomes. It is diffi- 
cult to conceive how purely mechanical forces can cause anything more 
than random pairing, while as a matter of fact the actual pairing is 
selective to the highest degree. That this association is not merely 
assortment according to size is shown by the pairing of unequal sex- 
chromosomes in the male, where X is several times as large as Y. 
Metz said further that the paired chromosomes are qualita- 
tively similar and ‘‘their association is dependent upon, although 
not necessarily caused by, this relation.”’ Convincingly support- 
ing this statement, he pointed out that in the tetraploid groups 
in Diptera, 
two of the four chromosomes are sister halves of the other two, and 
hence are respectively similar to them in make up. But all four of 
these chromosomes associate in essentially the same manner, iLe.,° 
paired chromosomes are indistinguishable from sister chromosomes in 
their manner .of association. 
Turning our attention to the case of Perla, we see at once that 
1. The ten chromosomes in the spermatogonial group may 
be grouped in pairs, and that 
2. Each of the pairs is represented by a single chromencine 
of corresponding appearance in the spermatocytic groups (ex- 
cepting the X- and Y-chromosomes). 
The X- and Y-chromosomes are seen actually coupling in 
the late prophase and in the metaphase of the first spermatocytic 
