70 
EDMUND B. WILSON 
an unnatural or hybrid pair, which consists of non-homologous 
members— the large idiochromosome and the third m-chromosome. 
The facts show most decisively that these two chromosomes do 
not play the part of synaptic mates towards each other, but retain 
each its own characteristic behavior. In synapsis the third 
m-chromosome invariably couples with its own kind to form a 
triad element while the large idiochromosome remains unpaired. 
Thus the substitution of one chromosome for another of a different 
kind has been followed by no regulative process, and a perma- 
nently new combination has been produced. The full force of this 
conclusion first becomes evident when we compare the present case 
with those in which there is present a single small supernumerary 
of the type described in my fifth Stud}^ In the diploid groups such 
a supernumerary is quite indistinguishable from a third m-chro- 
mosome — as we may see, for instance, upon comparison with figs. 
Ik, 7, t-y, photograph 29, of Study V.^ In the first spermatocyte- 
division also, in cases where a small supernumerary lies within the 
ring of large bivalents (as in photograph 6, fig. 7i of my fifth Study) 
side-views give a picture almost indistinguishable from such a 
condition as that shown in fig. 3c. Such a side view of terminalis 
No. 1, is given for comparison in fig. Sd, the two m-chromosomes 
just separating at the center of the group, and the supernumerary 
(s) just to the left. The resemblance between this figure and fig. 
3c is so close as to amount almost to identity. It seems incredible 
that the behavior of the third small chromosome in the ensuing 
division should not be identical in the two cases ; and it should be 
identical were the history of the individual chromosomes in matur- 
ation determined merely by their size or their mechanical rela- 
tion to the achromatic figure. In point of fact, however, the small 
supernumerary and the third m-chromosome show characteristic 
differences throughout the whole process. In the growth-period 
the former appears as a condensed chromosome-nucleolus, usually 
coupled with the idiochromosome-nucleolus, while the m-chro- 
mosome remains diffuse and usually free. In the first division 
the former divides as a univalent (i. e., it is typically uncoupled, 
though it may be in contact with the idiochromosomes) and is 
'Loc. cit.: '09c. 
