STUDIES ON CHROMOSOMES 93 
first spermatocyte-division, and passes with one half of the bivalent 
to one pole. Since the spermatogonial number in Leptynia (36) 
is an even one and twice that of the separate chromosomes present 
in the first spermatocyte-division, it may be inferred that the 
X-element is already united with one of the ordinary chromo- 
somes in the spermatogonia, though Sinéty does not state this. 
Somewhat later McClung (’05) discovered essentially similar rela- 
tions in the grasshoppers Hesperotettix and Anabrus (fig. 6, 
c-e),and in ease of the first named form was able to establish the 
important fact that it is always the same particular bivalent with 
which the X-chromosome is thus associated. In respect to the 
intimacy of this association, a progressive series seems to exist, 
since in Leptynia it seems to take place in the spermatogonia, in 
Hesperotettix only in the prophases of the first spermatocyte- 
division, while in Thyanta the union is only effected after the 
first division is completed. 
Finally, the recent observations of Boring (’09), Boveri (’09) 
and Edwards (’10) seem to establish the fact that in Ascaris megalo- 
cephala the X-element, whether in the diploid groups or in the 
maturation-divisions, may either appear as a separate chromo- 
some (which has the usual behavior of an accessory chromosome) 
or may be indistinguishably fused with one of the ordinary chromo- 
somes. 
These relations may, of course, be the result of a secondary 
coupling; and I myself formerly so interpreted them (’09c). But 
in view of what is seen in Thyanta or the reduvioids we may 
well keep in mind the possibility that they are expressions or 
remnants of a more primitive association, like that which I have 
assumed for an original XY-pair. Whatever be their origin, the 
effect is the same—a definite linking of the X-chromatin with 
that of one of the other pairs. 
Fig. 7 shows, in purely schematic form, the general conception 
of these relations that has been suggested above, the X-chromatin 
being everywhere represented in black. A is the primitive XY- 
pair from which all the other types may have been derived. By 
simple reduction of such a pair arises the ordinary or typical 
idiochromosome-pair (B); and from either A or B may be derived 
