STUDIES ON CHROMOSOMES 91 
Wilson) it consists of two unequal components, always separate, 
in the diploid groups but closely in contact (not fused) in both 
spermatocyte-divisions. The recent work of Guyer (10) indi- 
cates a similar condition in the X-element of man. In Agalena 
(Wallace) there are two equal components, always separate. 
Finally, in Ascaris lumbricoides (Edwards, ’10) there are five com- 
ponents, separate, and scattered in the diploid groups but closely 
associated in the spermatocyte-divisions. 
In all these cases the significant fact is that not only the number 
but also the size-relations of these components are constant; and 
in many of these forms this fact may be seen in such multitudes 
of cells, and with such schematic clearness, as to leave no manner 
of doubt. It seems impossible to understand this series of phe- 
nomena unless we assume that the single X-chromosome is essen- 
tially a compound body—~.e., one that consists of different con- 
stituents that tend to segregate out into separate chromosomes. 
We are led to suspect, further, that the composition of the X- 
element, even when it is a single chromosome, may differ widely 
in different species because of its great variations of size as between 
different species. For instance, in the family of Coreidae it is 
in some cases very large (Protenor), in others of middle size (Che- 
linidea, Narnia, Anasa), in others one of the smallest of the chromo- 
somes (Alydus). Similar examples might be given from other 
groups. 
In the case of Thyanta, therefore, it seems a fair assumption 
that the double X-element of the B form likewise represents at 
least a partial segregation of the X-chromatin from other con- 
stituents; and the latter may plausibly be regarded as represent- 
ing the ‘Y-chromatin’ of the original X-member of the pair. 
In other words, we may think of the triad element as a Y Y-pair, 
one member of which is accompanied by a separate X-chromosome. 
In accordance with this its formula should be X.Y.Y, while that 
of the A form is XY.Y; and this may also be extended to other 
forms of similar type. If this be admissible, the male formula, as 
regards essential chromatin-content, becomes in general XY.Y 
and the female XY.XY, both sexes being homozygous for the 
Y-constituents, while in respect to X the male is heterozygous, 
