The Sex Chromosomes. 259 
X-element may or may not be accompanied by a Y-chromosome. 
Examples of the second case are: Syromastes, where the X-element 
consists of two unequal components (Gross, Wilson); Phylloxera, 
where there are two unequal components, sometimes united, some- 
times separate (Morgan); Agalena, with two equal components 
(Wallace); and Ascaris, where there are no less than five 
components (Edwards). Examples of the second case (Y present) 
are Fitchia, Rocconota, Conorhinus (Payne), Thyanta calceata 
(Wilson), with two X-components; Sinea and Prionidus with 
three X-components (Payne); Gelastocoris, with four components 
(Payne); and, most remarkable of all, Acholla multispinosa, 
where X constantly consists of five components, two large and 
three very small (Payne). 
In all these cases (with the partial exception of Phylloxera) 
the remarkable fact is that the X-components are quite separate 
in the diploid chromosome-groups, but in the maturation of the 
male become associated in a coherent group which passes as a 
unit to one pole, so as to enter but half the spermatozoa. In 
all, again, the female diploid groups are found to contain two 
such X-groups, the male but one — a fact rendered certain, not 
only by the total number of chromosomes, but also in some cases 
by the characteristic size of the X-components. It results from 
this that the number of chromosomes in the female may exceed 
that of the male by two (Syromastes), three (Gelastocoris), four 
(Acholla), or even five (Ascaris lumbricoides); but all these cases 
conform to the formulas already given, namely, XX —= Female, 
and XY (or X) = Male. This will be made clear from the 
diagram, Fig. 4. 
I have thus far cited only cases in which the conditions in 
both sexes are known; but the same relations no doubt hold true 
in many other species where the male is known to be digametic 
but the female condition has not yet been determined. Among 
the most interesting of these is the case of man, where Guyer 
(1910) has recently discovered a double X-element closely similar 
to that of Syromastes. I think it probable, however, that cases 
will be found — perhaps many cases — in which a sexual 
dimorphism of the gametes exists in a physiological sense but 
is not visibly expressed in the chromosomes. I myself believed 
(Studies I, III, IV) that I had found two such cases in the 
