The Sex Chromosomes. 251 
certain Orthoptera; but Me Clung alone suspected its significance. 
This chromosome is the ”special‘‘, ”accessory“, ”odd“ or ”hetero- 
tropic‘‘ chromosome, afterwards called by Montgomery (1906) 
the’’ monosome“. I shall hereafter call it the ”X-chromosome“. 
Me Clung advanced the hypothesis that this chromosome is a sex- 
determining element, specifcally a male-determinant, and assumed 
that eggs fertilized by spermatozoa in which it is present produce 
males; but his hypothesis was based only upon arguments from 
analogy, since nothing was known of the conditions in the female. 
As the sequel showed, this particular assumption regarding the 
relation of the X-chromosome to sex was the reverse of the truth; 
but a consistent interpretation of the facts was at that time 
impossible, owing to the confused and contradietory state of the 
literature. 
The deecisive evidence in regard to this question was first 
produced by independent investigations upon Hemiptera and 
Coleoptera by Miss Stevens and myself in 1905/1906. Mv own 
work (Studies II, III) proved in case of several Hemiptera of the 
same type as Pyrrhocoris (Protenor, Anasa, Alydus, Harmostes) 
that the sexes differ in that the somatic or diploid nuclei of the 
male contain one chromosome fewer than those of the female; 
further, that this difference is owing to the presence of but one 
X-chromosome in the male while two are present in the female. 
Of the latter fact no doubt is left by the case (among others 
now known) of Protenor, where the X-chromosome is the largest 
of all the chromosomes; and I afterwards showed (1909a, 1909 d) 
that the same is true of Pyrrhocoris. 
In these cases it may unmistakably be seen that the male 
uniformly contains but one of these chromosomes, the female two 
(Fig. 1). In respect to this chromosome, therefore, the composition 
of the female is XX and that of the male X, the sexes being otherwise 
identical. This can only mean that eggs fertilized by spermatozoa 
containing X produce females, by those lacking X, males. For, 
in the female, upon reduction of the diploid chromosome-groups 
to haploid (in maturation) each egg must receive one X, while 
in the male but half the gametes receive this chromosome. The 
recent studies of Morrill (1910) upon several genera of Hemiptera 
(Protenor, Anasa, Chelinidea, Archimerus), made in my laboratory, 
prove that this conelusion regarding maturation in the female 
