The Sex Chromosomes. 259 
In turning to the more theoretical side of the question, it 
should be remarked that the observed relations of the X- and 
Y-chromosomes to sex are not theories but facts. The inter- 
pretation to be placed upon these facts from the standpoint of 
sex-determination is a distinet question, and at present it is 
admittedly a theoretical one. 
The force of the experimental evidence has now become 
irresistible that sex-determination must be treated as a form of 
heredity; and evidently the cytological facts provide a good basis 
for its analysis. Mendel suggested that sex may be inherited 
in a manner similar to that which he had discovered in other 
characters; and the same view was later developed by Bateson, 
Strasburger, Castle and many others. The interesting experi- 
ments of Correns on higher plants, and of other observers upon 
sex-limited heredity, render it nearly certain that the heredity 
of sex is closely analogous to that which appears when a hetero- 
zygous form (DR) is crossed with a homozygous recessive (RR) 
a cross which, like the sexual one, reproduces the two parental 
stages of the spermatogonia“, and which may thus ”be followed from 
generation to generation without, in the Hemiptera, undergoing those 
profound changes which characterize the other chromosomes after a mitosis“ 
(1904, p. 146). He failed to perceive the true relation of the XY pair to 
the odd or unpaired X-chromosome, and thus reached confused results 
regarding the large chromatie nucleolus of the growth-period. In Protenor 
he correctly identified the latter with the X-chromosome of the spermatogonia; 
but in other species of the same type (Alydus, Harmostes, Anasa) he still 
followed Paulmiers erroneous view that it arose from the m-chromo- 
somes. He therefore concluded (1901, 1905) that in these species the 
unpaired X-chromosome was not a ”heterochromosome“. On one point, 
however, his conclusions were consistent, namely, that innone ofthese 
forms does the unpaired X-chromosome appear asa 
chromosome-nucleolusinthespermatogonia. Nevertheless, 
even in his last general review (1906) he still maintained that the unequal 
"diplosomes“ (XY pair) are of the same nature as the m-chromosomes, and 
probably represent a modification of the latter (p. 143); and further, that 
the retention of a compact form in the "rest stages“ of the spermatogonia 
is characteristic of "many of them‘ (p. 146). 
In the mean time, Sin&ty (1901), Mc Clung (1900, 1902) and 
Sutton (1900, 1902) had reached correct conclusions regarding the chromo- 
some-nucleolus of the growth-period in Orthopteran spermatocytes, recognizing 
its identity with the univalent "special‘“‘, ”accessory‘‘ or X-chromosome, and 
its failure to appear as a chromosome-nucleolus in the spermatogonial nuclei. 
Me Clung, it is true, in his first paper on the accessory chromosome 
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