618 SIDNEY I. KORNHAUSER 
rate of metabolism of the germ cells of individuals in the various 
groups appeared (Riddle, ’17). Metabolic differences are de- 
monstrable and measurable between the sexes of highly special- 
ized forms which store yolk in large ova or provide nutriment to 
the zygote. This metabolic level is assumed to be an expression 
of sex rather than a causal factor. 
Genetic changes are most probably to be sought in changes 
in the chromatin. In the evolution of sex, genes located in 
various chromosomes have undoubtedly played a part. In 
nematodes, insects, spiders, and some mammals visible chromatic 
differences have led to the belief that the unpaired element 
of the male cells, the x-chromosome, is intimately connected 
with sexual differentiation. Let us inquire what is known 
about the x-chromosome and the location of other genes affect- 
ing principally sexual characteristics. In spermatocytes the 
x-chromosome would seem entirely different from ordinary 
chromosomes. It does not form a typical leptotene thread, and 
even if a y-chromosome is present it fails to form a double syn- 
detic thread with this element. Should a fairly long growth 
period follow syndesis and the autosomes become very indistinct, 
the x-chromosome remains a compact deeply staining mass. 
This is true for Thelia and is shown in figure 51, c and d. Still, 
the x-chromosome is not an inert mass of chromatic material 
differing in character from that of the autosomes. When paired 
as in the odcytes, they behave exactly like any of the other chro- 
mosomes. They form leptotene threads which conjugate. In 
Drosophila (Morgan and Bridges, ’16) linkage and crossing 
over in the paired x-chromosomes has been shown to occur just 
as in the autosomes. By investigations of linkage many genes 
which bring about sex-linked characteristics have been located 
in the x-chromosome and these genes seem to have a definite 
linear arrangement. Furthermore, Bridges’ (’16) remarkable 
observations on ‘non-disjunction’ have definitely demonstrated 
that one x-chromosome is absolutely necessary in the formation 
of the male and that two and only two are essential in the pro- 
duction of a female. From the behavior of the x-chromosome 
in spermatogenesis one might infer that this element were in- 
