REVIEW OF BIOLOGY OF SEX-DETERMINATION 
17 
characteristics. In the development of sex in vertebrates, the 
genes for the production of sex-hormones are probably second 
only in their importance and in their evolution to those genes 
which determine whether ova or sperm shall be formed in an 
individual. 
It has been clearly demonstrated that the genes for the secon- 
dary sexual characteristics lie in the autosomes and therefore 
each sex has also a double set of those for the opposite sex. the 
expression of one set or the other will depend on the sex genes. 
Thus for example in cases where the male is heterogametic, the 
presence of a single X-chromosome in all the cells of the indi- 
vidual, together with the normal secretion of the male gonads, 
causes the male genes for the secondary sexual characteristics to 
develop. By castration and transplantation of ovaries into an 
immature individual, the normal condition may be upset, and the 
female secondary sex genes brought into action, as in Steinach’s 
feminized rats. 
It has been well demonstrated in insects that castration even 
of very young individuals produces no effect upon the secondary 
sexual characteristics when the animal reaches its adult form. 
Even the implantation of gonads of the opposite sex results in 
no change. The growth and development of the soma seems 
fixed by its chromosomal complex and not alterable by any sex- 
hormone. This rigidity of sexual development in insects may 
be coupled with the fact that normal hermaphroditism is ex- 
tremely rare in this group. 
Crustacea and insects, when parasitized, may show alterations 
of the secondary sexual characteristics,, especially in the case of 
the males which then appear externally like females. In the 
Crustacea the best case is perhaps that of Inachus, described by 
Smith. The parasitized male becomes similar to the normal 
female in the form of its claw, abdomen, and abdominal appen- 
dages. Among insects Thelia himaculata, described by the au- 
thor, is a most striking example. Parasitized males resemble 
females even to the minute structure of the chitinous integu- 
ment. Such alterations are most likely due to an entire upset of 
the metabolism of the host and the internal environment is such 
