SEXUAL CHARACTERISTICS OF THELIA 601 
into adults normal in every way—in color, form, and psychie 
traits. Castration was often followed by the implantation of 
gonads of the opposite sex and still no effect was induced. Like- 
wise, the injection of gonad extracts into castrated individuals 
proved ineffective. The most noteworthy of these experiments 
has been reviewed in some detail in part 2. As intimated, these 
results are contrary to what is known in vertebrates. 
The literature dealing with castration and transplantation of 
gonads in vertebrates is so voluminous that no attempt will be 
made here to review the entire subject and only the general 
principles founded upon these researches will be cited. Several 
lines of work on vertebrates indicate that the interstitial cells of 
the gonads secrete important hormones which influence the 
development of the soma. Castration itself practiced on im- 
mature animals causes the retention of juvenal characteristics, 
and a partial or complete lack of the appearance of morpholog- 
ical and psychic sexual characteristics which normally appear at 
sexual maturity. The long bones of the body continue to grow 
and reach a length greater than the average in both sexes; in the 
male, an unusual amount of fat may be laid on, the external 
genitalia may be greatly reduced in size, and the male tempera- 
ment suppressed. However, some characteristics of the sex 
operated upon may be retained in part, such as the shape of 
the pelvic bones. Evidence that the interstitial cells and not 
the growing germ cells themselves play the important part in 
hormone production is presented by the following phenomena. 
In cryptorchism, or in testes subjected to ultraviolet rays, active 
germ cells may be entirely absent while the interstitial cells 
remain intact. In such animals the secondary sexual character- 
istics develop normally and it is believed that the activity of the 
interstitial cells accounts for this. Tandler and Gross (13) 
have presented this subject in a most illuminating manner. 
That hormones play an important part in the early embryonic 
development of mammals has been shown in the case of two 
sexed twins of cattle having anastomoses between the circula- 
tory vessels of the two individuals (Lillie, ’17; Chapin ’17). 
The more rapidly developing male by its hormones affects the 
