7. The effects of the removal of the ovaries or testes upon the development of secondary sexual characters differ for different species. In insects the secondary sexual characters are not altered by an operative removal of the sexual glands as in the caterpillar, e. g., Ocneria dispar, according to Oudemans. This result has been invariably confirmed by all subsequent workers, especially by Meisenheimer. Crampton grafted the heads of pupæ of butterflies upon the bodies of other specimens of the opposite sex, but the sexual characters of the head remained unaltered.

In vertebrates, however, there exists a distinct influence of a secre­tion from the sexual glands upon the development of certain of the secondary sexual characters, which do not develop until sexual maturity. In a way the observa­tions on arrhenoidy and thelyidy referred to above are indica­tions of this influence.

Bouin and Ancel had already suggested that the sexual glands of mammals have two independent constituents, the sexual cells and the interstitial tissue; and that the latter tissue is responsible for the development of the secondary sexual character. This has been proved definitely by Steinach,[199] who showed that when young rats are castrated certain secondary sexual characters are not fully developed. The seminal vesicles and the prostate remain rudimentary and the penis develops incompletely. Such animals when adult recognize the female and seem to follow it, but do not persist in their atten­tion and neither erec­tion nor cohabita­tion occurs. When, however, the testes are retransplanted into the muscles of the castrated young animal (so that they are no longer connected with their nerves) seminal vesicles, prostate, and penis develop normally, and these animals show normal sexual ardour and cohabitate with a female although the female cannot become pregnant since the males cannot ejaculate any sperm. When the retransplanted testes were examined it was found that all the sperm cells had perished, only the interstitial tissue of the testes remaining. It was, therefore, proved that the development of the seminal vesicles, the prostate, the penis, and the normal sexual instincts and activities depends upon the internal secre­tions from this interstitial tissue and not upon the sex cells proper. This agrees with the conclusions at which Bouin and Ancel had arrived by ligaturing the vasa deferentia of male animals.

Steinach in another series of experi­ments castrated young male rats and transplanted into them the ovaries of young females. These ovaries did not disintegrate, the eggs remaining, and corpora lutea were formed. In such feminized individuals the seminal vesicles, prostate, and penis did not reach their normal development, and it was thereby proved that the internal secre­tions from the ovary do not promote the growth of the secondary sexual male characters. On the contrary, Steinach was able to show that the growth of the penis was directly inhibited by the ovary, since in the feminized males this organ remained smaller than in the merely castrated animals. On the other hand the infantile uterus and tube when transplanted into the young male with the ovaries grow in a normal way, and Steinach thinks that pregnancy in such feminized males is possible if sperm be injected into the uterus. In some regards the feminized males showed the morpho­logical habitus of females. Soon after the transplanta­tion of ovaries into a castrated male the nipples of its mammary glands begin to grow to the large size which they have in the female and by which the two sexes can easily be discriminated. In addi­tion the stronger longitudinal growth of the body in the male does not occur in the feminized specimens, the body growth becomes that of a female; and likewise the fat and hair of the feminized male resemble that of a real female.

While the castrated males show an interest in the females, the feminized males are absolutely indifferent to females and behave like them when put together with normal males; and, what is more interesting, they are treated by normal males like normal females. The sexual instincts have, therefore, also been reversed in the feminized males by the substitu­tion of ovaries for testes.

The inhibi­tion of the growth of the penis by the ovary is of importance; it supports the idea already expressed that in hermaph­ro­dites this inhibi­tion of the growth of the secondary organs of the other sex is only feeble or does not exist at all.

We may finally ask whether there is any connec­tion between the cytological basis of sex determina­tion by special sex chromo­somes and the physio­logical basis of sex determina­tion by specific substances or internal secre­tions. It is possible that the sex chromo­somes determine or favour, in a way as yet unknown, the forma­tion of the specific internal secre­tion discussed in the second part of this chapter. In this way all the facts of sex determina­tion might be harmonized, and it may become clear that when it is possible to modify secre­tions by outside condi­tions or to feed the body with certain as yet unknown specific substances the influence of the sex chromo­somes upon the determina­tion of sex may be overcome.