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.

CHAPTER IX

MENDELIAN HEREDITY AND ITS MECHANISM1

I

I. The scientific era of the investigation of heredity begins with Mendel's paper on plant hybridization which was not appreciated by his contemporaries. Mendel invented a method for the quantitative study of heredity which consisted essentially in crossing two forms of peas differing only in one well-defined hereditary character; and in following statistically and separately the results of this crossing and that of the inbreeding of the second and third generations of hybrids. This led him to the recognition of one essential feature of heredity; namely, that while the hybrids of the first generation are all alike, each hybrid produces two types of sex cells in equal numbers, one for each of the pure breeds which has been used for the crossing. This takes place not only when the forms used for the crossing differ in regard to one

For the literature on the subject the reader is referred to Morgan, T. H., Sturtevant, A. H., Muller, H. J., and Bridges, C. B., The Mechanism of Mendelian Heredity. New York, 1915.

character only but also if they differ for two or more characters. The statement made is Mendel's law of heredity, or, more correctly, Mendel's law of the segregation of the hereditary characters of the parents in the sex cells of the hybrids.' Mendel's law allows us to tabulate and calculate beforehand the relative number of different forms which appear if the offspring of a mating of two varieties are bred among themselves. In order to do this it must be remembered also that while in some cases the hybrid is an intermediate between the two parent forms, in other cases it cannot be discriminated from one of the two parent forms. In such cases the character which appears in the hybrid was called by Mendel the dominant character and the one which disappeared the recessive character. According to Bateson, who was the first to systematize the phenomena of Mendelian heredity, recessiveness means generally the absence of a character which is present in the dominant type. When, e. g., the cross between a tall and a dwarf form of pea gives in the first generation only tall peas, on the basis of the presence and absence theory the dominant form contains a factor for growth which is lacking in the dwarf form. While this theory fits many cases it meets with difficulties in others. Thus the presence of a factor

Mendel, G., "Experiment in Plant-Hybridization," translated in W. Bateson's classical book on Mendel's Principles of Heredity. Cambridge, 1909.