The work of Steinach[[12, pp.225f.]] on rats is another well-known example of changing sex characters by surgery. Steinach found that an ovary transplanted into a male body changed its characteristics and instincts into the female type. The growth of the male sex organs he found to be definitely inhibited by the ovaries. He went so far as to transplant the whole uterus and tube into the male body, where it developed normally. One of the most interesting of his results is the observation of how the instincts were changed along with the type of body. The feminized males behaved like normal females toward the other males and toward females. Likewise they were treated as normal females by the males.
It would be impossible to give here any just idea of the vast amount of rigid scientific experimentation which has been carried on in this field, or the certainty of many of the results. Sex is really known, about as well as anything can be known, to arise from the chemical causes discussed above. That is, the endocrine explanation is the correct one.
One of the most significant results of the transplantation experiments is the evidence that each individual carries the fundamental bases for both sexes. When Goodale changed a male bird into a female as to secondary characters and instincts by replacing one secretion with another, he was faced with the following problem: How can a single secretion be responsible for innumerable changes as to feather length, form and colouring, as to spurs, comb and almost an endless array of other details? To suppose that a secretion could be so complicated in its action as to determine each one of a thousand different items of structure, colour and behaviour would be preposterous. Besides, we know that some of these internal secretions are not excessively complicated—for instance adrenalin (the suprarenal secretion) can be compounded in the laboratory. We may say that it cannot possibly be that the ovarian or testicular secretion is composed of enough different chemical substances to produce each different effect.
There remains only the supposition that the female already possesses the genetic basis for becoming a male, and vice versa. This is in accord with the observed facts. In countless experiments it is shown that the transformed female becomes like the male of her own strain and brood—to state it simply, like the male she would have been if she had not been a female. If we think of this basis as single, then it must exhibit itself in one way in the presence of the male secretions, in another way under the influence of the female secretions. In this way a very simple chemical agent in the secretion might account for the whole difference—merely causing a genetic basis already present to express itself in the one or the other manner.
This may be illustrated by the familiar case of the crustacea Artemia salina and Artemia Milhausenii. These are so unlike that they were long supposed to be different species; but it was later discovered that the genetic basis is exactly the same. One lives in 4 to 8% salt water, the other in 25% or over. If, however, the fresh-water variety is put in the saltier water with the salt-water variety, all develop exactly alike, into the salt-water kind. Likewise, if the salt-water variety is developed in fresh water, it assumes all the characteristics of the fresh-water kind. Thus the addition or subtraction of a single chemical agent—common salt—makes all the difference.
If this basis for sex is single, it is represented by the male plumage in domestic birds, the secretions from the sex-glands acting as modifiers. But a great deal of evidence has been produced to show that the genetic basis, in man and some other forms at least, is double. That is, we must think of two genetic bases existing in each individual—each representing one of the two types of secondary sex characters. The primary sex (i.e., the sex glands) would then determine which is to express itself. In the domestic birds described above, the male type of body appears in the absence of the ovarian secretion, and the female type in its presence. In man and the more highly organized mammals, we must use "secretions" in the plural, since a number of them, from different glands, act together in a "complex." Goodale, experimenting with birds, was unable to definitely decide whether the basis for sex was single or double in that material, though he favoured the latter explanation.
Dr Bell, the English gynecologist, using human surgical cases as a basis, commits himself strongly to the dual basis.[[2, p.13.]] "Every fertilized ovum," he says, "is potentially bisexual," but has "a predominating tendency ... toward masculinity or femininity." But "at the same time," he remarks, "it is equally obvious that latent traits of the opposite sex are always present." After discussing mental traits observed in each sex which normally belong to the other, he concludes as follows: "If further evidence of this bisexuality, which exists in everyone, were required, it is to be found in the embryological remains of the latent sex, which always exist in the genital ducts."
In some lower forms, dual sexuality is apparent until the animal is fairly well developed. In frogs, for example, the sex glands of both sexes contain eggs in early life, and it is not possible to tell them apart with certainty, until they are about four months old.[[12, p.125.]] Then the eggs gradually disappear in the male.
However, we need not depend upon non-mammalian evidence for either the secretory explanation or the dual basis. An ideal case would be to observe the effects of circulating the blood of one sex in a developing embryo of the other. This blood-transfusion occurs in nature in the "Free-Martin" cattle.[[21]]
Two embryos (twins) begin to develop in separate membranes or chorions. At an early stage in this development, however, the arteries and veins of the two become connected, so that the blood of each may circulate through the body of the other. "If both are males or both are females no harm results from this...," since the chemical balance which determines the bodily form in each case is of the same type. But if one is a male and the other a female, the male secretory balance dominates the female in a very peculiar fashion. The female reproductive system is largely suppressed. She even develops certain male organs, and her general bodily appearance is so decidedly masculine that until Dr Lillie worked out the case she had always been supposed to be a non-functional male. She is sterile. The blood transfusion not only alters the sex-type of her body, but it actually modifies the sex glands themselves, so that the ovary resembles a testicle, though dissection proves the contrary.