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]