THE BOVINE FREE-MARTIN The part played by the sex-hormones in mammalian inter sexuality is best illustrated by the bovine free-martin, a genetic female (XX) co-twin to a normal male, the reproductive system of which becomes abnormal during the period of sexual differ entiation as a result of the action of the sex-hormone of a male co-twin in utero. Twins in cattle may consist of two normal males, two normal females, one male and one female each normal, or one male and the other an individual with an abnormal repro ductive system and known as a "free-martin." In all save one of 126 cases of twins in cattle thoroughly examined by Lillie, two corpora lutea (yellow scars on the surface of the ovaries at the points at which ova have been extruded) were found. This shows that twins are almost invariably binovular (fraternal) in this animal.
The two fertilised ova pass into the bicornuate uterus and become attached. As the zygotes increase in size, the embryonic membranes of the two foetuses meet and in many cases fuse. If so, an intermingling of their blood vessels can result. In the case of twinning involving one normal male and one normal female it is found that fusion of the blood systems does not occur.
Thus the sex-hormone of each developing individual is at liberty to pass into the tissues of its co-twin. The sex-hormone is the instrument which models the sex-organisation. The internal secretions of other organs can also pass from each individual to the other, but these are mainly concerned in the general and not in the special development of the individual and will be alike in both twins. But if the twins are bisexual and if vascular
intercommunication becomes established (as it does in seven cases out of eight), the sex-differentiation of both individuals will be directed by that sex-hormone which is more potent or formed earlier. The testis becomes differentiated earlier than the ovary, and so the male sex-hormone is liberated before the female. The female twin will pursue her sex-differentiation under the direction of the male sex-hormone of her co-twin and will therefore come to possess more or less completely a male organisation. The assumption of the male characters in the case of the foetuses examined is imperfect ; the external genitalia are of female pat tern, the internal organs more or less completely male. The male sex-hormone is liberated before the embryonic gonads of the female have undergone differentiation into ovaries; such differ entiation is prevented and so there is no question of a competitive action between male and female sex-hormones. Hartman (192o) has produced evidence which may mean that the reverse type of hormonic intersex—a male rendered abnormal by the sex-hormone of the female—may occur in the opossum and in man.