The absence of ovaries has been often noticed in twin heifers, and most
commonly associated with deficiency or absence of the womb, and even of
the anterior part of the vagina. The condition is especially common,
though not constant as some have supposed, when the other twin was a
male. Such females are known as _free martins_ and fail to breed. Even
when the ovaries are present in such twins they remain undeveloped, and
are no larger than a bean or hazel nut. These usually have a firm,
fibrous structure, and though there may be interspaces filled with a
transparent fluid, no true Graafian follicles are formed. In birds, the
left ovary only is developed and physiologically active. The absence of
ovary has been noted also in the ewe, and less frequently in the mare
and other species, and appears to be more common in twins than in single
pregnancy. In cattle only has the influence of the male on the female
twin of the same pregnancy been specially noted.
It has been noted that females with ovaries undeveloped, tend to show
many male characters, in head, horns, and neck in cattle, in plumage in
birds, and in voice in both.