Kind of long read, but interesting about freemartins:
Lillie {46j 47, 48) 4^y 50, 51), who examined a large number of twin
fetuses in a packing house, offered a theory explaining the occurrence
of the freemartin in cattle which has been rather widely accepted.
More than 96 percent of the bovine twins he examined were monochorial
(inclosed in a common placenta) ; the two fetuses usually had
developed in such a way that the blood vessels of the two circulations
were joined and a constant interchange of blood had taken place
between the two developing fetuses. If both fetuses were male or
both were female neither was detrimentally affected by this common
circulation, but if one was male and the other female a sterilization
of the female took place. In some such cases the development of
the female reproductive organs was suppressed, and in extreme cases
certain male organs developed in the twin that was originally female.
He explained the occasional occurrence of fertility in the female of
mixed twins on the theory that, even though monochorial, the vascular
systems of the two fetuses do not necessarily become joined.
Tandler and Keller found that twin cattle fetuses
were inclosed in a common chorion and usually had a joint circulatory
system resulting in typical freemartin genital organs in the female
if the twins were originally of opposite sex. Nevertheless, for purposes
of brevity the foregoing explanation of the cause of the freemartin
will be referred to in the following discussion as Lillie's theory.
The histological studies of Lilhe's specimens by Chapin (4) showed
that the interstitial cells of the testis, the secretion of which supposedly
determines the development of secondary sexual characters, are
produced earlier in fetal life than the corresponding cells of the ovary.
Thus, in the case of vascular fusion in mixed twins the sex hormones
of the male fetus pass into the circulation of the female embryo early
enough to interfere with the development of the mechanism controlling
female secondary sexual characters, and the sex organs of the
female, which are in an indifferent stage, develop toward the male
condition. Chapin attributed the great variation in the degree of
alteration or reversal in the reproductory organs of the freemartin
to differences in the stage at which the interstitial secretions of the
male are introduced into the female embryo, and to the amounts
introduced.
Buyse (3) attributed the extreme modification he found in a 2^-
year-old Brown Swiss freemartin to the fact that it was one of triplets
of which the other two were males. However, Pearl (55) showed that
one male of triplets was capable of sterilizing two feniales. Similar
results were shown by Bissonnette (2), Hutt (38) cited a case of
bovine quadruplets in which one male apparently naodiñed three
females and caused them to become freemartins. Lillie (5i) and
590 Journal of Agricultural Research voi. ei, NO. S
Bissonnette (2) concluded that the amount of hormone does not
determine the degree of sex modification but that it is an ''all-or-none''
reaction as far as freemartin formation is concerned. The extremely
small quantity of the hormone required to transform the female into
a freemartin is particularly noteworthy. Lillie (51) showed that in
one case a male fetus weighing less than 4 gm. had produced male
hormones sufficient to bring about a practical inversion of the ovary
in the female twin.