Abstract
It is known since the work of Keller and Tandler 1 and Lillie 2 that in heterosexual twins of cattle with fused chorionic membranes the male part exerts a very characteristic influence on the differentiation and the development of the gonads of its female cotwin. This action results in the following distinctive features of the free-martin gonad:
(1) Sterility in consequence of the absence of germ cells.
(2) Reduced or completely suppressed ovarial cortex.
(3) Hypertrophied medulla with more or less pronounced male type of differentiation.
Experiments with amphibians joined in parabiosis at an early embryonic stage were expected to provide possibilities of a detailed analysis of the mechanism of this effect. This hope was rather disappointed, however, when it appeared from the results of Burns' 3 and Witschi's 4 investigations that Amblystoma and Rana behave otherwise than cattle twins and represent two different and new types of reactions. In heterosexual combinations of Amblystoma either one of the sexes may already in the primary sex differentiation assume the absolute leadership, so that pairs of males and pairs of females are obtained exclusively. In frogs, on the other hand, the two sexes first develop independently. Later on the male becomes predominant and causes a regular sex transformation in the female cotwin. The case of Rana comes closer to the cattle type than the case of Amblystoma, because of the finally established dominance of the male sex, however, it differs from it by the accomplishment of perfect sex inversion in the female cotwin.
A new group of experiments with the Calif ornian salamander, Triturus torosus, yielded results more nearly comparable with the conditions in cattle. Of twenty-two pairs of twins, joined in parabiosis at the stage immediately preceding the beginning of muscular movements and raised until close to metamorphosis, one half proved to be unisexual (4 
and 7 
), while the other half was heterosexual (69 
and 5 
).
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