Sex-reversal can result from a disturbance of the general physi ology of the individual during embryonic life.
Burns (1925) joined young embryos of Amblystoma in the tailbud stage in parabiosis and instead of getting the expected chance combinations of the sexes, 6 6 ,18 9 9 6 9 9, he obtained exclusively one sexed pairs. He suggested that the reason for this 44 6 6 : 36 9 9 or 1 : 1 ratio was that in one-half of the original 6 9 and 9 6 associations the males became transformed into females, whereas in the other half the females became trans formed into males.
Witschi (1927), using four different species of frogs, joined embryos 50-70 hours old, and shortly after the closure of the medullary tube, in parabiosis. The controls exhibited the first signs of sex-differentiation during the third week of development ; in the case of the parabiotic twins it was somewhat delayed. The twins were preserved at intervals during the larval period and the stage of metamorphosis. The sex-ratio among the controls was 966a : ioo 99 or r. Among the 56 twins there would be expected the following sex-combinations: 146 6, 14 6 9, 14 9 9. There was found on examination 1666, 17 6 9 (with 7 of the 9 9 exhibiting some stage in sex-reversal), io 96 (with 4 of the 9 9 undergoing sex-reversal), 13 9 The combination of a female with a male twin undergoing sex-reversal was not encountered.
Witschi therefore concluded that the male sex-differentiating agencies predominated and that sex-reversal did not take place before the time of sex-differentiation.
Disharmonies in the development time-relationships of sex differentiation may provide opportunity for sex-reversal. For example, in Myaine (Schreiner, 1904), growth would seem to proceed undirected to the stage when the individual is her maphroditic, before the processes of sex-differentiation set in to convert the individual either into a functional male or else into a functional female. In the young males of the Stone-fly Perle both ovarian and testicular tissues are to be found but the ovarian tissue undergoes atrophy, so that in the adult none but testicular remains ( Junker, 1923). In such circumstances as these the morphological basis of a possible sex-reversal is re vealed and it becomes entirely conceivable how physiological dis turbances during embryonic and early post-embryonic life can change the course of differentiation and lead to the conversion of a genetic male into a functional female, and vice versa.
Sex-reversal can result from a disturbance of the general physi ology of the post-embryonic individual. The conditions necessary for such sex-transformation are (I) there must be a switch-over from one type of metabolism to the other, from the female to the male, or vice versa; (2) the component structures of the sex equipment must be capable of transformation or replacement, one kind of sex-gland tissue must be replaced by the other, ovary must become or be replaced by testis, or vice versa, the accessory sexual apparatus, the external organ of reproduction, the rest of the secondary sex characters must be remodelled or replaced.
Complete sex-reversal therefore cannot occur in any individual or form in which the internal and external organs of reproduction, being fashioned early in embryonic life, thereafter lose their plasticity and become unresponsive to any stimulus which, had it been exhibited at the time of their differentiation, would have controlled these processes. Nor can it occur in those cases in which the differences between male and female sex-equipments are based upon the differential development of two different sets of structures, one of which, in either sex, undergoes complete atrophy. No more can it take place in those forms in which sex dimorphism involves a differential mode of development of one and the same set of structures, for if one plan of differentiation is pursued, the steps cannot be retraced and the alternative route then followed.
Harms (1923) and Guyenot and Ponse (1923, 1925) have shown that if young castrated male toads are fed on a diet contain ing an excess of fat, lipoids, and lecithin for a considerable period of time, the hind portion of Bidder's organ becomes differentiated as an ovary and the fore portion as a new organ of Bidder, and that oviducts and uteri arc: developed while the pointed head becomes transformed into the blunted characteristic of the female. Ponse (1925) succeeded in rearing 9 metamorphosed offspring of one such transformed male functioning as a female and of these 6 were males and 3 females. Harms raised 184 such offspring and of the 161 the sex of which could be identified there were 104 males and 57 females. If in the toad the male is diga metic, and if the YY zygote is non-viable, the expected sex ratio is 266 :1 9.