. The biology of the amphibia. Amphibians. THE UROGENITAL SYSTEM 279 bers of each pair was always the same. As it was very unlikely this could have come about through the chance selection of individuals of the same sex, a sex reversal would seem to have occurred in some instances. Humphrey (1929a) implanted the preprimordium of the gonad of one sex into the embryo of another after removing the corresponding gonad preprimordium from the latter and found that it usually differentiated according to the sex determination of the donor. Later a modification or sex reversal of the graft or host gonad
. The biology of the amphibia. Amphibians. THE UROGENITAL SYSTEM 279 bers of each pair was always the same. As it was very unlikely this could have come about through the chance selection of individuals of the same sex, a sex reversal would seem to have occurred in some instances. Humphrey (1929a) implanted the preprimordium of the gonad of one sex into the embryo of another after removing the corresponding gonad preprimordium from the latter and found that it usually differentiated according to the sex determination of the donor. Later a modification or sex reversal of the graft or host gonad may be effected. When an ovary or testis of similar size and species developed together in an animal it was always the ovary which suffered modification. A hormone from the testis apparently exerted an inhibitory influence on the growth of the ovarian cortex very similar to the. Fig. 101.—The urogenital systems of four adult frogs, Rana temporaria, representing stages in the transformation of females into males. K., kidney; , Mullerian duct; Ov., ovary; , seminal vesicle; T., testis; „, Wolf- fian duct. {After Witschi.) action of heat in the case of the Wood Frog, and further changes in the male direction were a result of the cessation of cortical activity. If the gonad developed from a male donor was small in size or retarded in development, it was not able to dominate the ovary, but on the other hand, it underwent a partial sex rever- sal. It would thus seem that there are both male and female determining substances in the developing gonads. Amphibia pass through an indifferent and apparently later a bisexual state before genetic factors give the ascendency to the tissues which shape the germ cells into either ova or sperm. Up to a certain stage various environmental factors can reverse this dominance, and gonads of the opposite sex will develop. This shift of balance is not, however, transmitted to the next generation. Female frogs which have changed into mal
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