1. Steroid hormones were administered to trout alevins by adding them to the culture medium in alcoholic solution, the concentration of hormone in the cultures being within the range 50 y to 400 y/1., and that of alcohol, O O5%o to O-3%o. A control culture was maintained in O-5°/oo of alcohol.

  2. Oestradiol (50 y and 300 y/I.) and testosterone (60 y/1.) had closely similar effects, which included strong inhibition of the development of germinal tissue except in one male treated with the higher oestradiol concentration, where stimulation occurred posteriorly. There was no evidence of sexual inversion, and vasa deferentia appeared in a normal proportion of individuals. With the partial exception of some males, a large area of the somatopleure in the individuals treated became, as in female breeding trout, a ciliated columnar epithelium resembling the internal epithelium of the gonoducts of other teleosts. The connective tissue of the gonadal area was unusually vascular, and in the body-fluids there were large amounts of deeply chromophil material. The tubules of the pronephros were much enlarged. Those of the mesonephros were increased in diameter and reduced in number in the culture treated with the higher concentration of oestradiol. Both concentrations of oestradiol caused the collapse of the gall-bladder, this being the only obvious difference from the effect of testosterone. There are strong reasons for believing that oestradiol caused enlargement of the cells of the interrenal (‘anterior interrenal’) gland, and some reason to think that in some instances testosterone could also cause their hypertrophy.

  3. Progesterone and more particularly desoxycorticosterone acetate inhibited the development of germinal tissue, except in some males treated with the former hormone, where in addition vasa deferentia developed precociously and were much enlarged. Vasa deferentia of a normal size were present in most other individuals lacking oocytes. There was a significant excess of females over males after treatment with DOCA, but the possibility of this being due to a selective mortality cannot be ruled out. There was evidence of such a mortality having occurred in the culture treated with progesterone. Deeply chromophil material was usually present in the body-fluids.

  4. Slight sterol-mimetic effects were observed in the controls treated with alcohol. Possible causes are discussed.

  5. The results are compared with those obtained from similar experiments in other teleosts and in tetrapods.

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