If the optic glands are removed from mature male octopuses, the weight of the testis and its ducts decline. Spermatophore production eventually ceases after five or six weeks. The effect of the operation is most marked in the case of large animals. If the glands are activated by removing the source of their nerve supply in the subpedunculate lobe of the suprapesophageal brain, testis and duct enlargement follows enlargement of the glands themselves ; the number of spermatophores found in the male ducts doubles within a month of the operation. Castration of the animals is not followed by a change in the weight of the male ducts, indicating that the optic gland secretion affects the state of the ducts directly rather than through the testis.Octopus vulgarisCuvier andO. cyaneaGray were used in the experiments.