Summary.(1) The extrusion of the genital products of echinoids is due to the contraction of muscle fibres in the gonad walls, more abundant in the ovary than in the testis. The contraction of these muscles can be artificially stimulated.(2) A spawning male stimulates ripe individuals of both sexes which are in the neighbourhood to spawn. A spawning female stimulates ripe males to spawn.(3) After spawningStrongylocentrotus lividusre‐forms ripe eggs in nine days at a temperature of 17–