Two types of neurosecretory cells (α and β) are present in the brain in Hirudo and Theromyzon . The α cells contain secretions which stain with paraldehydefuchsin and contain considerable amounts of cystine. They have an annual cycle of activity, showing greater amounts of secretion in cell bodies and axons in spring and summer. The β cells contain secretions which are acidophilic and poor in cystine. They are consistently present in Theromyzon ; their presence (particularly that of the β1 subclass) is variable in Hirudo . The annual cycle of spermatogenesis in Hirudo is described. Immature stages are found in the testes throughout the year; mature stages are observed during the months of May through October, the greatest number being present in August. Removal of the brain is found to interfere with the maturation of the sperm in both species. This indicates the origin of a gonadotropic influence in the brain. Results of a single experiment suggest that this influence is a hormone. The o cells are suggested to be the source of the gonadotropic factor.