Endocrine programmed development and reproduction in Nereis

The pattern of cerebral endocrine activity in Nereis diversicolor is characterized by the maintenance of a high plateau of activity prior to, and during most of, the time occupied by gametogenesis. During the closing stages of the life cycle, the rate of secretion is progressively reduced, resulting in the production of a homogeneous population of gametocytes and subsequently in their final maturation. Secretion has been assayed in terms of the regeneration-promoting, maturation-inhibiting, and gametotrophic activities of the hormone. Assays involved transplantation of living brains, in some cases between animals at different stages of maturity, but evidence is presented that the activity of such brains reliably reflects their rate of secretion in situ. Cerebral control of growth and maturation is probably mediated by a single hormone, an ordered sequence of events occurring as different thresholds for hormone action are reached in turn.