The breeding season extends from the second week in June to the middle of September. The gametes are produced continuously and spawning occurs more than once. A rhythm is present which is interpreted to be a lunar periodicity. The rate of embryonic development is variable but under usual conditions the larva metamorphoses in about three weeks. Growth during the first two years is comparatively rapid and average adult size is attained at the end of the second year. Fortunate individuals may continue to live and grow for four years. Sexual maturity is reached in one year and spawning is abundant although greater quantities of gametes are produced during the second year. The egg of Cumingia is subject to polyspermy and abnormal development often occurs if too heavily inseminated. The unfertilized egg will retain its vitality for approximately nine to twelve hours during which it is capable of developing normally. The spermatozoa when set free in sea water usually die within four to nine hours, a few die within three hours and a few live for twelve or fifteen hours. The Cumingia egg is favorable material for experimental study in that it is obtainable in abundance, is readily fertilizable artificially, and it withstands laboratory manipulation without injury.