The relationship of gonadal development to the life cycles of the paired species of lamprey, Lampetra fluviatilis (L.) and Lampetru planeri (Bloch)

Gonadal developnient has been studied throughout the larval period of Lumpetru fluviatitis and comparisons have been made with conditions in the larvae of the closely relatcd non‐parasitic species, L. planeri . The two forms differ in the time of onset or mitotic division in germ cells and in the period when oogenesis is initiated. These phases begin in L. pluneri when the ammocoetes are one year old and in L. fluviatilis at the end of the second year. The course of sex differentiation appears to be sinlilar in both forms. While in the early stages, the oocytes of L. planeri are larger than those of L. fluviatilis. at comparable ages, by the end of the third year they are sinlifar in size in both spccics. It is suggested that in L. planeri , during the extension of the larval period, gonad growth is retarded and during this phase the ammocoete accumulates body reserves which are used to meet the metabolic requirements of the rapidly developing gonads after metamorphosis. The differences observed in early gonadogenesis are believed to account for the reduced fecundity of the L. planeri ammococtc and their adaptive significance is discussed.