Sex differentiation and development of the gonad in the Japanese river larnprey, Lmnpeira japozicu

Sex differentiation and development of the gonad were examined histologi-cally during larval, metamorphosing, and young adult stages in the Japanese river lamprey, Lampetra japonica, collected throughout the year in the Assabu River, southern Hokkaido. Gonads of all ammocoetes with total lengths below 30 mm were sexually indifferent. During the subsequent course of larval growth, gonial germ cells developed to form definite cysts in all of the gonads examined, revealing the occurrence of the cystic stage in initial sex differentiation of the lamprey. In all ammocoetes with total length of 70-90 mm, germ cells in the cysts more or less showed meiotic nuclear changes, though the degree of appearance of the premeiotic cysts varied in different specimens observed. Differentiation of ovaries with grow-ing oocytes followed the cystic stage in some larvae of total length of 70-90 mm. Testicular differentiation was observed to occur either with the breakdown of germ cell cysts and rearrangement of surviving germ cells at the cystic stage or wit