Cyclic changes in the testis of the brook stickleback Eucalia inconstans (Kirtland)

The cyclic changes in the testis of the five‐spined stickleback Eucalia inconstans (Kirtland) were studied histologically. Specimens were trapped between July 1965 and July 1967 in a shallow pond near London, Ontario. A three‐dimensional microscopic study showed a main vas deferens and a system of primary, secondary and tertiary tubules. The testis cycle was divided into seven arbitrary stages. Spawning takes place from mid‐April to mid‐July. This is followed by the division of primary spermatogonia which are located along the walls of the tubules, producing cysts of spermatogonia enclosed in connective tissue which is surrounded by a thin epithelium. Both primary and secondary spermatocytes develop within these cysts. Breakdown of the cysts occurs with the development of spermatids and spermiogenesis occurs while spermatids are free in the tubules. Over‐wintering of mature sperm takes place. Development of mature sperm from primary spermatogonia takes about 156 days. Germinal epithelium is absent but primary germ cells are believed to be those cells occupying the spaces between the tubules of the testis. No tissue which might be implicated in hormone production was observed. Phagocytic invasion of the testis has been studied. Massive infiltration by phagocytes is believed to be responsible for the sudden increase in testis weight observed during spawning. These cells ingest sperm nuclei and groups of them have been observed in the lumen of the tubules and the vas deferens, probably on their way out of the body.