Basically, the fine structure of the centrosome at the spindle poles of dividing spermatogonia and spermatocytes of Lymnaea stagnalis is similar to that of mitotic centrosomes in animal somatic cells. The presence of two centrioles at the mitotic poles of spermatogonia indicates that correlation between the nuclear and the centriolar cycles is maintained. The poles of the meiosis I and meiosis II spindles possess two centrioles, thus suggesting that the nuclear and centriolar cycles are uncoupled in the secondary spermatocytes. Centrioles replicate between two meiotic divisions, whereas DNA replication does not proceed. In the secondary spermatocytes the final stages of elongation of the daughter centriolar cylinders are lacking: at the poles of the meiosis II spindle the length of the daughter centrioles is no more than half the length of the mother centrioles. Complex transformations of the centriolar apparatus are observed during spermiogenesis in L. stagnalis. In the early spermatid a single basal body has a centriolar structure. Later, microtubules cannot be found in the basal body cylinder, which consists of nine amorphous columns. The diameter and length of the basal body are approximately twice as large as those of the centriolar cylinders. An additional cylinder of increased electron density appears inside the basal body and is also preserved in the spermatozoon. During spermiogenesis, an electron-dense structure can be observed near the proximal end of the basal body. The sperm basal body oft. stagnalis has been suggested to be the result of transformation of two centrioles, which enter the spermatid during meiosis II. (C) 1994 Academic Press, Inc.