Fine Structure and Differentiation of the Acrosome‐Like Structure in the Solitary Ascidians, Pyura haustor aud Styela plicata1

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Abstract
An acrosome‐like structure has been recognized at the apex of mature spermatozoa of both Pyura haustor and Styela plicata. The acrosome‐like structure of P. haustor is a slightly depressed ellipsoid, approximately 90 nm × 80 nm × 50 nm, in length, width and height, respectively, while that of S. plicata is an antero‐posteriorly elongated, flattened vesicle, approximately 200 nm × 100 nm × 50 nm, in length, width and height, respectively.During spermiogenesis, two vesicles (50–80 nm in diameter) are found in a blister at the apex of early spermatids of both species. These vesicles, presumably derived from the Golgi apparatus, contain moderately electron‐dense material. In late spermatids, these two vesicles appear to fuse to form an acrosome‐like structure.Because of its extremely reduced size and the paucity of its contents, it is unlikely that the acrosome‐like structure of these sperm contain a significant amount of chorion lysin(s). A well developed Golgi apparatus and many Golgi vesicles of various sizes are found in the cytoplasm of spermatids in both P. haustor and S. plicata. It is hypothesized that ascidian spermatozoa contain a poorly developed acrosome, and that the chorion lysin(s) are intercalated into the plasmalemma enclosing the sperm head.