Occurrence of an accessory cavity in the testis of the topmouth gudgeon, Pseudorasbora parva, and its relation to juvenile intersexuality.

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The structure of the testis-sperm duct system of the topmouth gudgeon, <I>Pseudorasbora parva</I>, was examined histologically. In almost all of the young and adult males examined, a cavity was found to exist between the dorsomedian side of the testis and the opposed peritoneal wall. The testicular cavity was generally flat in shape and ran along the entire length of the testis, with a blind end at the level of the common sperm duct. It appeared that the testicular cavity was not implicated in the transport and storage of spermatozoa in the fish. By studying the process of differentiation and development of gonads in juvenile fish, it was confirmed that some germ cells in differentiating testes underwent oogenesis in many cases, and that the cavity was formed by fusion of the distal edge of the testes with the peritoneal wall in quite the same manner as the ovarian cavity. Thus, the testicular cavity of the topmouth gudgeon is homologous with the ovarian cavity and is apparently a preserved vestige of juvenile intersexuality occurring in this species of cyprinid teleosts.
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