Ultrastructural Observations on the Spermatozoon, Oocyte and Fertilization Process in Gonapodasmius , a Gonochoristic Trematode (Trematoda: Digenea: Didymozoidae)

Observations were performed in the uterus of a female Gonapodasmius sp., a gonochoristic didymozoid Trematode. The oocyte is a round cell 6 μm in diameter, which shows a ‘nucleolus‐like cytoplasmic body’ and cortical granules. The spermatozoon is filiform, mobile and about 50 μm long. There is no acrosome. The anterior tip of the spermatozoon contains two centrioles made up of singlets and cortical microtubules with associated glycocalyx. The centrioles are continued as two axonemes of the classical 9 + ‘1’ pattern of flatworms, accompanied by a mitochondrion and a short row of cortical longitudinal microtubules. It is the posterior part of the sperm cell which contains the nucleus. At the outset of fertilization, the anterior part of the spermatozoon coils around the oocyte and penetrates it by lateral fusion. The posterior region of the spermatozoon, with the nucleus, is the last part to enter the oocyte, after passing through a perforation in the forming eggshell. The whole spermatozoon thus penetrates the female cell.