Zosterisessor ophiocephalus is subject to remarkable fishing effort even during the reproductive period. Therefore, basic knowledge of its biology is an essential preliminary to any attempt to manage either natural or artificial reproductive stocks. This study is meant to provide a basis for identifying reproductive stages and for relating them to the natural seasonal cycle or to the effect of treatments for promoting maturation. Oocyte staging was performed on conventional thick and TEM sections. Previtellogenic, vitellogenic and maturative stages were further subdivided according to the classic criteria such as meiotic chromosome pairing, nucleus and nucleolus appearance and position, Balbiani's body formation, occurrence and storage of yolk granules as summarized in Table I. Development of theca and granulosa cells was also taken into account. Attaching filament formation and chorionic micropylar specializations are described by TEM and SEM. The results are compared with a previous approximate LM description in this species and with the more recent ultrastructural features reported in gobies and in other teleosteans.
The planarian Dugesia lugubris is a balanced hermaphrodite meaning that male genetic factors are in equilibrium with female factors . Differentiation of the gonads is controlled by the region in which they develop. According to the classical theory of germ cell formation these cells stem from neoblasts that are induced to differentiate by factors specific to the gonadal regions factors presumably due to gradients formed by neurosecretory activity of the cephalic ganglia and longitudinal nerve cords . A more recently proposed theory holds that germ cells in regenerates originate not from neoblasts but from dedifferentiated cells and that characteristics of the gonadal regions are determined by direct interactions of cells here . Results of our experiments with homo-and autoplastic grafst support the classical theory . Prepharyngeal portions grafted onto posterior body portions retained their ability to maintain or induce development of ovaries . Postpharyngeal portions grafted onto anterior portions produced only testes even though the brain developed normally in these regenerates . Under these experimental conditions gonad regeneration took longer than it does in normal regeneration (i .e. that in which body regions are not displaced) .
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