Oogenesis and sexuality of the chiton, Lepidochitona cinereus (Mollusca, Polyplacophora), The development of the oocytes of Lepidochitona cinereus (L.) has been studied by light microscopical techniques. This development comprises 5 stages, which can be characterized by histological features. During development the egg ceils protrude gradually from the germ epithelium and, finally, become separated from the epithelium. Preceding main vitellogenesis, the oocyte surface is enlarged by cytoplasmic protrusions, which are later reduced. The oocyte cortex or hull is multilayered and is set with appendages.

Nukleolenh6fe auf einem Schnitt durch die Grenzregion yon Karyo-und Zytoplasma (H6he der Oozyte 0,078 ram) Abb. 7: Oozyte des Wittlings im ersten Wachstum (Breite mit Follikel 0,130 mm) Wachstumsvorg/inge ein. Sie werden offenbar unterst/itzt durch eine Vergr6gerung der Kernobertl/iche, die sich wellt. Ein Tell der Nukleoli tritt bereits auf dieser friihen Phase aus dem Kernplasma heraus (Abb. 7). Das Zellplasma beginnt sich zu schichten. Auf die perinukleare Zone folgt eine homogene, dann

On the ultrastructure of the oocytes of marine teleosts. The fine structure of various stages of development of oocytes of several marine teleosts was studied by electron microscope. The primary membrane of the oocyte represents a typical cell membrane. The contact between the oocyte and the folIicIe is particularly close in early stages. The membrane of the oocyte and the intercellular space disappear in closely restricted areas. Outside the oocyte a homogeneous substance is applied between the microvilli introducing the forming of the cortical layers. The cortex consists of two layers: the outer one may bear ledgeqike elevations; the inner one may form bundles of fibrils, which strengthen the cortex. During growth of the oocyte the surface of the nucleus is enlarged by indentations in which a nucleolus is almost aIways located. Nucleolar material is supplied to the cytoplasm. Mitochondria first appear near the nucleus; later they fill up the cytoplasm. Through concentricaI regrouping of their inner structure, the mitocbondria are transformed into multi-Iayered, membranous bodies which have a close topographical relation to the sparse ergastoplasm. Aflcer regrouping of their inner structures, the mitochondria deta& small elongated vesicles into the cytoplasm. In this way, not only membranous material is delivered but also an immediate contact between the inner chondrioplasm and the cytoplasm is reached. There are two ways of yolk production: in the unconfined cytoplasm (lipid yolk) and within certain areas surrounded by membranes (protein yolk). The yolk nucleus of teleost oocytes is granulous and bears no other elements of structure. The intercellular space between the oocyte and the follicular epithelium is transformed into zona pellucida by storing of special mucopolysaccharides. During ovulation the oocyte is separated from the follicle along the basement membrane. The barrier between blood and the oocyte consists of at least 5 layers. The basement membrane, which serves as ultra filter, has one or more layers, depending on the species. The high resistance of demersal eggs is related to their thick cortical layers. Planktontic eggs are of more delicate structure, the details of their construction, however, vary among species. EINLEITUNG

Reproduction and oocyte-development in Zoarces ~ai~viparus (Pisces, Osteichthyes). The oocytes of Zoarces viviparus L., obtained from south-eastern parts of the North Sea (Helgoland, Sylt), undergo four cytological stages. They are ready for fertilization in July-August. In the ovaries of gravid females, embryos appear in September. The number of embryos depends mainly on the female body length. During oogenesis the ripening oocytes and the adjacent cell layers are transferred into the lumen of the ovary. Each of the ripening eggs is fixed to the germ epithelium by a funiculus and an adhering region. A cle~ filled with liquor originates in the theca folliculi. The oocyte and its surrounding layers are embedded in this liquor. The layers are the same as in oviparous fishes: cortex radiatus internus and externus, follicle epithelium, basal lamina and theca folliculi. The main difference is the smaller thi&ness of the cortical layers in Zoarces oocytes. Another difference is the higher grade of vascularization in the connective tissue of the follicle. Both features are adaptations characteristic to ovoviviparity. The duration of development and the possible function of the fotlicle layers are discussed.

Investigations on reproductive biology in three Littorina species of the southern North Sea. Populations of Iu'ttorina mariae, L, obtusata and L. saxatilis from the coast near Roscoff (Brittany), from the North Sea islands Helgoland and Sylt were investigated with regard to several characters of their reproductive biology. Important factors are body size and weight, maturation size, shell thickness, sex ratio and reproductive effort of adults as well as number, size and mortality of embryos. Mature L. saxatilis individuals inhabiting sandy shores are smaller and have thinner shells and higher reproductive effort than those on rocky coasts. On shores with loose and semipermanent boulders the adults are clearly larger. Interannual variations in some life history parameters may be explained by changes in food supply. In populations with small adults, ~ 9 are bigger than c~ ~. In localities where L, mariae and L. obtusata occur sympatrically, a tendency to accentuate interspecific contrast is recognized. Life history patterns, especially in L. saxatilis populations, show two tendencies: (a) a rapid increase of shell-size and thickness, which offers better chances of survival and of maximizing clutch-size and clutch-numbers during life-time; (b) early entrance into maturity leads to accelerated reproduction, but life expectation is reduced. Both tendencies are balanced by the action of biotic and abiotic factors. EINLEITUNG