Die Ei- und Embryonalentwicklung vonCorydendrium parasiticum mit besonderer Berücksichtigung der Oocyten-Feinstruktur während der Vitellogenese

Egg and embryo development ofCorydendrium parasiticum (L.) (Hydroida, Athecata) has been examined with special regard to ultrastructural changes of the oocytes during vitellogenesis. It has been established that ectodermal interstitial cells migrate through the mesolamella into the endoderm and differentiate there to germ cells. Despite differences in the origin of the interstitial cells during early development there is still justification to speak of ectodermal descent of hydroid germ cells. The gonophores are equivalent to hydranth buds. By early immigration, oocytes determine the development of buds into gonophores. After completion of vitellogenesis and rupture of the follicle epithelium eggs undergo meiosis. The total and equal segmentation starts already during spawning. After forming a solid morula, gastrulation takes place as moruladelamination. Interstitial cells and cnidoblasts are formed in the prospective endoderm and migrate early into the ectoderm. A polar differentiation becomes evident shortly before hatching of the planula. The planulae keep their solid endoderm until their transformation into primary polyps. Metamorphosis can occur after one day. The delayed beginning of metamorphosis up to 12 days under lab conditions is probably due to the absence of certain bacteria. According to developmental stages of the oocytes ultrastructural changes during vitellogenesis are described. The endproducts of vitellogenesis are heterogeneous yolk (“Komplexdotter”), lipid droplets and glycogen. The “Komplexdotter” represents the bulk of reserve material which is formed by several components of different origin. Of special interest in this respect are the cytosomes resembling the microbodies of liver and kidney because of their paracristalline core and close relationship to the rough endoplasmic reticulum. Comparable cores exist in the “Komplexdotter” which may correspond to enzymes histochemically identified in the yolk of other species.