Contribution to the study of egg development and derivation in Oscarella lobularis (Porifera, Demospongiae)

Electron-microscope examinations carried out on Oscarella lobularis specimens allowed us to elucidate cytological details of the development of the female gamete. The young oocytes, dispersed in wide areas of the sponge mesohyl, present an eccentric nucleus, microvillar projections and rare pseudopodia. The cytoplasm contains a dense population of microvesicles, vacuoles with amorphous material, glycogen rosettes, and mitochondria which usually appear clustered. During successive stages of development, the oocytes develop longer and more numerous microvilli, and yolk granules become visible. Microvilli can interdigitate with analogous projections protruding from endopinacocytes, thus suggesting that the latter cells may play a role in supplying oocytes with nutrients. Morphological changes of some choanocytes, which were only recorded during the reproductive period, may indicate the transformation of these elements into oogonia.