Ultrastructure of primary embryonic envelopes in Proteocephalus longicollis (Cestoda : Proteocephalidea)

Ultrastructure of fertilized oocytes of the tapeworm Proteocephalus longico llis (Zeder, 1800) and development of embryonic envelopes during cleavage a nd early preoncospheral stages were examined by transmission electron micro scopy. The developing embryo is surrounded with three basic envelopes. The outermost envelope is the capsule, which consists of two closely apposed me mbranes. The appearance of the capsule changes during the more advanced sta ges. The outermost envelope encloses outer and inner embryonic envelopes, e ach of which is syncytial. Vitelline cells take part in formation of embryo nic envelopes. The outer envelope contains numerous ribosomes, polysomes, c isternae of granular endoplasmic reticulum, large lipid droplets and rosett es of alpha-glycogen. The inner envelope is formed by three mesomeres. In t he inner envelope, abundant ribosomes and some mitochondria are present tog ether with the large nuclei of mesomeres. Differences in the fine structure of the primary embryonic envelopes of P. longicollis and other cestodes ar e discussed.