One of the most remarkable features of the reproductive system of
flatworms (Neoophora) is the presence of specialized cells, vitellocytes,
participating in the process of egg-shell formation. The embryonic development
in cestodes of the recently established order Diphyllobothriidea
is polylecithal and occurs in external aquatic environment, similar
to other lower eucestodes, i.e. caryophyllideans, bothriocephalideans,
spathebothriideans, diphyllideans and trypanorhynchs. Diphyllobothriidean
cestodes produce a large number of polylecithal eggs, which are characterized
by a thick and hard egg-shell. A great amount of nutritive reserves
(lipids and glycogen) is required during embryonic development of
these cestodes, which allows them to remain in the water until the
embryonation is complete. Therefore, the ultrastructural aspects
of thematuration of vitelline cells participating in the egg-shell formation
are considered an important step toward better understanding of the
reproductive biology of cestodes.