[Structure and formation of embryonic envelopes in acanthocephalans].

The data on the fine structure and formation of embryonic envelopes in acanthocephalans have been reviewed. Three forms of eggshell organization were recognized according to the ultrastructure of the embryonic envelopes which corresponded to the taxonomic status of acanthocephalans at the class level. The third embryonic envelope is the most variable; its particular structure can be considered a morphological adaptation of the eggs to environmental conditions. Actual formation proceeds as the subsequent formation of embryonic envelopes and intervening space start from the outer envelope. Details of the "fertilization membrane" histogenesis are discussed.