Fine Structure of the Gravid Paruterine Organ and Embryonic Envelopes of Mesocestoides lineatus (Cestoda)

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Abstract
M. lineatus tetrathyridia were removed from the body cavity of lizards, Anolis carolinensis, and fed to hamsters in which they developed to adults. Gravid proglottids were taken from feces for study. Intact proglottids or excised paruterine organs (PO) were processed by standard techniques for TEM [transmission electron microscopy]. For SEM [scanning electron microscopy]. PO contents were exposed by subjecting whole proglottids to freon cryofracture or sectioning in Paraplast. Gravid PO were thick-walled with enclosed eggs distributed throughout a cellular matrix. The wall was of parenchymal origin, containing muscle bundles and various cellular products. Each egg consisted of an oncosphere enclosed by a double-unit oncospheral membrane and a syncytial cytoplasmic envelope containing an embryophore. A discrete outer envelope and capsule were absent. Eggs were separated by PO matrix cells with prominent nuclei, mitochondria, intercellular junctions, and much extracellular material. These matrix cells may be related ontogenetically to individual eggs and thus homologous to embryonic envelope cells normally lying outside the embryophore of other cyclophyllids. Some differences in embryonic envelopes presumably was associated with containment of all eggs in the PO which may eliminate the need for individual protective shells. Other components of the PO matrix such as large reticular bodies are of unknown significance.