Ultrastructure of the embryonic envelopes and associated maternal structures of Distoichometra bufonis (Platyhelminthes, Cestoidea, Nematotaeniidae)

Eggs within paruterine capsules of gravid proglottids of Distoichometra buf onis were examined by light and transmission electron microscopy. The embry onic capsule was membranous, but was immediately underlain by a non-uniform subcapsular lamina that was an intracellular component of the outer embryo nic envelope. The subcapsular lamina was thick and semi-rigid on anterior a nd posterior poles, but thin and membranous laterally, giving the entire eg g a laterally oblong shape. In contrast, the embryophore was spherical and uniform in thickness. The paruterine capsule walls were derived from layers of flattened processes of medullary parenchyma cells lined internally with a layer of uterine epithelium. All these layers extended inward to form pa renchyma-uterine partitions segregating each egg into an individual chamber . The uterine epithelium was very thin, syncytial, and contained numerous v esicles. Little uterine secretory product occurred in the uterine lumen or on the outer surface of the embryonic capsule. Except for the unique subcap sular lamina, most features of the eggs and paruterine capsule resembled th ose of other nematotaeniid species. The paruterine capsule wall was similar to that of Mesocestoides lineatus, a species whose paruterine organ lacks parenchyma-uterine partitions.