Development and Morphology of the Oncosphere of Mesocestoides corti, a Tapeworm of Mammals

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Abstract
I. Embryonic development (1). Oogenesis. Oocytes are derived from plastin cells (primordial oocytes) matured in the ovary. As the oocytes grow, their cytoplasm becomes granular. Mature oocytes contain large spheres of vitelline substance. Meiosis occurs with the sperm nucleus present while oocytes are in the uterus. (2). Cleavage. The zygote, enclosed by a capsule of vitelline material, divides into 2 nearly equal blastomeres. One blastomere divides slowly, is excluded from the morula and forms no part of the embryo. The other blastomere divides into cells which cleave at various rates to form a morula of small cells. (3). Cellular differentiation in the embryo. An embryonic epidermis, the embryophore, develops from small, squamous, granule-filled cells at the surface of the embryo and separates from the inner cell mass. The latter develops 6 hooks and becomes the oncosphere. The granules and membrane of the embryophore are metachromatic and give a positive reaction with the periodic acid-Schiff procedure suggesting the presence of glyco-protein or mucopolysaccharide substances. II. Morphology of the Oncosphere. The morphology and development of the hooks from oncoblasts is considered. The body parenchyma, covered by a thin cuticle, is composed of small "somatic" cells. Typical compact plastin cells differentiated from granular cells in the parenchyma. They form a germinative cell mass in the presumptive scolex region of the oncosphere. The morphology of sectioned oncospheres is described. The behavior and visible structures in living oncospheres are presented. Flame cells are absent. Granular areas ("penetration glands") as found in Hymenolepis are absent. The basic knowledge necessary for comparative embryonic studies in tapeworms is suggested. The tapeworm tissues are considered to be predominantly "mesodermal" in morphology and, probably, in origin.
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