In a recent paper (Ogren, 1957) the embryogeny of this tapeworm was described. The present paper deals exclusively with the structural and developmental changes of the oncosphere although both topics were summarized in an abstract (Ogren, 1956b). It has not been generally realized that the hexacanth embryo contains differentiated structures and undifferentiated embryonic cells. Moreover, the changes by which the sphere of blastomeres becomes an hexacanth embryo have not been described. In fact, this series of changes was not realized by the writer until after publication of the development of Mesocestoides (Ogren, 1956a). The oncosphere is a muscular embryo that manifests a series of cellular changes during its development before being eaten by the intermediate host. A knowledge of these changes helped in understanding how it is that oncospheres are prepared for infectivity and what original structures are present. Knowledge was also obtained of secretory activity by embryonic cells. The oncosphere of Oochoristica is a large spherical, muscular embryo (Fig. 19) characterized by the possession of 6 large hooks, a body of mesenchymal cells and irregular granular epidermal areas. Each oncosphere is enclosed by an outer shell and an inner non-cellular pseudoembryophore. The purpose of this paper is to describe the development of these structures. Answers to the following questions