Growth and Hatching of Taeniid Eggs

Abstract

In our work on the effects of radiation upon Hydatigera taeniaeformis Batsch, 1786, we have had difficulty obtaining heavy infections, because of apparent variability among the eggs. In an approach toward control of this variability, we tried to determine the characteristics of a mature egg, as to both its development in the gravid proglottid and its hatchability in artificial media. The taeniid egg has been described by a number of workers. Wardle and McLeod (1952) described the egg of Hydatigera taeniaeformis, the cat tapeworm, and Silverman (1954) described the egg of Taeniarhynchus saginatus (= Taenia saginata) Goeze, 1782, the beef tapeworm of man, and of Taenia pisiformis, Bloch, 1780, the dog tapeworm. Ogren (1953, 1957) outlined the early embryology and development of the embryophore in taeniid cestodes, and gave the taeniid ontogeny a place in his proposed system of embryological patterns. Lee et al (1958) added a note on the origin of the embryophore by secretion. Other workers have described the oncospheres and membranes of members of this important group of tapeworms. In Hydatigera, the developed oncosphere is contained within 3 membranes, an outer chorionic enclosing yolk material, a striated embryophore consisting of hexagonal prisms arranged radially and cemented together, and an oncospheral membrane closely investing the 6-hooked larva, or oncosphere. During development, the embryophore probably arises by secretion from a layer of polyploid cells (actually a detached epidermis derived from the embryo during its early cleavage); as the egg matures, the embryophore thickens. The mature egg, liberated by rupture of the proglottid in the host's intestine, loses its outer (chorionic) membrane, and when recovered by concentration from cat feces possesses the characteristic striated appearance of all taeniid embryos. We undertook the present study in order to determine whether or not the eggs of Hydatigera pursue a regular course of development to maturity within the proglottid. As we and others have noted, however, morphological maturity is no guarantee of hatchability in taeniid eggs. Therefore we undertook further studies on the hatching of eggs in artificial media. Silverman (1954) artificially hatched and activated eggs of Taeniarhynchus saginatus with consistently good results, reaching 78 percent hatching in some samples. Whatever differences in percent hatched and activated embryos he obtained were attributed to differences in the percent of mature eggs in the samples used. Our hatching experiments had an unforeseen result, different from Silverman's, which we think has evolutionary implications. We found variation in hatching accompanying apparent uniformity in stages of development.