Quarterly Journal of Microscopical Science, Vol s2-76, 573-614, Copyright © 1934 by Company of Biologists

Memoirs: Studies on the Germ-cell Cycle of Cryptocotyle lingua

II. Germinal Development in the Larval Stages

¹ University College, New York University

The writer's earlier study (Cable, 1931) on the germ-cell cycle in the adult stage of Cryptocotyle lingua is supplemented by an investigation of germinal development in the larval stages occurring in the marine snail, Littorina littorea. The miracidium-mother-sporocyst was not found although very young rediae were abundant in the material studied. The primordial germ-cells of the young redia are observed in an undifferentiated condition in the body-cavity, which is not well defined due to an abundance of connective tissue. After a period of differentiation, including growth, progressive nuclear changes, and condensation of cytoplasm, the germ-cells multiply by equal division, a process which is interpreted as polyembryony. Germinal differentiation exhibits a distinct anterior-posterior gradient. The mature germinal cells give rise directly to cercarial embryos without germ-mass formation and dissociation or any maturation processes. Although germinal lineage may be traced in the redia, it seems to be interrupted in the cercaria, due to delayed segregation of germ-cells. The soma of the redia does not produce germ-cells at any stage. Evidence is afforded by this and other studies that germinal lineage with sudden intercalations of polyembryonic stages (germ-masses) cannot explain the germinal cycle of the trematodes as a group. In an alternative hypothesis, based on the phylogeny of the Digenea, it is suggested that the ancestors of this group became sexually mature in the mollusc and completed the cycle in that host, possibly before the appearance of vertebrates; and that, with the evolution of the trematodes, sexual phenomena have gradually been lost, while accessory stages and new hosts have been included in the life-cycle. Cryptocotyle lingua is assumed to have been modified to an intermediate extent since sexual reproduction, germ-masses, and the maximum number of intercalary stages are lacking in the larval generations.