GASTRULATION IN THE TURBELLARIAN HYDROLIMAX GRISEA (PLATYHELMINTHES; PLAGIOSTOMIDAE): FORMATION OF THE EPIDERMAL CAVITY, INVERSION AND EPIBOLY

¹ Department of Zoology, University of North Carolina, Chapel Hill, North Carolina 27514

1. Three events characterize the gastrulation of Hydrolimax grisea: formation of the epidermal cavity, inversion, and epiboly.

2. The epidermal cavity, the outstanding feature of gastrulation in this animal, forms in the center of the embryo among the lightly-staining cells of the future epidermis. The cavity expands, pushing aside the yolk cells which surround the embryo. Eventually the cavity becomes open at the surface of the yolk-cell mass. It is subsequently obliterated by compression from the other embryos within the cocoon.

3. Inversion begins when epidermal cells, which lined the epidermal cavity, spread around the yolk-cell mass from the opening of the collapsed cavity. The embryonic mass rises from the center to the surface of the yolk-cell mass, contributing to the progress of the inversion.

4. When the embryo is located to one side of the yolk-cell mass, epiboly begins. The epidermal cells stretch posteriorly to entrap the yolk-cell mass. A few parenchymal cells accompany the spreading epidermis.