An electron microscope study of primary epidermis formation in freshwater planarian embryos

Abstract

Early development in freshwater planarians is generally considered to be highly modified to the point of being unique. A careful examination by TEM, however, suggests that the primary epidermis (Skaer, 1965) is formed in a rather regular manner but is partially inverted with respect to the definitive body axes. After formation of the yolk cell syncytium, the blastomeres enclosed within it increase in number in the central area. Some of these blastomeres then move peripherally as a group and fuse to form another syncytium, the primordium of the primary epidermis. This primordium contacts the surface of the yolk-cell syncytium at the place where the primordium will subsequently flow out. The primordium spreads to the opposite pole through the spaces among the syncytial and non-syncytial yolk cell masses.

We have shown that the formation of the primary epidermis in planarian embryos involves an epiboly-like movement of cells similar to that described for other neoophoran groups (Thomas, 1986). This suggests that triclad development may not have diverged as markedly from that of the other turbellarians as previously thought and that epidermal epiboly in the triclads might be homologous to gastrulation in the archoophora (Kato, 1940), though the direction of cell migration is reversed.