Embryogenesis in mouth-breeding cichlids (Osteichthyes, Teleostei) structure and fate of the enveloping layer

The eggs of African mouth-brooders are of unusual size and shape. Studying their development may help to more clearly understand epiboly, gastrulation, and the relation between enveloping layer (periderm) and epidermis. When epiboly has progressed over just one fifth of the yolk mass, the germ ring and embryonic shield are already well established. Behind the germ ring very few deep cells are present at this early stage of epiboly, except in the embryonic shield. When the blastodisc covers the animal half of the yolk mass, the future body is already well established with notochord, somites and developing neural keel. Apart from these structures, no deep cells can be detected between enveloping layer and yolk surface; not even a germ ring remains behind the advancing edge of the enveloping layer. Epiboly over the greater part of the yolk is achieved only by the enveloping layer and the yolk syncytial layer. As the margin of the enveloping layer begins to reduce its circumference when closing around the vegetal pole, groups of cells in the advancing edge become spindle-shaped, with a single cell in between of each of these groups broadening along the edge. The enveloping layer (called periderm after epiboly) remains intact until after hatching, when, together with the underlying ectoderm, it forms the double-layered skin of the larval fish. Thereafter, cells deriving from the subperipheral ectoderm gradually replace the decaying periderm cells to form the final epidermis. Thus, in the cichlids studied, the enveloping layer alone forms the yolk sac to begin with, and it covers the larval body until some days after hatching.