The origin of pericarderm and epidermis has been studied in trout embryos from stage 20(2 days after fertilization at 10degrees c) to hatching (stage 410). Between stages 20 and 50, the blastodisc consists of an inner mass of blastomeres covered by a superficial layer of closely packed blastomeres, the peripheral layer. This layer gives rise to both peripheral cells and to cells joining the inner mass of blastomeres. Between stages 50 and 110, junctions differentiate between peripheral cells. This newly formed superficial epithelium the enveloping layer, no longer gives rise to inward migrating cells. From stage 110 on, a basement membrane differentiates beneath a one-cell thick subperipheral layer, which thus becomes the ectodermal basal layer, the prospective epidermal basal layer. From these and ultrastructural observations, it is concluded that 1)the epidermis apparently originates, at least in part from the peripheral cells, between stages 20 and 50; 2) the periderm assumes a protective function over the body of the embryo and also a secretory function over the yolk sac (probably producing the hatching enzymes); 3)the periderm, which is a temporary structure, appears to play the role of an embryonic membrane in teleosts.