ENVELOPING LAYER AND PERIDERM OF TROUT EMBRYO (SALMO-TRUTTA-FARIO L)

The origin of periderm and epidermis was studied in trout [S. trutta fario] embryos from stage 20 (2 days after fertilization at 10.degree. C) to hatching (stage 410). Between stages 20-50, the blastodisc consisted of an inner mass of blastomeres covered by a superficial layer of closely packed blastomeres, the peripheral layer. This layer gave rise to both peripheral cells and to cells joining the inner mass of blastomeres. Between stages 50-110, junctions differentiated between peripheral cells. This newly formed superficial epithelium, the enveloping layer, no longer gave rise to inward migrating cells. From stage 110 on, a basement membrane differentiated beneath a 1-cell thick subperipheral layer, which thus became the ectodermal basal layer, the prospective epidermal basal layer. The epidermis apparently originated at least in part, from the peripheral cells, between stages 20-50. The periderm assumed a protective function over the body of the embryo and also a secretory function over the yolk sac (probably producing the hatching enzymes). The periderm, which was a temporary structure, appeared to play the role of an embryonic membrane in teleosts.