RESUMEN
To investigate a possible role of the yolk syncytial layer (YSL) in the development of the medaka embryo, blastoderms were isolated at different stages of embryogenesis either with or without the layer and were incubated in a culture medium. The blastoderms from cleavage stage embryos (stage 8-9), in which the YSL had not yet formed, developed into an irregular mass of cells. But some of the blastoderms isolated with the YSL from the blastula embryos (stage 10) developed into embryo-like structures with apparent body axes and contained differentiated organs, such as the eye, ear, contractile heart, yolk sac-like sphere and posterior body trunk with notochord. The proportion of such explants increased as the developmental stage proceeded. However, the proportion was much smaller when blastoderms were isolated at the blastula stage without the YSL. These results suggest that the YSL is essential for the development of embryonic structures. At stage 12 (early gastrula), the frequency of formation of such structures was the same among blastoderms with or without the YSL, so that these embryos are apparently committed for pattern formation.