Fig. 4

Sections of early 4-cell stage zebrafish and medaka embryos. Semi-thin section of zebrafish (A) and medaka (B) embryos visualized in a light microscope. Arrows indicate the areas shown in C-F representing the cleavage positions. The insets represent the orientation of the sections. Low magnification at the electron microscope showing the region of the germ plasm of zebrafish (C), and the corresponding position in medaka (D). At a higher magnification, the germ plasm of zebrafish is visible as distinct amorphous inclusions with a regular spherical shape and a filamentous fine structure lacking a surrounding membrane (E). At this magnification germ plasm resembling structure composed of irregularly shaped amorphous inclusions with a granular fine structure can be observed in medaka embryos. As described for zebrafish [28], the putative germ plasm of medaka can be followed in serial sections.