/. Embryo!, exp. Morph. Vol 32, 3, pp. 795-804, 1974 795 Printed in Great Britain Studies on the gastrulation of amphibian embryos: pseudopodia in the gastrula of Bufo bufo japonicus and their significance to gastrulation By NORIO NAKATSUJI 1 From the Department of Zoology, University of Kyoto SUMMARY The course of gastrulation in embryos of Bufo bufo japonicus was studied by use of 1 /*m Epon sections. Tt was observed that the cells of the presumptive pharyngeal endoderm and mesoderm form pseudopodia coinciding with their invagination. Some of the inner cells of the ectodermal layer also form pseudopodia. The cells of the endodermal mass and the surface of the embryo rarely form such structures. The observed pseudopodia seem to correspond to the hitherto reported pseudopodia formed by the dissociated cells from the gastrula of amphibians. Formation of the pseudopodia seems to suggest that the invagination of the presumptive pharyngeal endoderm and mesoderm is brought about by active migration of the individual cells along the inner surface of the ectodermal layer. INTRODUCTION When morphogenetic movements, such as gastrulation, occur in embryos, many factors, such as migration, adhesion, contraction and expansion of embryonic cells, are thought to play important roles. In the case of echinoderm gastrulation, cells of the invaginating archenteron protrude fllopodia toward the blastocoelic wall and these filopodia play a main role in the invagination (Gustafson & Kinnander, 1956; Dan & Okazaki, 1956; Gustafson & Wolpert, 1967). During the epibolic movement in teleost embryos the deep cells form pseudopodia and migrate actively (Lenz & Trinkaus, 1967; Trinkaus & Lenz, 1967; Wourms, 1972; Trinkaus, 1973). Invaginating mesodermal cells of chick embryos, in the primitive streak and between the epiblast and hypoblast, are ftbroblast-like and form filopodia (Trelstad, Hay & Revel, 1967; Granholm & Baker, 1970). In the case of amphibian gastrulation, the cells lining the top of the invaginat- ing archenteron resemble a bottle in shape. These cells have been termed 'bottle cells' or 'flask cells' and it was proposed that they play a main role in the invagination of the archenteron (Holtfreter, 1943a, b; Baker, 1965). Epibolic 1 Author's address: Department of Zoology, Faculty of Science, University of Kyoto, Sakyo-ku, Kyoto 606, Japan.