Knowledge of the embryonic morphogenetic movements of any group of vertebrates must be explored by experimental means in order to answer the questions and correct the errors left over from simply watching development or studying a series of stages in serial sections. The holostean fishes are among numerous important groups of vertebrates that have not previously been explored by the newer cell-marking techniques. Because of superficial resemblances of the epibolic blastodisc and yolk plug of the Amia embryo to those of amphibia, earlier reports usually postulated for this species an invagination of cells over a dorsal lip. Vital staining of the surface cells calls this into question, showing that most if not all of the mesoderm and endoderm is formed from originally internal cells without contribution from the outer layers that form the general ectoderm and the nervous system. The prostomal thickening, a cluster of cells under the so-called dorsal lip, was earlier considered to arise from various sources and seen as the agent which formed more or less of the endoderm by a process called invagination. Evidence is presented that the prostomal thickening is not a constant feature in Amia embryos, when present has not been derived from the embryo's external epithelial covering, and has little if anything to do with formation of the endoderm.