The sequence of events leading to the closure of the neural tube has been investigated in hamster embryos. This consisted of: (1) approach and fusion of the neural folds at the level of the presumptive neural-crest cells; (2) active proliferation of the neural-crest cells at the area of fusion and formation of a distinct wedge by them; (3) fusion and closure of the surface ectoderm; (4) reduction of the number of neural-crest wedge cells due to their active migration from it; (5) fusion of the neurectoderm beginning at the ependymal surface, progressing dorsally until the neural tube was completely closed; and (6) fusion and closure of the mesoderm between the surface ectoderm and the closed neural tube. The time required to complete this sequence of events, the amount of embryonic tissue involved in it at a given time, and the number of neural-crest cells formed at the area of fusion varied from one region of the embryo to another. It is postulated that the mesoderm appeared to play a double role in the process of closure—a direct one late in the closure process (see 6) and an indirect one, perhaps as a buttress supporting the other cellular elements during the entire closure process.