Printed in Great Britain Experiments on /3-mercaptoethanol as an inhibitor of neurulation movements in amphibian larvae

The morphogenetic movements of the ectoderm during neurulation include: (1) the movements taking place within the neural plate, which becomes longer and more concentrated in a medio-lateral direction (Jacobson, 1962); and (2) those found in the lateral epidermis layer which, in an epibolic way, moves in a dorsal direction, thus exerting a pushing effect on the lateral edges of the neural plate (Lewis, 1947). The former is, to a great extent, realized by a change of form of the neuroepithelium cells, from cuboidal in early neurulae to the high columnar cells observed during later phases of neural-tube closure. In the epidermis, on the other hand, the case is the reverse. The dorsal spreading of the layer is made possible by a flattening of the cells. In a series of papers, Brächet and his group have show that /?-mercapto-ethanol (HSCH2CH2OH; in this article, called ME) inhibits neurulation (for review, see Brächet, 1964). The aim of this investigation was to determine if this thiol acts on the whole morphogenetic complex or if only parts of the processes involved are blocked. Therefore, we extirpated the neural plates of the amphibian Amblystoma and observed the differences in development between operated and unoperated animals, both in ME solutions and in normal media. It is known that if the neural-plate epithelium is extirpated in early neurulae, the epidermal part of the ectoderm has the power to move up over the exposed chorda-mesoderm and fuse in the dorsal midline (Jacobson, 1962). MATERIALS AND METHODS Experiments were performed on two different species of the genus Ambly-stoma: A. opacum and A. mexicanum. Operations were made on stage-14 neurulae (open neural plate, ridges appearing), since experiments had shown that larvae of later stages continue their change of form for a time even afte