ABSTRACT
This paper reviews evidence relating to the question, at what stage in the development of the frog embryo are segment boundaries specified? Current evidence leads to the hypothesis that a spatiotemporal series of cell states leading to segmentation is continuously initiated at a position 200 to 300 μm from the posterior end of the presomitic mesoderm, about nine somite intervals before the formation of a definitive somite. The evidence suggests, though by no means proves, that segment boundaries are specified close to this time. This hypothesis relies critically on evidence concerning the effects of disruptive agents, the extent of cell mixing prior to the early gastrula stage, fate map data, and a comparison with development in the mouse where a similar fate map can be related to morphological evidence of somitomeric segmentation.
Evidence regarding the organization of the posterior, undifferentiated zone of the mesoderm in the frog embryo indicates that the cells are not proliferating rapidly, but are undergoing cell movements and rearrangements associated with caudal extension. The speculation that the segment pattern derives from inductive interactions in this region is discussed.
