ABSTRACTWe have repeated classic dorsoventral somite rotation experiments (Aoyama and Asamoto, 1988, Development 104, 15-28) and included dorsal and ventral gene expression markers for the somitogenic tissue types, myotome and sclerotome, respectively. While the histological results are consistent with those previously published, gene expression analysis indicates that cells previously thought to be ‘sclerotome’ no longer express Pax1 mRNA, a sclerotome marker. These results, together with recent quail-chick transplantation experiments indicating that even very late sclerotome tissue fragments are multipotential (Dockter and Ordahl, 1998, Development 125, 2113-2124), lead to the conclusion that sclerotome tissue remains phenotypically and morphogenetically plastic during early embryonic somitogenesis. Myotome precursor cells, by contrast, appear to be determined within hours after somite epithelization; a finding consistent with recent reports (Williams and Ordahl, 1997, Development 124, 4983-4997). Therefore, while these findings support a central conclusion of Aoyama and Asamoto, that axis determination begins to occur within hours after somite epithelialization, the identity of the responding tissues, myotome versus sclerotome, differs. A simple model proposed to reconcile these observations supports the general hypothesis that determinative aspects of early paraxial mesoderm growth and morphogenesis occur in early and late phases that are governed principally by the myotome and sclerotome, respectively.