Abstract
In 16-cell Xenopus embryos, horseradish peroxidase (HRP) was injected into blastomere D1.2 on one side. No Rohon-Beard neurons originated from D1.2 in either of the two patterns of cleavage that were studied (Jacobson, M. (1981) J. Neurosci. 1: 918–922). In other embryos, after injection of HRP into D1.2, the neighboring ventral blastomere V1.2, from which 68 to 90% of Rohon-Beard neurons normally originate, was removed. In the cases that developed normally to larval stages 32 to 34, the number and sizes of Rohon-Beard neurons were normal, but 14.9 to 73.9% of Rohon-Beard neurons were labeled, showing that they originated from the injected blastomere D1.2. Labeling also occurred in cells of the spinal dorsal root ganglia that normally descend from V1.2 but not from D1.2. This proves that individual blastomeres at the 16- cell stage are not committed to form specific types of neurons or restricted parts of the central nervous system.