After injection of horseradish peroxidase into single blastomeres in Xenopus embryos at 2- to 512-cell stages, all of the descendants could be traced and counted at tailbud stages. Progenitors of Rohon-Beard neurons and of primary spinal motoneurons were identified, and all neurons of each type that originated from individual progenitors were counted. From these data were derived the geometric ...
