Nuclear activation at the onset of amphibian gastrulation

A quantitative radioautographic study of RNA synthesis in the diverse cell types of the Xenopus blastula and gastrula is described. Nuclear activation, studied as increase in the number of grains in nuclear RNA after pulse‐labelling with uridine H 3 , was found to occur abruptly just before gastrular movements become observable. Presumptive endoderm and mesoderm participate in this widespread activation, while presumptive ectoderm becomes activated shortly after, at mid‐gastrulation. It is shown that per‐nucleus rates of high molecular weight, non‐ribosomal RNA synthesis increase at least 20 fold within one hour in endoderm and mesoderm as the embryo prepares to enter gastrulation.