Pattern regulation in isolated halves and blastomeres of early Xenopus laevis.

Xenopus embryos at the 2-cell stage were cut into right and left halves, those at the 4-cell stage into dorsal and ventral halves or individual blastomeres, and those at the 8-cell stage into lateral, animal and vegetal halves. Defect embryos, that is, 8-cell embryos from which a particular pair of blastomeres had been removed, were also prepared. These halves, blastomeres and defect embryos were cultured in 50% Leibovitz (L-15) medium supplement was determined from their macroscopic appearance when controls reached stage 26 (early tailbud stage) or later. The only halves that could develop into normal larvae or frogs were lateral ones of 2- and 8-cell embryos. An interesting finding was that these halves of 2-cell embryos developed into only half-embryos when cultured in the above Leibovitz medium beyond the beginning of gastrulation. On the other hand, most or all the dorsal and ventral halves at the 4-cell stage and the animal and vegetal quartets at the 8-cell stage did not form normally proportioned embryos. Defect embryos lacking any two blastomeres of the animal half gave rise to nearly normal embryos, whereas those lacking two dorsal or two ventral blastomeres of the vegetal half did not. From the present results and those of studies now in progress, it is concluded that development of blastomeres and halves from these early embryos, except lateral halves from 2- and 8-cell embryos, is not regulative as expected earlier, and that a certain combination of blastomeres is essential for complete pattern regulation.