A cascading development model for amphibian embryos.

The mesodermal tissue of some amphibian gastrula develops into a dorsal-to-ventral sequence of notochord, somite, pronephros, and lateral plate cell types. The cellular proportions regulate with respect to embryo size. The dorsal blastoporal lip appears to function as an organizer for the embryo. The transplantation of a donor lip to the ventral side of a host causes a second, opposed embryo to form and the system commits similar total proportions of cells as do normally developing embryos. Transplantation of donor somite to the ventral side of a host causes a reduction in the proportion of host somite developed. A modified reaction-diffusion system governing embryo development is proposed. Developmental simulations consistent with experimental observations are presented and analyzed. The results suggest that the degree of somite inhibition is positively correlated with the size of the somite transplant. Further predictions are that sufficiently large somite transplants would induce ectopic, ventral pronephros to form and ventral pronephros transplants would inhibit host pronephros development.