The organizer region and pattern regulation in amphibian embryos.

The cellular proportions in the dorsal-to-ventral, mesodermal sequence of pattern elements in the gastrula of certain amphibian embryos regulate with respect to embryo size. The dorsal, blastoporal lip serves as an organizer for this developmental sequence and the grafting of an additional organizer into a ventral location results in a symmetric pattern of cell types. A size-regulating, reaction-diffusion model is presented which produces positional information for development consistent with experimental observations in normal amphibian development and in the presence of an additional, ventrally-located, organizer region.