A novel development pattern for frogs: gastrulation produces an embryonic disk

Abstract

According to von Baer's laws of 1828^1,2, early embryos of closely related animals tend to resemble each other. While the application of these laws to a large taxonomic group, such as the vertebrates, has been strongly criticized³, early development appears similar within classes of terrestrial vertebrates. One reason for the similarities may be that embryos of a given taxonomic group are subjected to similar environmental conditions. It follows that similar animals which provide their embryos with different environments may have different developmental patterns. Amphibians present several modes of reproduction^4,5 and are therefore particularly suited to the examination of this issue. While many amphibians have small eggs and aquatic development, the egg-brooding hylid frogs exhibit maternal incubation of embryos, multinucleate oogenesis^6,7, and very large eggs^5,8. We have compared early development of the egg-brooding hylids, Gastrotheca riobambae and Gastrotheca plumbea, with Eleutherodactylus coqui and Xenopus laevis. We report here that early developmental pattern in Gastrotheca differs from other frogs, particularly with respect to gastrulation, which results in the formation of an embryonic disk.