Cleavage and gastrulation in the egg-brooding, marsupial frog, Gastrotheca riobambae

The marsupial frog Gastrotheca riobambae has several reproductive adaptations, most prominent of which is the incubation of the embryo in a pouch on the mother's back. We have followed cleavage and gastrulation by microscopical observation and by vital staining, and have found several alterations in these processes which may reflect the reproductive adaptations. The large, yolky egg has a cap of yolk-poor cytoplasm at the animal pole which is incorporated into a translucent blastocoel roof consisting of a single cell layer. The epithelium of the yolk sac is derived from the roof. The inconspicuous blastoporal lips form near the vegetal pole from cells of the marginal region. Gastrulation movements include the epibolic stretching of the surface towards the blastopore and a contraction of the vegetal surface. The blastoporal lips close over a small archenteron, and the cells of the lips become the embryonic disc, a discrete group of small cells which give rise to most of the embryo's body. The great size difference between animal and vegetal blastomeres during cleavage, the single-celled blastocoel roof, the dissociation in time between archenteron formation and its expansion, the embryonic disc and the slow development distinguish G. riobambae embryos from those of other frogs. The importance of the marginal region which produces the embryonic disc and the unimportance of the most animal region whose fate is primarily yolk sac emphasizes the role of the marginal region in amphibian development.