Developmental anatomy of the tailed frog (As cap hits truei): a primitive frog with large eggs and slow development



The embryonic and early larval development of Ascaphus was studied by culturing embryos at 11 °C. Twenty stages of normal development are morphologically defined by using a standard staging system for anuran amphibians.

Cleavage stages are distinctive. The first cell division is unique because two separate furrows develop and then fuse to form a single cleavage plane. The large, yolky cytoplasm continues to influence cell divisions, and a blastula develops with very large vegetal cells and small, irregular shaped animal cells. There is a conspicuous, translucent blastocoel in the late blastula, and during gastrulation this structure is displayed forward by the internal migration of cells. Except for slight differences in proportions, the gastrula and neurula stages resemble the typical anuran pattern. Circulation (stage 20) begins when large vitelline veins develop on the yolk sac and direct the return of blood flow to the heart.

At hatching, the tadpole has a large yolk sac, little skin pigmentation, and no gills. Mouthparts and opercular coverings of the branchial arches develop slowly. Hind limb buds develop at the base of the tail, but they are soon concealed as a skin membrane develops and covers these structures.

The reproductive strategy for this primitive and highly aquatic frog is unusual: there is a small number of large eggs that develop slowly and produce tadpoles that continue this slow developmental process.