DEVELOPMENT OF THE AMPHIBIAN EAR RUDIMENT IN EXPLANTS

¹ Marine Biological Laboratory, Woods Hole, Massachusetts

1. Two series of operations were performed on larvae of Taricha rivularis, T. torosa, Ambystoma maculatum, A. tigrinum and Rana sylvatica. In the E Series, a piece of ectoderm with attached ear rudiment was excised. and the mesendoderm and some, if not all, of the mesectoderm were removed. In the EM Series, mesendoderm and mesectoderm were included with the ear rudiment and ectoderm. These excised tissues were cultured, together with the donor larvae, in sterile Holtfreter's solution for varying lengths of time and, in most cases, at 15° or 16° C.

2. Study of serial sections of the explants showed that time ear rudiments of T. rivularis, T. torosa, A. maculatum and R. sylvatica were capable of independent development and differentiation up to the stage when the auditory vesicle possesses a distinct endolymphatic duct and sensory epithelium. This corresponds to Stage 40 in the salamanders and Stage 22 in the frog. In a few cases, there was some indication of the beginning of formation of a lateral pouch and separation of two areas of sensory epithelium. The explants of A. tigrinum were cultured for only a short period and consequently showed only the earliest signs of differentiation.

3. At Stages 40 (salamander) and 22 (frog), the cells of the auditory vesicles no longer contain visible yolk granules. They have presumably. used up all of their intrinsic food material and are dependent on the developing circulatory and digestive systems for the supply of further nutrients.

4. A review of the literature indicates that these stages in the developing amphibian larva are marked by an increase in the heart rate and the rate of oxygen consumption, as well as an increase in the activity of specific enzymes which synthesize new food materials to replace those originally present in the yolk granules.

5. It is concluded that the degree of differentiation attained by the ear rudiment when cultured as an explant in a balanced salt solution may not necessarily represent the limit of its potentiality for independent development. Further development might be possible if the explant were cultured in an adequate nutrient solution.