ABSTRACT
The differentiation of the ectoderm in Ambystoma mexicanum (Harrison stage 26--27) was examined under in vivo and in vitro conditions by scanning electron microscopy under different experimental conditions. About one out of three flank epidermal cells was found to be ciliated in the undistrubed or control embryos. The shape of ciliated cells in the explants from the animal region was only slightly affected. In no case was it possible to find two adjacent ciliated cells, implying that these cells prevent the appearance of cilia in the cells in direct contact. Transformation to ciliated cells is suppressed by hypertonicity but favoured in a hypotonic medium. The differentiation of epidermis is also dependent upon the synthesis of RNA and some kind of sulphated glucosaminoglycan, corroborated by the inhibitory effect of actinomycin and selenate. The differences between the test series and the controls are discussed with regard to factors controlling embryonic epidermal differentiation.