The teleost chorion serves as a natural barrier to environmental agents. In this investigation, the chorions of the fathead minnow, Pimephales promelas, were subjected to light microscopy, transmission electron microscopy, and scanning electron microscopy during different stages of embryogenesis. The chorion is composed of an outer layer of 22 lamellae, and an inner, more electron‐dense layer. During embryogenesis, there is a great increase in the thickness of the inner layer of the chorion, with a consequential decrease in the thickness of the outer 22 lamellae. The outside surface also changes during embryogenesis. It is suggested that the chorion be taken into consideration in environmental toxicity studies, and that the number of lamellae might be a way of classifying embryos.

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This study is concerned with the morphogenesis of the carpal elements in the regenerating forelimb of the adult newt. Blastema cells surrounding the remnant bony stumps begin to differentiate into cartilage on the twentieth post-amputation day. Subsequently, masses of carilage build up from the radial and ulnar stumps. The radial mass is larger and differentiates more rapidly than the ulnar mass. By the fifty-fifth post-amputation day, the eight basic carpal elements are formed, with fusion of two of the units, intermedium with ulmare, occurring by the seventieth day. The completed regenerate possesses the seven carpal elements characteristic of the normal adult limb. The present results show that during limb regeneration in the adult newt the carpal elements are restored to their original number and position and that the pattern of such carpal differentiation proceeds in a proximodistal direction influenced by the stump remnants of the radius and ulna.