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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