The early stages of nuclear differentiation in spermatids of the house cricket are described with regard to the fine structural elements and chemical components which occur. Particular attention is given to the loss of nonhistone protein from the nucleus and its relation to chromatin structure. Granular elements about 25 to 80 mµ in diameter, and fibers about 8 mµ in diameter occur in the earliest spermatid nucleus. The fibers are found in diffuse and condensed chromatin while granules are found only in diffuse material. DNA and histone parallel the chromatin fibers in distribution, while nonhistone protein and RNA parallel the granules in distribution. The granules and most of the nonhistone protein are lost, simultaneously, after the early spermatid stage. The protein loss occurs without detectable change in the structure of chromatin fibers. Chromatin fibers first show a structural change in mid spermiogenesis, when they become thicker and very contorted. Unusually thin fibers (about 5 mµ) also appear in mid spermatid nuclei; they are apparently composed of nonhistone protein and free of DNA and histone.

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