Dense fibrous material is first seen in association with mitochondria in tadpole oogonia but is most prominent in oocytes during the extended first meiotic prophase when it aggregates into dense bodies in the perinuclear cytoplasm. The origin of this material has been attributed to 350-A nuclear granules which form cytoplasmic streamers of fibrous material upon passage through nuclear pores. This has commonly been interpreted as the transfer of ribonucleoprotein to the cytoplasm for storage. However, cytochemical reactions for nucleic acids have indicated an absence of detectable RNA in this dense material, and the results of radioautographic studies with labeled uridine, thymidine, or actinomycin D argue against the presence of nucleic acids. When sites of incorporation of tritiated amino acids were radioautographically localized, an appreciable number of silver grains were present over the dense bodies. Uptake of certain amino acids occurs fairly promptly but the degree of labeling levels off after about 6 hr, suggesting a rapid turnover of the material in the dense bodies. Attention is drawn to the similarity of the dense bodies to structures present in germ cells of a number of other species, and possible functions of the dense bodies in germ cell differentiation are considered.

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