The nucleoids of Escherichia coli, independently of the physiological state of the bacteria, are shown to be preserved as a fine-stranded fibrillar nucleoplasm by an OsO4 fixation under defined conditions: acetate-veronal buffer pH 6, presence of Ca++ and amino acids, stabilization with uranyl-acetate before dehydration. The same fixation procedure applied to the DNA of vegetative phage reveals a pool of homogeneous fibrillar structure very similar to the nucleoplasm. The "versene test," which produces a coarse coagulation of these plasms, emphasizes the similar behaviour of the pool and the nucleoids.

The heads of mature phage are preserved in their true polyhedral shape by the standard fixation procedure, although they may be badly distorted when fixed under different conditions. Lanthanum nitrate and uranyl-acetate are shown to increase markedly the contrast of both phage and cytoplasm.

The consequences of the fibrillar structure of the genetic material are discussed in relation to the probable division process.

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