Chromosomes in the nuclei of living salivary glands of Simulium have been observed with Nomarski optics and polarized light before and during irrigation with various fixatives. All cause loss of chromosomal organization to some degree. The best fixative is redistilled glutaraldehyde; even so, although it leaves the nucleus visually unaltered, a network forms in the nuclear sap and birefringence is lost. Calcium ions in the fixative cause chromosomal movements. The fixatives may alter nucleoli and may dissolve them completely. New fixatives are needed for reliable fine-structural studies of chromosomal organization.

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