Smear Preparations for the Electron Microscopy of Animal Chromosomes

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Abstract
Prepns. of Drosophila salivary gland and human pachytene spermatocyte chromosomes are made by fixing in 2% orcein and 60% acetic acid; dissecting and crushing under a coverslip on a slide; after freezing the coverslip is removed, the tissue wet with 95% glycerine and 5% saturated aqueous lanthanum acetate and pressed on a slide or coverslip previously coated with a 200 A formvar film. Suitable specimens selected by microscopic examination are placed for 6-12 hrs. in a fixative composed of 3 parts saturated picric acid and 1 part formalin; the slide and cover separate, leaving the tissue attached to the film which is washed in water, dehydrated with alcohols, treated with 0.5% collodion in absolute alcohol, and air dried. Appropriate portions are transferred to a supporting grid, and regions selected under the light microscope are studied in the electron microscope. Results obtained with this material are in good agreement with those obtained by sectioning and replica methods.
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