Tissue sections are prepared and coated with NTB 3 Eastman Kodak emulsion for radioautography in the usual way. After they have been exposed for a suitable time they are processed with no agitation in the following solutions, which must be freshly prepared. Place for 3 min at 24° C in a developer solution which contains per litre 40 g sodium carbonate, 2 g sodium sulphite, 1 g potassium bromide, 2-5 g ‘genochrome’, 40 ml ethylene glycol, and 100 ml acetone in which 3 g of a-naphthol is dissolved. Wash for 2 min in running tap-water. Place for 2 min in a solution containing 30 g potassium ferricyanide and 15 g potassium bromide per litre. Wash for 2 min in running tap-water. Place for 2 min in a solution containing 40 g sodium sulphite, 240 g sodium thiosulphate, and 100 ml commercial formalin per litre. Wash in running tap-water for 2 min. Place in 0.02% neutral red for 5 min. Wash in distilled water for 1 min. Dip in a solution containing 10 g gelatin and 1 g chromealum per litre, which has been heated in a water-bath until liquid, and then allowed to cool to room temperature. Stand each slide vertically to dry in air, then mount a coverslip with Canada balsam. This method gives blue grains: if red ones are required substitute p-nitrophenylacetonitrile (Eastman Kodak) for α-naphthol.

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