Two main modifications of Holmes's method are described. In both the amount of silver nitrate in the impregnating bath is increased, and larger quantities of higher pyridine derivatives are substituted for pyridine. The preferred method for general purposes employs 2.4% 2:6 lutidine at pH 8.4, and gives a bluish picture with improved selectivity of fibre staining. The use of 1.6% 2:4:6 collidine gives a reddish picture, with highly selective fibre staining, and sharp resolution of neurosecretory cells. Minor modifications of pre-treatment and reduction procedures are also described.

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