A general method for silvering invertebrate central nervous systems

The best-known silvering methods are less successful with invertebrate than with vertebrate central nervous tissue. They fall into two classes with conflicting merits. In the first, metallic impregnation is assisted by accessory chemical agents, and subsequent development deposits only limited silver derived from the impregnating bath ; in the second, impregnation is in a simple silver nitrate solution, and the development is physical, the silver deriving from the developer. The most important part of the first process is impregnation, development adding relatively little; impregnation is comparatively slight in the second process, which relies much more on development. If an impregnation of the first type is followed by physical development, the results have the advantages of both methods. The final picture is as good as that produced by the classic methods, and the reliability of the result greatly increased ; the method is successful with a variety of difficult invertebrate material. A method is described, using Blest’s modification of Holmes’s impregnator (class I) and Samuel’s physical developer (class II). The effects of temperature, pH, and duration of impregnation are described and related to the needs of different tissues. Increases in both temperature over the range 30° to 70° C and pH from 7·0 to 9·0 have similar and summating effects: increase in intensity of axon staining, increase in background staining, and decrease in fibre specificity.