Spheres that stain with neutral red in cephalopod neurons show changes with variations in the tonicity of the surrounding medium. In hypertonic solutions the surface becomes shrunken and the stainable matter restricted to crescents or granules at the side of the sphere. These processes are reversible. They may be interpreted by considering that the surface of the spheres consists of material with properties partly analogous to those of a semi-permeable layer and partly to a membrane, that is to say a layer with tangential cohesive forces. These analogies are not perfect, however, and in particular the cohesive properties of the ‘membrane’ are limited and its material tends to flow together to form drops when the inner pressure is reduced.

The appearances produced by the classical methods for staining the Golgi apparatus are a result of distortions of these droplets similar to those produced by changes in tonicity. The outer more osmiophil or argentophil component adopts forms reminiscent of those shown by the component that stains more deeply with neutral red.

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