that osmotic action tended to stop, or to reverse, on change of curvature, it would follow that this phenomenon would give rise to internal currents; and the change of pressure consequent on these would tend to intensify the change of curvature when once started57.
The sperm-cells of the Decapod crustacea exhibit various singular shapes. In the Crayfish they are flattened cells with stiff curved processes radiating outwards like a St Catherineãs wheel; in Inachus there are two such circles of stiff processes; in Galathea we have a still more complex form, with long and slightly twisted processes. In all these cases, just as in the case of the blood-corpuscle, the structure alters, and finally loses, its charôÙacôÙterôÙisôÙtic form when the nature or constitution (or as we may assume in particularãthe density) of the surrounding medium is changed.
Here again, as in the blood-corpuscle, we have to do with a very important force, which we had not hitherto considered in this connection,ãthe force of osmosis, manifested under conditions similar to those of Pfefferãs classical experiments on the plant-cell. The surface of the cell acts as a ãsemi-permeable membrane,ã {274} permitting the passage of certain dissolved substances (or their ãionsã) and including or excluding others; and thus rendering manifest and measurable the existence of a definite ãosmotic pressure.ã In the case of the sperm-cells of Inachus, certain quantitative experiments have been performed58. The sperm-cell exhibits its charôÙacôÙterôÙisôÙtic conformation while lying in the serous fluid of the animalãs body, in ordinary sea-water, or in a 5 per cent. solution