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312 perhaps erroneous, and that, in any case, when stained with gold, some of the nervous channels show themselves in the form of fully differentiated nerves. Now this fact, it is needless to say, greatly enhances the interest of the previous experiments. If, as I formerly said, the proof of vicarious action being possible to an almost unlimited extent in these incipient nerve-fibres appeared to me one of the most interesting among the additions to our knowledge of invertebrate physiology, much more interesting does this proof become if we further learn that these incipient nerve-fibres are only incipient in the sense of constituting the earliest appearance of nerve-fibres in the animal kingdom. For if these true nerve-fibres admit, from the peculiarly favorable plan of their anatomical distribution, of being proved to be not improbably capable of vicarious action to so extraordinary a degree, we may become the more prepared to believe that nerve-fibres elsewhere are similarly capable of vicarious action. But the interest does not end here, for Mr. Schäfer's numerous preparations all show the highly remarkable fact that the nerve-fibres which so thickly overspread the muscular sheet of Aurelia do not constitute a true plexus, but that each fibre is comparatively short, and nowhere joins with any of the other fibres. That is to say, although the constituent fibres of the network cross and recross one another in all directions—sometimes, indeed, twisting around one another like the strands of a rope—they can never be actually seen to join, but remain anatomically isolated throughout their length. So that the simile by which I have represented this nervous network—the simile, namely, of a sheet of muslin overspreading the whole of the muscular sheet—is as a simile even more accurate than has hitherto appeared; for just as in a piece of muslin the constituent threads, although frequently meeting one another, never actually coalesce, so, in the nervous network of Aurelia, the constituent fibres, although frequently in contact, never actually unite.

Now, if it is a remarkable fact that in a fully differentiated nervous network the constituent fibres are not improbably capable of vicarious action to almost any extent, much more remarkable does this fact become when we find that no two of these constituent nerve-fibres are histologically continuous with one another. Indeed, it seems to me that we have here a fact as startling as it is novel. There can scarcely be any doubt that some influence is communicated from a stimulated fibre a to the adjacent fibre b at the point where these fibres come into close apposition. But what the nature of the process may be whereby a disturbance in the excitable protoplasm of a sets up a sympathetic disturbance in the anatomically separate protoplasm of b, supposing it to be really such—this is a question concerning which it would as yet be premature to speculate. But if, for the sake of a name, we call this