P.S.—I should have said that neither gold nor silver brings out any nervous tissue.
Medusa muscle is not doubly refracting, but then none that I have here seen is striated, and unstriated muscle is not doubly refracting anywhere, is it?
Dunskaith: June 24.
Many thanks for your long and suggestive letter. The poisons also are most acceptable. I have waited before writing to try effect of the latter, but the weather has been so stormy that no jelly-fish could be got.
The most interesting observations I have made since writing before are the following. Unmutilated Sarsia in a dark room seek a beam of light thrown into the bell-jar containing them, and this as keenly as do moths. But when the so-called eye-specks are cut out, the animal no longer cares for light.
I have only come across two species of luminous Medusæ—both, I believe, as yet undescribed—and in these the light is emitted from the margin alone, and, with electrical stimulus, is strictly confined to the intra-polar regions, being strongest at the two poles.
There is no doubt at all about the muscular nature of the fibres we saw. In the larger kinds of Medusæ (the covered-eyed) these fibres are much coarser, and are clearly seen to be arranged in concentric bundles, having four or five fibres in each bundle. Alternating with these bundles, and about the same width as these, are strands of undifferentiated protoplasm. These strands are not spontaneously contractile, although their dimensions are altered by the contraction of the muscular branch on each of their sides. No part of the tissue is doubly refracting in the fresh state. Is there any way of treating it with a view of bringing out this property if latent, so to speak? The peculiarity is not due to the transparency of the tissue, for I find that the muscular fibre of the transparent osseous fish Leptocephalus is as doubly-refracting as could be wished. There are no signs of striæ, but Agassiz says that in some of the Mediterranean species striæ are well marked. But if both striated and unstriated fibres are elsewhere doubly-refracting, it does not, I suppose, much signify whether or not the muscles of Medusæ are striated—so far, I mean, as the peculiarity in question is concerned.
I wish you would say what you think about this peculiarity in relation to a subject that I have been working up. You no doubt remember that in ——'s paper that we heard read, he said that the snail's heart had no nerves or ganglia, but nevertheless behaved like nervous tissue in responding to electrical stimulation. He hence concluded that in undifferentiated tissue of this kind, nerve and muscle were, so to speak, amalgamated. Now it was principally with the view of testing this idea about 'physiological continuity' that I tried the mode of spiral and other sections mentioned in my last letter. The result of these sections, it seems to me, is to preclude, on the one hand, the supposition that the muscular tissue of Medusæ is merely muscular (for no muscle would respond to local stimulus throughout its substance when so severely cut), and, on the other hand, the supposition of a nervous plexus (for this would require to be so very intricate, and the hypothesis of scattered cells is without microscopical evidence here or elsewhere). I think, therefore, that we are driven to conclude that the muscular tissue of Medusæ, though more differentiated into fibres than is the contractile tissue of the snail's heart, is, as much as the latter, an instance of 'physiological continuity.' (Whether or not the interfascicular protoplasmic substance before spoken of is the seat of this physiological continuity is here immaterial.) Dr. Foster fully agrees with me in this deduction from my experiments, and is very pleased about the latter, thus affording additional support to his views. But what I want to ask you is, supposing the interfascicular substance to have no share in conducting stimulus (and I have no evidence of its presence in Sarsia), and hence that the properties of nerve and muscle are united in the contractile fibres of Medusæ—supposing this, do you think that the peculiarity you observed in the molecular conformation of this tissue, considered as muscular, is likely to have anything to do with this peculiarity in its function?
I know you do not like theory, so I shall return to fact. There can be no doubt whatever that the seat of spontaneity is as much localised in the margin as the sensibility to stimulus is diffused throughout the bell. There must, therefore, be some structural difference in the tissue here to correspond to this great functional difference. Agassiz is very positive in describing a chain of cells running round the inner part of the marginal canal. Now, although I sometimes see a thin cord-like appearance here, I should not dare to say it was nervous. Gold certainly stains it, but it also stains many other parts of the tissue, and until I can see cells here I cannot be sure about a visible nervous cord. The cord I do see may be the wall of the marginal canal. I intend to persevere, however, trying your suggestions, also osmic acid.
I can get no indications of electrical disturbance during contraction in the way you suggest—at least not with Sarsia; but I intend to try with some of the larger Medusæ.