420 



NATURE 



[Sept. 2, 1880 



a network of nene-fibres, placing the various parts of the body 

 in nervous communication. 



2. The process by which nerves became connected with 

 muscles, so tliat a stimulus received by a nerve-cell could be 

 communicated to and cause a contraction in a muscle. 



Recent investigations on the anatomy of the Coelenterata, 

 especially of jelly-fish and sea-anemones, have thrown some 

 light on tliese points, although there is left much that is still 

 obscure. 



In our own country Mr. Romanes has conducted some inter- 

 esting physiological experiments on these forms ; and Prof. 

 Schafer has made some important histological investigations 

 upon them. In Germany a series of intere.-.ting researches have 

 also been made on them by Professors Kleinenberg, Claus, and 

 Eimer, and more especially by the brothers Ilertwig, of Jena. 

 Careful histological investigations, especially tliose of the last- 

 named authors, have made us acquainted with the forms of 

 some very primitive types of nervous system. In the common 

 sea-anemones there are, for instance, no organs of special sense, 

 and no definite central nervous system. There are, however, 

 scattered throughout the skin, and also throughout the lining of 

 tlie digestive tract, a number of specially modified epithelial cells, 

 which are no doubt delicate organs of sense. They are provided 

 at their free extremity w ith a long hair, and are prolonged on 

 their inner side into a fine process which penetrates the deeper 

 part of the epithelial layer of the skin or digestive wall. They 

 eventually join a fine network of protoplasmic fibres which 

 forms a special layer immediately within the epithelium. The 

 fibres of this network are no doubt essentially nervous. In 

 addition to fibres there are, moreover, present in the network 

 cells of the same character as the multipolar ganglion-cells in 

 the nervous system of Vertebrates, and some of these cells 

 are characterised by sending a process into the superjacent 

 epithelium. Such cells are obviously epithelial cells in the act 

 of becoming nerve-cells ; and it is probable that the nerve-cells 

 are, in fact, sense-cells which have travelled inwards and lost 

 their epithelial character. 



There is every reason to think that the network just described 

 is not only continuous with the sense-cells in the epithelium, 

 but that it is also continuous with epithelial cells which are 

 provided with muscular prolongations. The nervous system 

 thus consists of a network of protoplasmic fibres, continuous on 

 the one hand with sense-cells in the epithelium, and on the other 

 with muscular cells. The nervous network is generally distri- 

 buted both beneath the epithelium of the skin and that of the 

 digestive tract, but is especially concentrated in the disk-like 

 region between the mouth and tentacle-;. The above observa- 

 tions have thrown a very clear light on the characters of the 

 nervous system at an early stage of its evolution, but they leave 

 unanswered the questions (i) how the nervous network first 

 arose, and (2) how its fibres became continuous with muscles. 

 It is probable that the nervous network took its origin from 

 processes of the sense-cells. The processes of the different cells 

 probably first met and then fused together, and becoming more 

 arborescent, finally gave rise to a complicited network. 



The connection between this network and the muscular cells 

 also probably took place by a process of contact and fusion. 



Epithelial cells with muscular processes were discovered by 

 Kleinenberg before epithelial cells with nervous processes were 

 known, and he suggested that the epithelial part of such cells 

 wa? a sense-organ, and that the connecting part between this 

 and the contractile processes was a rudimentary nerve. This 

 ingenious theory explained completely the fact of nerves being 

 continuous with nmscles ; but on the further discoveries being 

 made which I have just described, it became obvious that this 

 theory would have to be abandoned, and that some other 

 explanation would have to be given of the continuity between 

 nerves and muscles. The hypothetical explanation just offered 

 is that of fusion. 



It seems very probable that many of the epithelial cells were 

 originally provided with processes, the protoplasm of which, 

 like that of the Protozoa, carried on the functions of nerves and 

 muscles at the same time, and that these processes united 

 amongst themselves into a network. By a process of differen- 

 tiation parts of this network may have become specially con- 

 tractile, and other parts may have lost their contractility and 

 become solely nervous. In this way the connection between 

 nerves and muscle-; might be explained, and this hypothesis fits 

 in very well with the condition of the neuro-mnscular system as 

 we find it in the Cadenterata. 



The nervous system of the higher Metazoa appears then to 

 have originated from a differentiation of some of the superficial 

 epithelial cells of the body, though it is possible that some parts 

 of the system may have been formed by a differentiation of the 

 alimentary epithelium. The cells of the epithelium were most 

 likely at the same time contractile and sensory, and the differen- 

 tiation of the nervous system may very probably have com- 

 menced, in the fir^t instance, from a specialisation in the 

 function of part of a network formed of neuro-muscular pro- 

 longations of epithelial cells. A simultaneous differentiation of 

 other parts of the network into muscular fibres may have led to 

 the continuity at present obtaining between nerves and muscles. 



Local differentiations of the nervous network, which was no 

 doubt distributed over the whole body, took place on the 

 formation of organs of special sense, and such differentiations 

 gave rise to the formation of a central nervous system. The 

 central nervous system was at first continuous with the epidermis, 

 l)ut became separated from it and travelled inwards. Ganglion- 

 cells took their origin from sensory epithelial cells provided with 

 prolongations continuous with the nervous network. Such 

 epithelial cells gradually lost their epithelial character, and 

 finally became completely detached from the epidermis. 



Nerves, such as we find them in the higher types, originated 

 from special differentiations of the nervous network, radiating 

 from the parts of the central nervous system. 



Such, briefly, is the present state of our knowledge as to the 

 genesis of the nervous system. I ought not, however, to leave 

 this subject without saying a few words as to the hypothetical 

 views which the distinguished 'evolutionist Mr. Herbert Spencer 

 has put forward on this subject in his work on Psychology. 



For Herbert Spencer nerves have originated, not as processes 

 of epithelial cells, but from the passage of motion along the lines 

 of least resistance. The nerves would seem, according to this 

 view, to have been formed in any tissue from the continuous 

 passage of nervous impulses through it. "A wave of molecular 

 disturbance," he says, "passing along a tract of mingled colloids 

 closely allied in composition, and isomerically transforming the 

 molecules of one of them, will be apt at the same time to form 

 s Dme new molecules of the same type, " and thus a nerve becomes 

 established. 



A nervous centre is formed, according to Herbert Spencer, at 

 the point in the colloid in which nerves are generated, where a 

 single nervous wave breaks up, and its part-; diverge along 

 various lines of least resistance. At such points some of the 

 nerve-colloid will remain in an amorphous state, and as the wave 

 of molecular motion will there be checked, it will tend to cause 

 decompositions amongst the unarranged molecules. The de- 

 compositions must, he says, cause " additional molecular motion 

 to be disengaged ; so that along the outgoing lines there will be 

 discharged an augmented wave. Thus there will arise at this 

 point something having the character of a ganglion corpuscle." 



These hypotheses of Herbert Spencer, which have been widely 

 adojJted in this country, are, it appears to me, not borne out 

 by the discoveries to which I have called your attention to-day. 

 The discovery that nerves have been developed from processes of 

 epithelial cells, gives a very different conception of their genesis 

 to that of Herbert Spencer, which makes them originate from 

 the passage of nervous impulses through a tract of mingled col- 

 loids ; while the demonstration that ganglion -cells arose as epi- 

 thelial cells of special sense, which have travelled inwards from 

 the surface, admits still less of a reconciliation with Herbert 

 Spencer's view on the same subject. 



Although the present state of our knowledge on the genesis of 

 the nervous system is a great advance on that of a few years ago, 

 there is still much remaining to be done to make it complete. 



The subject is well worth the attention of the morphologist, 

 the physiologist, or even of the psychologist, and we must not 

 remain satisfied by filling up the gaps in our knowledge by such 

 hypotheses as I have been compelled to frame. New methods 

 of research will probably be required to grapple with the 

 problems that are still unsolved ; but when we look back and 

 survey what has been done in the past, there can be no reason for 

 mistrusting our advance in the future. 



Department of Anthropology 

 Address by F. W. Rudler, F.G.S., Vice-President of 



THE SECTIO.N 



After referring to the etlmologically mixed state of the 

 population of South Wales, Mr. Rudler went on— What then 



