The Origin of Vertebrates

by Walter Holbrook Gaskell

If we choose to speak of the body as made up of two syncytia in this way, we must at the same time recognize the fundamental difference in character between them. In the one case the elements are connected together only by what may be called non-living material; there is no direct metabolic activity caused by the action of one cell over a more distant cell in consequence of such connection, it is not a true syncytium; in the second case there is a living connection, the metabolism of one part is directly influenced by the activity of another, and the whole utility of the system depends upon such functional connection.

The tissues composing this second syncytium may be spoken of as the master-tissues of the body, and we may express this conception of the building up of the body of the higher Metazoa by saying that it is composed of a syncytial host formed of the master-tissues, which contains within its meshes a system of free-living cells, none of which have any connection with the nervous system. This syncytial host is in the adult composed of a number of double elements, a nerve-cell element, and an epithelial element, such as muscle-cell, gland-cell, etc., connected together by nerves; and if such connection is always present as we pass from the adult to the embryo, if there is no period when, for example, the neural element exists alone free from the muscle-cell, no period when the two can be seen to come together and join, then it follows that when the single-layered blastula stage is reached, muscle-cell and nerve-cell must have fused together to form a neuro-muscular cell. Similarly with all the other neuro-epithelial organs; however far apart their two components may be in the adult, they must come together and fuse in the embryo to form a neuro-epithelial element.

The close connection between muscle and nerve which has always been recognized by physiologists, together with the origin of muscle from a myo-epithelial cell in Hydra and other Cœlenterata, led the older physiologists to accept thoroughly Hensen's views of the neuro-epithelial origin of all tissues connected with the central nervous system. Of late years this conception has been largely given up owing to the statement of His that the nervous system arises from a number of neuroblasts, which are entirely separate cells, and have at first no connection with muscle-cells or any peripheral epithelial cells, but subsequently, by the outgrowing of an axial fibre, find their way to the muscle, etc., and connect with it. I do not think that His' statement by itself would have induced any physiologist to give up the conception of the intimate connection of muscle and nerve, if the work of Golgi, Ramón y Cajal, and others had not brought into prominence the neurone theory, i.e. that each element of the central nervous system is an independent element, without real connection with any other element and capable of influencing other cells by contact only. These two statements, emanating as they did from embryological and anatomical studies respectively, have done much to put into the background Hensen's conceptions of the syncytial nature of the motor, neural, and sensory elements, which make up the master-tissues of the body, and have led to the view that all the elements of the body are alike, in so far as they are formed of separate cells each leading an independent existence, without any real intimate connection with each other.

The further progress of investigation is, it seems to me, bringing us back to the older conception, for not only has the neuroblast theory proved very difficult for physiologists to accept, but also Graham Kerr, in his latest papers on the development of Lepidosiren, has shown that there is continuity between the nerve-cell and the muscle-cell from the very first separation of the two sets of elements; in fact, Hensen is right and His wrong in their respective interpretation of the earliest stages of the connection between muscle and nerve. So also, it seems to me, the intimate connection between the metabolism of the gland-cell, as seen in the submaxillary gland, and the integrity of its nervous connection implies that the connection between nerve-cell and gland-cell is of the same order as that between nerve-cell and muscle-cell. Graham Kerr also states in his paper that from the very commencement there is, he believes, continuity between nerve-cell and epithelial cell, but so far he has not obtained sufficiently clear evidence to enable him to speak positively on this point.

Further, according to the researches of Anderson, the cells of the superior cervical ganglion in a new-born animal will continue to grow healthily as long as they remain connected with the periphery, even though entirely separated from the central nervous system by section of the cervical sympathetic nerve, and conversely, when separated from the periphery, will atrophy, even though still connected with the central nervous system. So, also, on the sensory side, Anderson has shown that the ganglion-cells of the posterior root-ganglion will grow and remain healthy after separation of the posterior roots in a new-born animal, but will atrophy if the peripheral nerve is cut, even though they are still in connection with the central nervous system. Further, although section of a posterior root in the new-born animal does not affect the development of the nerve-cells in the spinal ganglion, and of the nerve-fibres connecting the posterior root-ganglion with the periphery, it does hinder the development of that part of the posterior root connected with the spinal ganglion.

These experiments of Anderson are of enormous importance, and force us, it seems to me, to the same conclusion as that to which he has already arrived. His words are (p. 511): "I suggest, therefore, that the section of peripheral nerves checked the development of motor and sensory neurones, not because it blocked the passage of efferent impulses in the first case and the reception of stimuli from the periphery in the second, but for the same reason in both cases, viz. that the lesion disturbed the chemico-physical equilibrium of an anatomically continuous (neuro-muscular or neuro-epithelial) chain of cells, by separating the non-nervous from the nervous, and that the changes occurring in denervated muscle, which I shall describe later (and possibly those in denervated skin), are in part due to the reciprocal chemico-physical disturbance effected in these tissues by their separation from the nervous tissues; also that the section of the posterior roots checked the development of those portions of them still attached to the spinal ganglia, because the chemico-physical equilibrium in those processes is maintained not only by the spinal ganglion-cells, but also by the intra-spinal cells with which these processes are anatomically continuous."

What is seen so strikingly in the new-born animal can be seen also in the adult, and in Anderson's paper references are given to the papers of Lugaro and others which lead to the same conclusion.

These experiments seem to me distinctly to prove that the connection between the elements of the peripheral organ and the proximate neurone is more than one of contact.

We can, however, go further than this, for, apart from the observations of Apathy, there is direct physiological evidence that the vitality of other neurones besides the terminal neurones is dependent upon their connection with the peripheral organ, even though their only connection with the periphery is by way of the terminal neurone. Thus, as is seen from Anderson's experiments, section of the cervical sympathetic nerve in a very young animal causes atrophy of many of the cells in the corresponding intermedio-lateral tract, cells which I supposed gave origin to all the vaso-constrictor, pilomotor, and sweat-gland nerves. A still more striking experiment given by Anderson is the effect of the removal of the periphery upon the medullation of those efferent fibres which arise from these same spinal cells, for, as he has shown, section of the nerves from the superior cervical ganglion to the periphery in a very young animal delays the medullation in the fibres of the cervical sympathetic—that is, in preganglionic fibres which are not directly connected with the periphery but with the terminal neurones in the superior cervical ganglion. So also on the afferent side a sufficiently extensive removal of sensory field will cause atrophy of the cells of Clarke's column, so that, just as in the case of the primary neurones, the secondary neurones show by their degenerative changes the importance of their connection with the peripheral organs.

In this way I can conceive the formation of a series of both efferent and afferent relays in the nervous system by proliferation of the original neural moiety of the neuro-epithelial elements, every one of which is dependent upon its connection with the peripheral epithelial elements for its due vitality, the whole system being a scheme for co-ordination of a larger and larger number of peripheral elements. Thus the cells of the vasomotor centre are in connection with the whole system of segmental vaso-constrictor centres in the lateral horns of the thoracic region of the cord, so that to cause atrophy of these cells a very extensive removal of the vascular system would be required. Each of the segmental centres in the cord supplies a number of sympathetic segments, the connection with all of which would have to be cut in order to ensure complete removal of the connection of each of its cells with the periphery, and finally each of the cells in the sympathetic ganglia supplies a number of peripheral elements, all of which must be removed to ensure complete severance.