The Origin of Vertebrates

by Walter Holbrook Gaskell

Thus, if we take any arbitrary number, such as 4, to represent the number of peripheral organ-elements with which each terminal neurone is connected, and suppose that each neurone has proliferated into sets of 4, then a cell of the third order, such as a cell of the vasomotor centre, would require the removal of 64 peripheral elements to cause its complete separation from the periphery, one of the second order (a cell of the thoracic lateral horn) 16 elements, one of the first order (a cell of a sympathetic ganglion) 4 elements.

Such intimate inter-relationship between the neurones, both afferent and efferent, and their corresponding peripheral organs does not imply that all nerve-cells are necessarily as closely dependent upon some connection with the periphery, for just as the proliferation of epithelial or muscle-cells forms an epithelial or muscular sheet, the elements of which are so loosely, if at all, connected together that their metabolism is in no way dependent upon such connection, so also a similar proliferation of the neural elements may form connections between nerve-cell and nerve-cell of a similarly loose nature.

It is this kind of proliferation which, in my opinion, would bind together the separate relays of efferent and afferent neurones, and so give origin to reflex actions at different levels. Such neurones would not be in the direct chain of either the afferent or efferent neurones, and so not directly connected with the periphery, and could therefore be removed without affecting the vitality of either the efferent or afferent chain of neurones. In other words, the vitality of the cells on the efferent side ought not to be dependent on the integrity of the reflex arc. With regard to the development of the anterior roots, Anderson has shown that this is the case, for section of all the posterior roots conveying afferent impulses from the lower limb in a new-born animal does not hinder the normal development of the anterior roots supplying that limb. Also Mott, who originally thought that section of all the posterior roots to a limb caused atrophy of the corresponding anterior roots, has now come to the same conclusion as other observers, and can find no degeneration on the efferent side due to removal of afferent impulses.

Again, the process of regeneration after section of a nerve is not in favour of the neuroblast theory. There is no evidence that the cut end of a nerve can grow down and attach itself to a muscular or epithelial element without the assistance of a nerve tube down which to grow. When the cut nerves connected with the periphery degenerate, that applies only to the axis-cylinder and the medullary sheath, not to the neurilemma; the connective tissue elements remain alive and form a tube into which the growing axon finds its way, and so is conducted to the end-plate or end-organ of the peripheral structure.

Possibly, as suggested by Mott and Halliburton, the products of degeneration of the axis-cylinder and medullary sheath stimulate these connective tissue sheath-cells into active proliferation, and so bring about the great multiplication of cells arranged as cell-chains, which are so often erroneously spoken of as forming the young nerves. These sheath-cells are then supposed to re-form and secrete a pabulum which is important for the process of regeneration of the down-growing axis-cylinder and medullary sheath. Without such pabulum regeneration does not take place, as is seen in the central nervous system, where the sheath of Schwann is absent.

Again, it is becoming more and more doubtful whether the peripheral terminations of nerves are ever really free. As far as efferent nerves are concerned the nervous element may entirely predominate over the muscular or glandular, as in the formation of the electric organs of the Torpedo and Malapterurus, but still the final effect is produced by the alteration of the muscle or gland-cell. On the afferent side especially free nerve-terminations are largely recognized, or, as in Barker's book, nerves are spoken of as arising in connective tissue. Thus the numerous kinds of special sense-organs, such as Pacinian bodies, tendon-organs, genital corpuscles, etc., are all referred to by Barker under the heading of "sensory nerve beginnings in mesoblastic tissues." Yet the type of these organs has been known for a long time in the shape of Grandry's corpuscles or the tactile corpuscles in the duck's bill, where it has been proved that the nerve terminates in special large tactile cells derived from the surface-epithelium.

So also with all the others, further investigation tends to put them all in the same category, all special sensory organs originating from a localized patch of surface-epithelium. Thus Anderson has shown me in his specimens how the young Pacinian body is composed of rows of epithelial cells, into each of which a twig from the nerve passes. He has also shown me how, in the case of the tendon-organ, each nerve-fibre passes towards the attachment of the tendon and then bends back to supply the tendon-organ, thus indicating, as he suggests, how the nest of epithelial cells has wandered inwards from the surface to form the tendon-organ. Again, Meissner's corpuscles and Herbst's corpuscles are evidently referable to the same class as those of Grandry and Pacini.

Yet another instance of the same kind is to be found in the chromatophores of the frog and other animals which are under the influence of the central nervous system and yet have been supposed by various observers to be pigmented connective tissue cells. The most recent work of Leo Loeb and others has conclusively shown that such cells are invariably derived from the surface-epithelium.

Finally, in fishes we find the special sense-organs of the lateral line and other accessory sensory organs, all of which are indisputably formed from modified surface epithelial cells.

The whole of this evidence seems to me directly against Barker's classification of sensory nerve-beginnings in mesoblastic tissues; in none of these cases are we really dealing with free nervous tissue alone, the starting point is always a neuro-epithelial couple.