The Origin of Vertebrates

by Walter Holbrook Gaskell

Criticism of Balanoglossus theory.—Theory of parallel development.—Importance of the theory advocated in this book for all problems of Evolution.

The discussion in the last chapter on the "Principles of Embryology" completes the evidence which I am able to offer up to the present time in favour of my theory of the "Origin of Vertebrates." There are various questions which I have left untouched, but still are well worth discussion, and may be mentioned here. The first of these is the significance of the giant nerve-cells and giant nerve-fibres so characteristic of the brain-region of the lower vertebrates. In most fishes two very large cells are most conspicuous objects in any transverse section of the medulla oblongata at the level of entrance of the auditory nerves. Each of these cells gives off a number of processes, some of which pass in the direction of the auditory nerves and one very large axis-cylinder process which forms a giant-fibre, known by the name of a Mauthnerian fibre. Each Mauthnerian fibre crosses the middle line soon after its origin from the giant-cell, and passes down the spinal cord on the opposite side right to the tail. Here, near the end of the spinal cord, it breaks up into smaller fibres, which are believed by Fritsch and others to pass out directly into the ventral roots to supply the muscles of the tail. Thus Bela Haller says: "The Mauthnerian fibres are known to give origin to certain fibres which supply the ventral roots of the last three spinal nerves, so that their terminal branches serve, in all probability, for the innervation of the muscles of the tail-fin." They do not occur in the eel, according to Haller, or in Silurus, according to Kölliker. Their absence in those fishes, in which a well-developed tail-fin is also absent, increases the probability of the truth of Fritsch's original conclusion that these giant-fibres are associated axis-cylinders for certain definite co-ordinated movements of the fish, especially for the lateral movement of the tail.

In Ammocœtes, instead of two Mauthnerian fibres, a number of giant-fibres are found. They are called Müllerian fibres, and arise from giant-cells which are divisible into two groups. The first group consists of three pairs situated headwards of the level of exit of the trigeminal nerves. Two of these lie in front of the level of exit of the oculomotor nerves, and one pair is situated at the same level as the origin of the oculomotor nerves. The second group consists of a number of cells on each side at the level of the entrance of the fibres of the auditory nerves.

The Müllerian fibres largely decussate, as described by Ahlborn, and then become the most anterior portion of the white matter of the spinal cord, forming a group of about eight fibres on each side (Fig. [73]). A few fibres are also found laterally, and slightly dorsally, to the grey matter. These giant-fibres pass down the spinal cord right to the anal region; their ultimate destination is unknown. Mayer considers that in the first part of their course they correspond to those tracts of fibres known as the "posterior longitudinal bundles" in other vertebrates. I imagine, therefore, that the spinal part of their course represents the two antero-lateral descending tracts. The second group of giant-cells, which appears to have some connection with the auditory nerves, may represent "Deiter's nucleus." The whole system is probably the central nervous part of a co-ordination mechanism, which arises entirely in the pro-otic or prosomatic region of the brain—the great co-ordinating and equilibrating region par excellence.

If we turn now to the arthropod it is a striking coincidence that in the crayfish and in the lobster the work of Retzius, of Celesia, of Allen, and of many others demonstrates the existence of an equilibration-mechanism for the swimming movements of the tail-muscles, which is carried out by means of giant-fibres. These giant-fibres are the axis-cylinder processes of giant-cells, situated exclusively in the brain-region, and they run through the whole ventral ganglionic chain in order to supply the muscles of the tail. In the ventral nerve-cord of the crayfish, according to Retzius, two specially large giant-fibres exist, each of which breaks up, at the last abdominal ganglion, into smaller fibres, which pass directly out with the nerves to the tail-fin. Allen has shown that, in addition to these two specially large giant-fibres, there are a number of others, some of which, similarly to the Müllerian fibres of Ammocœtes, cross the middle line, while some do not. Each of these arises from a large nerve-cell and passes to one or other of the last pair of abdominal ganglia. The latter fibres, he says, send off collaterals, while the two specially large giant-fibres do not. The cells which give origin to all these large, long fibres are situated in or in front of the prosomatic region of the brain, similarly to the giant-cells, which give rise to the corresponding Müllerian fibres of Ammocœtes. I do not know how far this system is represented in Limulus or Scorpio.

It is, to my mind, improbable that the Mauthnerian fibres pass out directly as motor fibres to the muscles of the tail-fin; it is more likely that they are conducting paths between the equilibration-mechanism in connection with the VIIIth nerve and the spinal centres for the movements of the tail. Similarly, with respect to the arthropod, it is difficult to believe that the motor fibres for the tail-muscles arise in the brain-region. In either case, the striking coincidence remains that the movements of the tail-end of the body are regulated by means of giant-fibres which arise from giant-cells in the head-region of the body in both the Arthropoda and the lowest members of the Vertebrata.

The meaning of this system of giant-cells and giant-fibres in both classes of animals is well worthy of further investigation.

Another important piece of comparative work which ought to help in the elucidation of this problem is the comparison of the blood- and lymph-corpuscles of the vertebrate with those of the invertebrate groups. As yet, I have not myself made any observations in this direction, and feel that it is inadvisable to discuss the results of others until I know more about the facts from personal observation.

The large and important question of the manner of formation of the vertebrate skin has only been considered to a slight extent. A much more thorough investigation requires to be made into the nature of the skin of the oldest fishes in comparison with the skin of Ammocœtes on the one side, and of Limulus and the Palæostraca on the other.

The muscular system requires further investigation, not so much the different systems of the striated voluntary musculature—for these have been for the most part compared in the two groups of animals in previous chapters—as the involuntary unstriped musculature, about which no word has been said. The origin of the different systems of unstriped muscles in the vertebrate is bound up with the origin of the sympathetic system and its relation to the cranial and sacral visceral systems. The reason why I have not included in this book the consideration of the sympathetic nervous system is on account of the difficulty in finding any such system in Ammocœtes. Also, so far as I know, the distribution of unstriped muscle in Ammocœtes has not been worked out.