Chapter XV: Final Remarks (2)

{29}We must endeavour to dismiss from our imagination such forms as the salmon and shark as representatives of the fish-tribe, and the lobster and spider of the arthropods, and try to picture the kind of animals living in the seas in the early Devonian and Upper Silurian times, and then we find, to our surprise, that instead of the contrast between fishes and arthropods being so striking as to make any comparison between the two seem an absurdity, the difficulty in the last century, and even now, is to decide in many cases whether a fossil is an arthropod or a fish.

I have shown what kind of animal the palæostracan was like. What information is there of the nature of the earliest vertebrate?

The most ancient fishes hitherto discovered have been classified by Lankester and Smith Woodward into the three orders, Heterostraci, Osteostraci, and Antiarcha. Of these the Heterostraci contain the genera Pteraspis and Cyathaspis, and are the very earliest vertebrates yet discovered, being found in the Lower Silurian. The Osteostraci are divided into the Cephalaspidæ, Tremataspidæ, etc., and are found in the Upper Silurian and Devonian beds. The Antiarcha, comprising Pterichthys and Bothriolepis, belong to the Devonian and are not found in Silurian deposits. This, then, is the order of their appearance--Pteraspis, Cephalaspis, and Pterichthys.

In none of these families is there any resemblance to an ordinary fish. In no case is there any sign of vertebræ or of jaws. They, like the lampreys, were all agnathostomatous. Strange indeed is their appearance, and it is no wonder that there should have been a difficulty in deciding whether they were fish or arthropod. Their great characteristic is their buckler-plated cephalic shield, especially conspicuous on the dorsal side of the head. Figs. 11, 14, 15, 16, give the dorsal shields of Pteraspis, Auchenaspis, Pterichthys, and Bothriolepis.

In 1904, Drevermann discovered a mass of _Pteraspis Dunensis_ embedded in a single stone, showing the same kind of head-shield as _P. rostrata_, but the rostrum was longer and the spine at the extremity of the head-shield much longer and more conspicuous. The whole shape of the animal as seen in this photograph recalls the shape of a Hemiaspid rather than of a fish. It is, then, natural enough for the earlier observers to have looked upon such a fossil as related to an arthropod rather than a fish.

{30}

{31}In Figs. 12 and 13 I have placed side by side two Silurian fossils which are found in the same geological horizon. They are both life size and possess a general similarity of appearance, yet the one is a Cephalaspidian fish known by the name of _Auchenaspis_ or _Thyestes verrucosa_, the other a Palæostracan called _Bunodes lunula_.

_Fro._, narial opening; _l.e._, lateral eyes; _gl._, glabellum or plate over brain; _Occ._, occipital region.]

In a later chapter I propose to discuss the peculiarities and the nature of the head-shields of these earliest fishes, in connection with the question of the affinities of the animals which bore them. At this point of my argument I want simply to draw attention to the undoubted fact of the striking similarity in appearance between the {32}earliest fishes and members of the Palæostraca, the dominant race of arthropods which swarmed in the sea at the time: a similarity which could never have been suspected by any amount of investigation among living forms, but is immediately revealed when the ages themselves are questioned.

_An._, position of anus.]

I have not reproduced any of the attempted restorations of these old forms, as usually given in the text-books, because all such restorations possess a large element of fancy, due to the personal bias of the observer. I have put in Rohon's idea of the general shape of Tremataspis (Fig. 17) in order to draw attention to the lamprey-like appearance of the fish according to his researches (_cf._ Fig. 18).

The argument, then, from geology, like that from comparative anatomy and from the consideration of the importance of the central nervous system in the upward development of the animal race, not only points directly to the arthropod group as the ancestor of the {33}vertebrate, but also to a distinct ancient type of arthropod, the Palæostracan, the only living example of which is the King-Crab or Limulus; while the nearest approach to the trilobite group among living arthropods are Branchipus and Apus. It follows, therefore, that for the following up of this clue, Limulus especially must be taken into consideration, while Branchipus and Apus are always to be kept in mind.

AMMOCOETES RATHER THAN AMPHIOXUS IS THE BEST SUBJECT FOR INVESTIGATION.

It is not, however, Limulus that must be investigated in the first instance, but the vertebrate itself; for it can never be insisted on too often that in the vertebrate itself its past history will be found, but that Limulus cannot reveal the future of its race. What vertebrate must be chosen for investigation? Reasons have been given why our attention should be fixed upon the king-crab rather than on the lobster on the invertebrate side; what is the most likely animal on the vertebrate side?

From the evidence already given it is manifest that the earliest mammal belonged to the lowest group of mammals; that the birds on their first appearance presented reptilian characteristics, that the earliest reptiles belonged to a low type of reptile, that the amphibians at their first appearance were nearer in type to the fishes than were the later forms. As each of these groups advances in number and power, specialization takes place in it, and the latest developed members become further and further removed in type from the earliest. So also it must have been with the origin of fishes: here too, in the quest for information as to the structure and nature of the first-formed fishes, we must look to the lowest rather than to the highest living members of the group.

The lowest fish-like animal at present living is Amphioxus, and on this ground it is argued that the original vertebrate must have approached in organization to that of Amphioxus; it is upon the comparison between the structure of Amphioxus and that of Balanoglossus, that the theory of the origin of vertebrates from forms like the latter animal is based. For my own part, I think that in the first instance, at all events, Amphioxus should be put on one side, although of course its structure must always be kept in mind, for the following reasons:--

{34}Amphioxus, like the tunicates, does not possess the characteristics of other vertebrates. In all vertebrates above these forms the great characteristic is a well-defined brain-region from which arise nerves to organs of special sense, the eyes and nose. In Amphioxus no eyes exist, for the pigmented spot at the anterior extremity of the brain-region is no eye but only a mass of pigment, and the so-called olfactory pit is a very rudimentary and inferior organ of smell. In connection with the nearly complete absence of these two most important sense-organs, the most important part of the central nervous system, the region corresponding to the cerebral hemispheres, is also nearly completely absent.

Now, the history of the evolution of the central nervous system in the animal race points directly to its formation as a concentrated mass of nervous material at the anterior extremity of the body, in consequence of the formation of special olfactory and visual organs at that extremity. As already stated, the concentration of nervous material around the mouth as an oral ring was its beginning. In connection with this there arose special sense-organs for the guidance of the animal to its food which took the form of olfactory and optic organs. With the shifting from the radial to the elongated form these sense-organs remained at the anterior or mouth-end of the animal, and owing to their immense importance in the struggle for existence, that part of the central nervous system with which they were connected developed more than any other part, became the leader to which the rest of the nervous system was subservient, and from that time onwards the development of the brain-region was inevitably associated with the upward progress of animal life.

To those who believe in Evolution and the Darwinian theory of the survival of the fittest, it is simply inconceivable that a soft-bodied animal living in the mud, blind, with a rudimentary brain and rudimentary olfactory organs, such as is postulated when we think of Balanoglossus and Amphioxus, should hold its own and come victorious out of the struggle for existence at a time when the sea was peopled with powerful predaceous scorpion- and crab-like armour-plated animals possessing a well-developed brain, good eyes and olfactory organs, and powerful means of locomotion. Wherever in the scale of animal development Amphioxus may ultimately be placed, it cannot be looked upon as the type of the earliest formed fishes such as appeared in Silurian times.

{35}The next lowest group of living fishes is the Marsipobranchii which include the lampreys and hag-fishes. To these naturally we must turn for a clue as to the organization of the earliest fish, for here we find all the characteristics of the vertebrates represented: a well-formed brain-region, well-developed eyes and nose, cranial nerves directly comparable with those of other vertebrates, and even the commencement of vertebræ.

Among these forms the lamprey is by far the best for investigation, not only because it is easily obtainable in large quantities, but especially because it passes a large portion of its existence in a larval condition, from which it emerges into the adult state by a wonderful process of transformation, comparable in extent with the transformation of the larval caterpillar into the adult imago. So long does the lamprey live in this free larval condition, and so different is it in the adult stage, that the older anatomists considered that the two states were really different species, and gave the name of _Ammocoetes branchialis_ to the larval stage, while the adult form was called _Petromyzon planeri_, or _Petromyzon fluviatilis_.

This long-continued free-living existence in the larval or Ammocoetes stage makes the lamprey, more than any other type of lowly organized fish, invaluable for the present investigation, for throughout the animal kingdom it is recognized that the larval form approaches nearer to the ancestral type than the adult form, whether the latter is progressive or degenerate. Not only are the tissues formed during the stages which are passed through in a free-living larval form, serviceable tissues comparable to those of adult life, but also these stages proceed at so much slower a rate than do those in the embryo _in utero_ or in the egg, as to make the larval form much more suitable than the embryo for the investigation of ancestral problems. It is true enough that the free life of the larva may bring about special adaptations which are not of an ancestral character, as may also occur during the life of the adult; but the evidence is very strong that although some of the peculiarities of the larva may be due to such coenogenetic factors, yet on the whole many of them are due to ancestral characters, which disappear when transformation takes place, and are not found in the adult.

Thus if it be supposed that the amphibian arose from the fish, the tadpole presents more resemblance to the fish than the frog. If {36}it be supposed that the arthropod arose from the segmented worm, the caterpillar bears out the suggestion better than the adult imago. If it be supposed that the tunicate arose from a stock allied to the vertebrate, it is because of the peculiarities of the larva that such a supposition is entertained. So, too, if it be supposed that the fish arose from a member of the arthropod group, the larval form of the fish is most likely to give decisive information on the point.

For all these reasons the lowest form of fish to be investigated, in the hopes of finding out the nature of the earliest formed fish, is not Amphioxus, but Ammocoetes, the larval form of the lamprey--a form which, as I hope to satisfy my reader after perusal of subsequent pages, more nearly resembles the ancient Cephalaspidian fishes than any other living vertebrate.

COMPARISON OF CENTRAL NERVOUS SYSTEMS OF VERTEBRATE AND ARTHROPOD WITHOUT REVERSAL OF SURFACES.

So far different lines of investigation all point to the origin of the vertebrate from arthropods, the group of arthropods in question being now extinct, the nearest living representative being Limulus; also to the fact that of the two theories of the origin of vertebrates, that one which is based on the resemblance between the central nervous systems of the Vertebrata and the Appendiculata (Arthropoda and Annelida) is more in accordance with this evidence than the other, which is based mainly on the supposed possession of a notochord among certain animals.

How is it, then, that this theory has been discredited and lost ground? Simply, I imagine, because it was thought to necessitate the turning over of the animal. Let us, then, again look at the nervous system of the vertebrate, and see whether there is any such necessity.

As previously mentioned, the comparison of the two central nervous systems showed such close resemblances as to force those anatomists who supported this theory to the conclusion that the infundibular tube was in the position of the original oesophagus; they therefore looked for the remains of a mouth opening in the dorsal roof of the brain, but did not attempt to explain the extraordinary fact that the infundibular tube is only a ventral offshoot from the tube of the central nervous system. Yet this latter tube {37}is one, if not the most striking, of the peculiarities which distinguish the vertebrate; a tubular central nervous system such as that of the vertebrate is totally unlike any other nervous system, and the very fact that the two nervous systems of the vertebrate and arthropod are so similar in their nervous arrangements, makes it still more extraordinary that the nervous system should be grouped round a tube in the one case and not in the other.

Now, in the arthropod the oesophagus leads directly into the stomach, which is situated in the head-region, and from this a straight intestine passes directly along the length of the body to the anus, where it terminates. The relations of mouth, oesophagus, alimentary canal, and nervous system in these animals are represented in the diagram (Fig. 3).

Any tube, therefore, such as that of the infundibulum, which would represent the oesophagus of such an animal, must have opened into the mouth on the ventral side, and into the stomach on the dorsal side, and the lining epithelium of such an oesophagus must have been continuous with that of the stomach, and so of the whole intestinal tract.

Supposing, then, the animal is not turned over, but that the dorsal side still remains dorsal and ventral ventral, then the original mouth-opening of the oesophagus must be looked for on the ventral surface of the vertebrate brain in the region of the pituitary body or hypophysis, and on the dorsal side the tube representing the oesophagus must be continuous with a large cephalically dilated tube, which ought to pass into a small canal, to run along the length of the body and terminate in the anus.

This is exactly what is found in the vertebrate, for the infundibular tube passes into the third ventricle of the brain, which forms, with the other ventricles of the brain, the large dilated cephalic portion of the so-called nerve tube, and at the junction of the medulla oblongata and spinal cord, this dilated anterior part passes into the small, straight, central canal of the spinal cord, which in the embryo terminates in the anus by way of the neurenteric canal. If the animal is regarded as not having been turned over, then the conclusion that the infundibulum was the original oesophagus leads immediately to the further conclusion that the ventricles of the vertebrate brain represent the original cephalic stomach, and the central canal of the spinal cord the straight intestine of the arthropod ancestor.

{38}For the first time a logical, straightforward explanation is thus given of the peculiarities of the tube of the central nervous system, with its extraordinary termination in the anus in the embryo, its smallness in the spinal cord, its largeness in the brain region, and its offshoot to the ventral side of the brain as the infundibular channel. It is so clear that, if the infundibular tube be looked on as the old oesophagus, then its lining epithelium is the lining of that oesophagus; and the fact that this lining epithelium is continuous with that of the third ventricle, and so with the lining of the whole nerve-tube, must be taken into account and not entirely ignored as has hitherto been the case. If, then, we look at the central nervous system of the vertebrate in the light of the central nervous system of the arthropod without turning the animal over, we are led immediately to the conclusion that what has hitherto been called the vertebrate nervous system is in reality composed of two parts, viz. a nervous part comparable in all respects with that of the arthropod ancestor, which has grown over and included into itself, to a greater or less extent, a tubular part comparable in all respects with the alimentary canal of the aforesaid ancestor. If this conclusion is correct, it is entirely wrong to speak of the vertebrate central nervous system as being tubular, for the tube does not belong to the nervous system, but was originally a simple epithelial tube, such as characterizes the oesophagus, cephalic stomach, and straight intestine of the arthropod.

Here, then, is the crux of the position--either the so-called nervous tube of the vertebrate is composed of two separate factors, consisting of a true non-tubular nervous system and a non-nervous epithelial tube, these two elements having become closely connected together; or it is composed of one factor, an epithelial tube which constitutes the nervous system, its elements being all nervous elements.

If this latter hypothesis be accepted, then it is necessary to explain why parts of that tube, such as the roof of the fourth ventricle, the choroid plexuses of the various ventricles, which are parts of the original roof inserted into the ventricles, are not composed of nervous material, but form simple single-layered epithelial sheets, which by no possibility can be included among functional nervous structures. The upholders of this hypothesis can only explain the nature of these thin epithelial parts of the nervous tube in one of two ways; either the tube was originally formed of nervous {39}material throughout, and for some reason parts of it have lost their nervous function and thinned down; or else these thin epithelial parts are on their way to become nervous material, are still in an embryonic condition, and are of the nature of epiblast-epithelium, from which the central nervous system originally arose.

The first explanation is said to be supported by embryology, for at first the nerve-tube is formed in a uniform manner, and then later, parts of the roof appear to thin out and so form the thin epithelial parts. If this were the right explanation, then it ought to be found that in the lowest vertebrates there is greater evidence of a uniformly nervous tube than in the higher members of the group: while conversely, if, on the contrary, as we descend the vertebrate phylum, it is found that more and more of the tube presents the appearance of a single layer of epithelium, and the nervous material is limited more and more to certain parts of that tube, then the evidence is strong that the tubular character of the central nervous system is not due to an original nervous tube, but to a non-nervous epithelial tube with which the original nervous system has become closely connected.

The comparison of the brain region of the different groups of vertebrates (Fig. 19) is most instructive, for it demonstrates in the most conclusive manner how the roof of the nervous tube in that region loses more and more its nervous character, and takes on the appearance of a simple epithelial tube, as we descend lower and lower; until at last, in the brain of Ammocoetes, as represented in the figures, the whole of the brain-roof, from the region of the pineal eye to the commencement of the spinal cord, is composed of fold upon fold of a thin epithelial membrane forming an epithelial bag, which is constricted in only one place, where the fourth cranial nerve crosses over it.

Further, the brain of Ammocoetes (Fig. 20) shows clearly not only that it is composed of two parts, an epithelial tube and a nervous system, but also that the nerve-masses are arranged in the same relative position with respect to this tube as are the nerve-masses in the invertebrate with respect to the cephalic stomach and oesophagus. This evidence is so striking, so conclusive, that it is impossible to resist the conclusion that the tube did not originate as part of the central nervous system, but was originally independent of the central nervous system, and has been invaded by it.

{40}

_CB._, cerebellum; _PT._, pituitary body; _PN._, pineal body; _C. STR._, corpus striatum; _G.H.R._, right ganglion habenulæ. _I._, olfactory; _II._, optic nerves.]

{41}

A, dorsal view; B, lateral view; C, ventral view. _C.E.R._, cerebral hemispheres; _G.H.R._, right ganglion habenulæ; _PN._, right pineal eye; _CH_2_, _CH_3_, choroid plexuses; _I.-XII._ cranial nerves; _C.P._, _Conus post-commissuralis_.]

{42}The second explanation is hardly worth serious consideration, for it supposes that the nervous system, for no possible reason, was laid down in its most important parts--the brain-region--as an epithelial tube with latent potential nervous functions; that even up to the highest vertebrate yet evolved these nervous functions are still in abeyance over the whole of the choroid plexuses and the roof of the fourth ventricle. Further, it supposes that this prophetic epithelial tube originally developed into true nervous material only in certain parts, and that these parts, curiously enough, formed a nervous system absolutely comparable to that of the arthropod, while the dormant prophetic epithelial part was formed so as just to mimic, in relation to the nervous part, the alimentary canal of that same arthropod.

The mere facts of the case are sufficient to show the glaring absurdity of such an explanation. This is not the way Nature works; it is not consistent with natural selection to suppose that in a low form nervous material can be laid down as non-nervous epithelial material in order to provide in some future ages for the great increase in the nervous system.

Every method of investigation points to the same conclusion, whether the method is embryological, anatomical, or pathological.

First, take the embryological evidence. On the ground that the individual development reproduces to a certain extent the phylogenetic development, the peculiarities of the formation of the central nervous system in the vertebrate embryo ought to receive an appropriate explanation in any theory of phylogenetic development. Hitherto such explanation has been totally lacking; any suggestion of the manner in which a tubular nervous system may have been formed takes no account whatever of the differences between different parts of the tube; its dilated cephalic end with its infundibular projection ventrally, its small straight spinal part, and its termination in the anus. My theory, on the other hand, is in perfect harmony with the embryological history, and explains it point by point.

From the very first origin of the central nervous system there is evidence of two structures--the one nervous, and the other an epithelial surface-layer which ultimately forms a tube; this was first described by Scott in Petromyzon, and later by Assheton in the frog. In the latter case the external epithelial layer is pigmented, while the underlying nervous layer contains no pigment; a marked {43}and conspicuous demarcation exists, therefore, between the two layers from the very beginning, and it is easy to trace the subsequent fate of the two layers owing to this difference of pigmentation. The pigmented cells form the lining cells of the central canal, and becoming elongated, stretch out between the cells of the nervous layer; while the latter, on their side, invade and press between the pigmented cells. In this case, owing to the pigmentation of the epithelial layer, embryology points out in the clearest possible manner how the central nervous system of the vertebrate is composed of two structures--an epithelial non-nervous tube, on the outside of which the central nervous system was originally grouped; how, as development proceeds, the elements of these two structures invade each other, until at last they become so involved together as to give rise to the conception that we are dealing with one single nerve tube. It is impossible for embryology to give a clearer clue to the past history than it does in this case, for it actually shows, step by step, how the amalgamation between the central nervous system and the old alimentary canal took place.

Further, consider the shape of the tube when it is first formed, how extraordinary and significant that is. It consists of a simple dilated anterior end leading into a straight tube, the lumen of which is much larger than that of the ultimate spinal canal, and terminates by way of the neurenteric canal in the anus.

Why should the tube take this peculiar shape at its first formation? No explanation is given or suggested in any text-book of embryology, and yet it is so natural, so simple: it is simply the shape of the invertebrate alimentary canal with its cephalic stomach and straight intestine ending in the anus. Again embryology indicates most unmistakably the past history of the race. How are the nervous elements grouped round this tube when it is first formed? Here embryology shows that a striking difference exists between the part of the tube which forms the spinal cord and the dilated cephalic part. Fig. 21, A (2), represents the relation between the nervous masses and the epithelial tube in the first instance. At this stage the nervous material in the spinal cord lies laterally and ventrally to this tube, and at a very early stage the white anterior commissure is formed, joining together these two lateral masses; as yet there is no sign of any posterior fissure, the tube with its open lumen extends right to the dorsal surface.

{44}The interpretation of this stage is that in the invertebrate ancestor the nerve-masses were situated laterally and ventrally to the epithelial tube, and were connected together by commissures on the ventral side of the tube (Fig. 21, A (1)); in other words, the chain of ventral ganglia and their transverse commissures lying just ventrally to the intestine, which are so characteristic of the arthropod nervous system, is represented at this stage.

Subsequently, by the growth dorsalwards of nervous material to form the posterior columns, the original epithelial tube is compressed dorsally and laterally to such an extent that those parts lose all signs of lumen, the one becoming the posterior fissure and the others the _substantia gelatinosa Rolandi_ on each side. The original tube is thus reduced to a small canal formed by its ventral portion only (Fig. 21, A (3)). In this way the spinal cord is formed, and the walls of the original epithelial tube are finally visible only as the lining of the central canal (Fig. 21, A (4)).

When we pass to the brain-region, to the anterior dilated portion of the tube, embryology tells a different story. Here, as in the spinal cord, the nervous masses are grouped at first laterally and ventrally to the epithelial tube, as is seen in Fig. 21, B (2), but owing to the large size of its lumen here, the nervous material is not able to enclose it completely, as in the case of the spinal cord; {45}consequently there is no posterior fissure formed; but, on the contrary, the dorsal roof, not enclosed by the nerve-masses, remains epithelial, and so forms the membranous roof of the fourth ventricle and of the other ventricles of the brain (Fig. 21, B (3)). In the higher animals, owing to the development of the cerebrum and cerebellum, this membranous roof becomes pushed into the larger brain cavity, and thus forms the choroid plexuses of the third and lateral ventricles. In the lower vertebrates, as in Ammocoetes and the Dipnoi, it still remains as a dorsal epithelial roof and forms a most striking characteristic of such brains.

In this part of the nervous system, then, the nervous material is all grouped in its original position on the ventral side of the tube; and yet it is the same nervous material as that of the spinal cord, all the elements are there, giving origin here to the segmental cranial nerves just as lower down they give rise to the segmental spinal nerves, connecting together the separate segments each with the other and all with the higher brain-centres--the supra-infundibular centres--just as they do in the spinal region.

Why should there be this striking difference between the formation of the infra-infundibular region of the brain and that of the spinal cord? Do the advocates of the origin of vertebrates from Balanoglossus give the slightest reason for it? They claim that their view also provides a tubular nervous system for the vertebrate, but give not the slightest sign or indication as to why the nervous material should be grouped entirely on the ventral side of an epithelial tube in the infra-infundibular region and yet surround it in the spinal cord region. And the explanation is so natural, so simple: embryology does its very best to tell us the past history of the race, if only we look at it the right way.

The infra-infundibular nervous mass is naturally confined to the ventral side of the epithelial tube, because it represents the infra-oesophageal ganglia, situated as they are on the ventral side of the cephalic stomach, and, owing to the size of the stomach, they could not enclose it by dorsal growth, as they do in the case of the formation of the spinal cord (Fig. 21, B (1)). Still these nervous masses have grown dorsalwards, have commenced to involve the walls of the cephalic stomach even in the lowest vertebrate, as is seen in Ammocoetes, in which animal a ventral portion of the epithelial bag has been evidently compressed and its lumen finally obliterated {46}by the growth of the nerve-masses on each side of it. Throughout the whole vertebrate kingdom this obliterated portion still leaves its mark as the _raphé_ or seam, which is so characteristic of the infra-infundibular portion of the brain.

_Cr._, membranous cranium; _I_, olfactory nerves; _l.v._, lateral ventricles; _gl._, glandular tissue which fills up the cranial cavity.]

Here, again, it is seen how simple is the explanation of a peculiarity which has always puzzled anatomists--why should there be this seam in the infra-infundibular portion of the brain and not in the supra-infundibular or in the spinal cord? The corresponding compression in the upper brain-region forms the lateral ventricles, as is seen in the accompanying figure of the brain of Ammocoetes (Fig. 22).

In yet another instance it is seen how markedly the nervous masses are arranged in the same position with respect to the central tube as are the nerve ganglia with respect to the intestinal tube in the case of the invertebrate. Thus in birds a portion of the spinal cord in the lumbo-sacral region presents a very different appearance from the rest of the cord; it is known as the rhomboidal sinus, and a section of the cord of an adult pigeon across this region is given in Fig. 23. As is seen, the nervous portions are entirely confined to two masses connected together by the white anterior commissures which are situated laterally and ventrally to a median gelatinous mass; the small central canal is visible and {47}the whole dorsal area of the cord is taken up by a peculiar non-nervous wedge-shaped mass of tissue. At its first formation this portion of the cord is formed exactly in the same manner as the rest of the cord; instead, however, of the nervous material invading the dorsal part of the tube to form the posterior fissure, it has been from some cause unable to do so, the walls of the original non-nervous tube have become thickened dorsally, been transformed into this peculiar tissue, and so caused the peculiar appearance of the cord here. The nervous parts have not suffered in their development; the mechanism for walking in the bird is as well developed as in any other animal; their position only is different, for they still retain the original ventro-lateral position, but the non-nervous tube, the remains of the old intestine, has undergone a peculiar gelatinous degeneration just where it has remained free from invasion by the nervous tissue.

Throughout the whole of that part of the nervous system which gives origin to the cranial and spinal segmental nerves, the evidence is absolutely uniform that the nervous material was originally arranged bilaterally and ventrally on each side of the central tube, exactly in the same way as the nerve-masses of the infra-oesophageal and ventral chain of ganglia are arranged with respect to the cephalic stomach and straight intestine of the arthropod. But, in addition, we find in the vertebrate nervous masses, the cerebral hemispheres, the corpora quadrigemina and the cerebellum situated on the dorsal side of the central tube in the brain-region; this nervous material is, however, of a different character to that which gives origin to the spinal and cranial segmental nerves. How is the presence of these dorsal masses to be explained on the supposition that the dilated anterior part of the nerve-tube was originally the cephalic stomach of the arthropod ancestor? The cerebral hemispheres are simple enough, for they represent the supra-oesophageal ganglia, which of necessity, as they increased in size, would grow round the anterior end of the cephalic stomach and become more and more dorsal in position.

The difficulty lies rather in the position of the cerebellum and corpora quadrigemina, and the solution is as simple as it is conclusive.

Let us again turn to embryology and see what help it gives. In all vertebrates the dilated anterior portion of the nerve-tube does not, {48}as it grows, increase in size uniformly, but a constriction appears on its dorsal surface at one particular place, so as to divide it into an anterior and posterior vesicle; then the latter becomes divided into two portions by a second constriction. In this way three cerebral vesicles are formed; these three primary cerebral vesicles indicate the region of the fore-brain, mid-brain, and hind-brain respectively. Subsequently the first cerebral vesicle becomes divided into two to form the prosencephalon and thalamencephalon, while the third cerebral vesicle is also divided into two to form the region of the cerebellum and medulla oblongata.

These constrictions are in the position of commissural bands of nervous matter; of these the limiting nervous strands between the thalamencephalon and mesencephalon and between the mesencephalon and the hind-brain are of primary importance. The first of these commissural bands is in the position of the posterior commissure connecting the two optic thalami. In close connection with this are found, on the mid-dorsal region, the two pineal eyes with their optic ganglia, the so-called _ganglia habenulæ_. From these ganglia a peculiar tract of fibre, known as Meynert's bundle, passes on each side to the ventral infra-infundibular portion of the brain. In other words, the first constriction of the dilated tube is due to the presence and growth of nervous material in connection with the median pineal eyes. Here in precisely the same spot, as will be fully explained in the next chapter, there existed in the arthropod ancestor a pair of median eyes situated dorsally to the cephalic stomach, the pre-existence of which explains the reason for the first constriction.

The second primary constriction separating the mid-brain from the hind-brain is still more interesting, for it is coincident with the position of the trochlear or fourth cranial nerve. In all vertebrates without exception this nerve takes an extraordinary course; all other nerves, whether cranial or spinal, pass ventralwards to reach their destination. This nerve passes dorsalwards, crosses its fellow mid-dorsally in the valve of Vieussens, where the roof of the brain is thin, and then passes out to supply the superior oblique muscle of the eye of the opposite side. The two nerves form an arch constricting the dilated tube at this place. In the lowest vertebrate (Ammocoetes) the constriction formed by this nerve-pair is evident not only in the embryonic condition as in other vertebrates, but during the whole larval stage. As Fig. 20, A and B, shows, the whole of the dorsal {49}region of the brain up to the region of the pineal eye and _ganglion habenulæ_ is one large membranous bag, except for the single constriction where the fourth nerve on each side crosses over. The explanation of this peculiarity is given in Chapter VII., and follows simply from the facts of the arrangement of that musculature in the scorpion-group which gave rise to the eye-muscles of the vertebrate.

In Ammocoetes both cerebellum and posterior corpora quadrigemina can hardly be said to exist, but upon transformation a growth of nervous material takes place in this region, and it is seen that this commencing cerebellum and the corpora quadrigemina arise from tissue that is present in Ammocoetes along the course of the fourth nerve.

Here, then, again Embryology does its best to tell us how the vertebrate arose. The formation of the two primary constrictions in the dilated anterior vesicle whereby the brain is divided into fore-brain, mid-brain, and hind-brain is simply the representation ontogenetically of the two nerve-tracts which crossed over the cephalic stomach in the prevertebrate stage, in consequence of the mid-dorsal position of the pineal eyes and of the insertion of the original superior oblique muscles.

The subsequent constriction by which the prosencephalon is separated from the thalamencephalon is in the position of the anterior commissure, that commissure which connects the two supra-infundibular nerve-masses, and is one of the first-formed commissures in every vertebrate. This naturally is simply the commissure between the two supra-oesophageal ganglia; anterior to it, in the middle line, equally naturally, the anterior end of the old stomach wall still exists as the _lamina terminalis_.

The other division in the hind-brain region, which separates the region of the cerebellum from the medulla oblongata, is due to the growth of the cerebellum, and indicates its posterior limit. In such an animal as the lamprey, where the cerebellum is only commencing, this constriction does not occur in the embryo.

From such simple beginnings as are seen in Ammocoetes, the higher forms of brain have been evolved, to culminate in that of man, in which the massive cerebrum and cerebellum conceals all sign of the dorsal membranous roof, those parts of the simple epithelial tube which still remain being tucked away into the cavities to form the various choroid plexuses.

{50}In the whole evolution from the brain of Ammocoetes to that of man, the same process is plainly visible, viz. growth and extension of nervous material over the epithelial tube; extension dorsally and posteriorly of the supra-infundibular nervous masses (as seen in Fig. 19), combined with a dorsal growth of parts of the infra-infundibular nervous masses to form the cerebellum and posterior corpora quadrigemina.

Especially instructive is the formation of the cerebellum. It consists at first of a small mass of nervous tissue accompanying the fourth nerve, then by the growth of that mass surrounding and constricting a fold of the membranous roof, the _worm_ of the cerebellum is formed, as in the dog-fish. This very constriction causes the membrane to be thrown into a lateral fold on each side, as seen in Fig. 24, and in the dog-fish the nervous material on each side, known as the fimbriæ, is already commencing to grow from the ventral mass of the medulla oblongata to surround these lateral membranous folds. These _fimbriæ_ develop more and more in higher forms, and thus form the cerebellar hemispheres.

Not only does comparative anatomy confirm the teachings of embryology, but also pathology gives its quota in the same direction.

_v_, worm of cerebellum; _IV._, membranous roof of fourth ventricle continuous with the membranous folds on each side. Through these the fimbriæ (_fb._) can be dimly seen.]

One of the striking facts about malformations and disease of the central nervous system is the frequency of cystic formations; _spina bifida_ is a well-known instance. These cysts are merely epithelial non-nervous cysts formed from the epithelium of the central canal, difficult to understand if the whole nerve tube is one and entirely nervous, either actually or potentially, but natural and easy if we are really dealing with a simple epithelial tube on the outside of which the nervous material was originally grouped. The cystic formation belongs naturally enough to this tube, not to the nervous system.

Again, where animals such as lizards have grown a new tail, owing to the breaking off of the original one, it is found that the central canal extends into this new tail for some distance, but not {51}the nervous material surrounding it; all the nerves supplying the new tail arise from the uninjured spinal cord above, the central canal with its lining layer of epithelial cells alone grows into the new-formed appendage.

To all intents and purposes the same thing is seen in the termination of the spinal cord in a bird-embryo; more and more, as the end of the tail is approached, does the nervous matter of the spinal cord grow less and less, until at last a naked central canal with its lining epithelium is alone left to represent the so-called nerve-tube.

All these different methods of investigation lead irresistibly to the one conclusion that the tubular nature of the central nervous system has been caused by the central nervous system enclosing to a greater or less extent a pre-existing, non-nervous, epithelial tube.

This must always be borne strictly in mind. The problem, therefore, which presents itself is the comparison of these two factors separately, in order to find out the relationship of the vertebrate to the invertebrate. The nervous system without the tube must be compared to other nervous systems, and the tube must be considered apart from the nervous system.

THE PRINCIPLE OF CONCENTRATION AND CEPHALIZATION.

The central nervous system of the vertebrate resembles that of all the Appendiculata in the fact that it is composed of segments joined together which give origin to segmental nerves. There is, however, a great difference between the two systems: the division into separate segments is not obvious to the eye in the vertebrate nervous system, while in the invertebrate we can see that it is composed of a series of separate pairs of ganglia joined together longitudinally by nervous strands known as connectives and transversely by the nerve-commissures. Such a simple segmented system is found in the segmented worms, and in the lower arthropods, such as Branchipus, no great advance has been made on that of the annelid. In the higher forms, however, a greater and greater tendency to fusion of separate ganglia exists, especially in the head-region, so that the infra-oesophageal ganglia, which, in the lower forms are as separate as those of the ventral chain, in the higher forms are fused together to form a single nervous mass.

{52}This is the great characteristic of the advancement of the central nervous system among the Invertebrata, its concentration in the region of the head. It may be called the principle of cephalization, and is characteristic not only of higher organization in a group, but also of the adult as distinguished from the larval form. Thus in the imago greater concentration is found than in the caterpillar.

The segmented annelid type of nervous system consists of a supra-oesophageal ganglion, composed of the fused ganglia belonging to the pre-oral segments, and an infra-oesophageal chain of separate ganglia. With the concentration and modification around the mouth of the most anterior locomotor appendages to form organs for prehension and mastication of food, a corresponding concentration and fusion of the ganglia belonging to these segments takes place, so that finally, in the higher annelids, and in most of the great arthropod group, a fusion of a number of the most anterior ganglia has taken place to form the infra-oesophageal ganglion-mass.

The infra-oesophageal ganglia which are the first to fuse are those which supply the most anterior portion of the animal with nerves, and include always those anterior appendages which are modified for mastication purposes. To this part the name _prosoma_ has been given; in many cases it forms a well-defined, distinct portion of the animal.

Succeeding this prosoma or masticatory region, there occurs in all gill-bearing arthropods a respiratory region, in many cases more or less distinctly defined, which has received the name of _mesosoma._ The rest of the body is called the _metasoma_.

In accordance with this nomenclature the central nervous system of many of the Arthropoda may be divided as follows:--

1. Pre-oral, or supra-oesophageal ganglia.

2. Infra-oral, or infra-oesophageal ganglia and ventral chain, which consist of three groups: prosomatic, mesosomatic, and metasomatic ganglia.

The infra-oesophageal ganglion-mass, then, in most of the Arthropoda may be spoken of as formed by the fusion of the prosomatic or mouth-ganglia, the mesosomatic and metasomatic remaining separate and distinct. The number of ganglia which have fused may be observed by examination of the embryo, in which it is easy to see indications of the individual ganglia or _neuromeres_, although all such indication has disappeared in the adult; thus the {53}infra-oesophageal ganglia of the cray-fish have been shown to be constituted of six prosomatic ganglia.

In Fig. 25 I give figures of the central nervous system (with the exception of the abdominal or metasomatic ganglia) of Branchipus, Astacus, Limulus, Scorpio, Androctonus, Thelyphonus, and Ammocoetes. In all the figures the supra-oesophageal ganglia are lined horizontally, and their nerves shown, viz. optic (lateral eyes (II) and median eyes (II[prime])), olfactory (I) (first antennæ, camerostome, nose); then come the prosomatic ganglia (dotted), with their nerves (A) supplying the mouth parts, and the second antennæ or cheliceræ; then the mesosomatic (lined horizontally), with their nerves (B) supplying respiratory appendages. These figures show that the concentrated brain mass around the oesophagus of an arthropod which has arrived at the stage of Astacus, is represented by the supra-oesophageal ganglia and the fused prosomatic ganglia.

The next stage in the evolution of the brain is seen in the gradual inclusion of the mesosomatic ganglia, one after the other, into the infra-oesophageal mass of the already fused prosomatic ganglia. With this fusion is associated the loss of locomotion in these mesosomatic appendages, and their entire subservience to the function of respiration. Dana urges that cephalization is a consequence of functional alteration in the appendages, from organs of locomotion to those of mastication and respiration. Whether this be true or not, it is certainly a fact that in Limulus, the ganglion supplying the first mesosomatic appendage has fused with the prosomatic, infra-oesophageal mass. It is also a fact that the prosomatic appendages are the organs of mastication, their basal parts being arranged round the mouth so as to act as foot-jaws, while the mesosomatic appendages, though still free to move, have been reduced to such an extent as to consist mainly of their basal parts, which are all respiratory in function, except in the case of the first pair, where they carry the terminal ducts of the genital organs. In the next stage, that, of the scorpion, in which the mesosomatic appendages have lost all power of free locomotion, and have become internal branchiæ, another mesosomatic ganglion has fused with the brain mass, while in Androctonus two of the branchial mesosomatic ganglia have fused; and finally, in Thelyphonus and Phrynus, all the mesosomatic ganglia have coalesced with the fused prosomatic ganglia, while the metasomatic ganglia have themselves fused together in the caudal region to form what is known as the caudal brain.

{54}

{55}The brain in these animals may be spoken of as composed of three parts--(1) the fused supra-oesophageal ganglia, (2) the fused prosomatic ganglia, and (3) the fused mesosomatic ganglia. Such a brain is strictly homologous with the vertebrate brain, which also is built up of three parts--(1) the part in front of the notochord, the prechordal or supra-infundibular brain, which consists of the cerebral hemispheres, together with the basal and optic ganglia and corresponds, therefore, to the supra-oesophageal mass, with its olfactory and optic divisions lying in front of the oesophagus; (2 and 3) the epichordal brain, composed of (2) a trigeminal and (3) a vagus division, of which the first corresponds strictly to the fused prosomatic ganglia, and the second to the fused mesosomatic ganglia. Further, just as in the embryo of an arthropod it is possible, with more or less accuracy, to see the number of neuromeres or original ganglia which have fused to form the supra- and infra-oesophageal portions of its brain, so also in the embryo of a vertebrate we are able at an early stage to gain an indication, more or less accurate, of the number of neuromeres which have built up the vertebrate brain. The further consideration of these neuromeres, and the evidence they afford as to the number of the prosomatic and mesosomatic ganglia which have formed the epichordal part of the vertebrate brain, must be left to the chapter on the segmentation of the cranial nerves.

The further continuation of this process of concentration of separate segments, together with the fusion of the nervous system with the tube of the alimentary canal, leads in the simplest manner to the formation of the spinal cord of the vertebrate from the metasomatic ganglia of the ventral chain of the arthropod.

THE ANTAGONISM BETWEEN CEPHALIZATION AND ALIMENTATION.

This concentration of the nervous system in the head-region, together with an actual increase in the bulk of the cephalic nervous masses, constitutes the great principle upon which the law of upward progress or evolution in the animal kingdom is based, and it illustrates in a striking manner the blind way in which natural selection works; for, as already explained, the central nervous system arose as a ring round the mouth, in consequence of which, with the progressive {56}evolution of the animal kingdom, the oesophagus necessarily pierced the central nervous system at the cephalic end. At the same time, the very fact that the evolution was progressive necessitated the concentration and increase of the nervous masses in this very same oesophageal region.

Progress on these lines must result in a crisis, owing to the inevitable squeezing out of the food-channel by the increasing nerve-mass; and, indeed, the fact that such a crisis had in all probability arisen at the time when vertebrates first appeared is apparent when we examine the conditions at the present time.

Those invertebrates whose central nervous system is most concentrated at the cephalic end belong to the arachnid group, among which are included the various living scorpion-like animals, such as Thelyphonus, Androctonus, etc.

As already mentioned, the giants of the Palæostracan age were Pterygotus, Slimonia, etc., all animals of the scorpion-type--in fact, sea-scorpions. Now, all these animals, spiders and scorpions, without exception, are blood-suckers, and in all of them the concentrated cephalic mass of nervous material surrounds an oesophagus the calibre of which is so small that nothing but a fluid pabulum can be taken into the alimentary canal; and even for that purpose a special suctorial apparatus has in some species been formed on the gastric side of the oesophagus for the purpose of drawing blood through this exceedingly narrow tube.

In Fig. 25 this increasing antagonism between brain-power and alimentation, as we pass from such a form as Branchipus to the scorpion, is illustrated, and in Fig. 26 the relative sizes of the oesophagus and the brain-mass surrounding it is shown. The section shows that the food channel is surrounded by the white and grey matter of the brain as completely as the central canal of the spinal cord of the vertebrate is surrounded by the white and grey nervous material.

_A_, supra-oesophageal ganglia; _B_, infra-oesophageal ganglia; _Al_, oesophagus.]

{57}Truly, at the time when vertebrates first appeared, the direction and progress of variation in the Arthropoda was leading, owing to the manner in which the brain was pierced by the oesophagus, to a terrible dilemma--either the capacity for taking in food without sufficient intelligence to capture it, or intelligence sufficient to capture food and no power to consume it.

Something had to be done--some way had to be found out of this difficulty. The atrophy of the brain meant degeneration and the reduction to a lower stage of organization, as is seen in the Tunicata. The further development of the brain necessitated the establishment of a new method of alimentation and the closure of the old oesophagus, its vestiges still remaining as the infundibular canal of the vertebrate, meant the enormous upward stride of the formation of the vertebrate.

At first sight it might appear too great an assumption even to imagine the possibility of the formation of a new gut in an animal so highly organized as an arthropod, but a little consideration will, I think, show that such is not the case.