Chapter XII: The Organs of Excretion (10)

To attempt in the brief time which I have at my disposal to trace the influence of the Darwinian theory on all the branches of anatomy and physiology would be wholly impossible, and I shall confine myself to an attempt to do so for a small section only. There is perhaps no department of Biology which has been so revolutionised, if I may use the term, by the theory of animal evolution, as that of Development or Embryology. The reason of this is not far to seek. According to the Darwinian theory, the present order of the organic world has been caused by the action of two laws, known as the laws of heredity and of variation. The law of heredity is familiarly exemplified by the well-known fact that offspring resemble their parents. Not only, however, do the offspring belong to the same species as their parents, but they inherit the individual peculiarities of their parents. It is on this that the breeders of cattle depend, and it is a fact of every-day experience amongst ourselves. A further point with reference to heredity to which I must call your attention is the fact that the characters, which display themselves at some special period in the life of the parent, are acquired by the offspring at a corresponding period. Thus, in many birds the males have a special plumage in the adult state. The male offspring is not, however, born with the adult plumage, but only acquires it when it becomes adult.

The law of variation is in a certain sense opposed to the law of heredity. It asserts that the resemblance which offspring bear to their parents is never exact. The contradiction between the two laws is only apparent. All variations and modifications in an organism are directly or indirectly due to its environments; that is to say, they are either produced by some direct influence acting upon the organism itself, or by some more subtle and mysterious action on its parents; and the law of heredity really asserts that the offspring and parent would resemble each other if their environments were the same. Since, however, this is never the case, the offspring always differ to some extent from the parents. Now, according to the law of heredity, every acquired variation tends to be inherited, so that, by a summation of small changes, the animals may come to differ from their parent stock to an indefinite extent.

We are now in a position to follow out the consequences of these two laws in their bearing on development. Their application will best be made apparent by taking a concrete example. Let us suppose a spot on the surface of some very simple organism to become, at a certain period of life, pigmented, and therefore to be especially sensitive to light. In the offspring of this form, the pigment-spot will reappear at a corresponding period; and there will therefore be a period in the life of the offspring during which there is no pigment-spot, and a second period in which there is one. If a naturalist were to study the life-history, or, in other words, the embryology of this form, this fact about the pigment-spot would come to his notice, and he would be justified, from the laws of heredity, in concluding that the species was descended from an ancestor without a pigment-spot, because a pigment-spot was absent in the young. Now, we may suppose the transparent layer of skin above the pigment-spot to become thickened, so as gradually to form a kind of lens, which would throw an image of external objects on the pigment-spot. In this way a rudimentary eye might be evolved out of the pigment-spot. A naturalist studying the embryology of the form with this eye would find that the pigment-spot was formed before the lens, and he would be justified in concluding, by the same process of reasoning as before, that the ancestors of the form he was studying first acquired a pigment-spot and then a lens. We may picture to ourselves a series of steps by which the simple eye, the origin of which I have traced, might become more complicated; and it is easy to see how an embryologist studying the actual development of this complicated eye would be able to unravel the process of its evolution.

The general nature of the methods of reasoning employed by embryologists, who accept the Darwinian theory, is exemplified by the instance just given. If this method is a legitimate one, and there is no reason to doubt it, we ought to find that animals, in the course of their development, pass through a series of stages, in each of which they resemble one of their remote ancestors; but it is to be remembered that, in accordance with the law of variation, there is a continual tendency to change, and that the longer this tendency acts the greater will be the total effect. Owing to this tendency, we should not expect to find a perfect resemblance between an animal, at different stages of its growth, and its ancestors; and the remoter the ancestors, the less close ought the resemblance to be. In spite, however, of this limitation, it may be laid down as one of the consequences of the law of inheritance that every animal ought, in the course of its individual development, to repeat with more or less fidelity the history of its ancestral evolution.

A direct verification of this proposition is scarcely possible. There is ample ground for concluding that the forms from which existing animals are descended have in most instances perished; and although there is no reason why they should not have been preserved in a fossil state, yet, owing to the imperfection of the geological record, palæontology is not so often of service as might have been hoped.

While, for the reasons just stated, it is not generally possible to prove by direct observation that existing forms in their embryonic state repeat the characters of their ancestors, there is another method by which the truth of this proposition can be approximately verified.

A comparison of recent and fossil forms shews that there are actually living at the present day representatives of a considerable proportion of the groups which have in previous times existed on the globe, and there are therefore forms allied to the ancestors of those living at the present day, though not actually the same species. If therefore it can be shewn that the embryos of existing forms pass through stages in which they have the characters of more primitive groups, a sufficient proof of our proposition will have been given.

That such is often the case is a well-known fact, and was even known before the publication of Darwin's works. Von Baer, the greatest embryologist of the century, who died at an advanced age but a few years ago, discussed the proposition at considerable length in a work published between the years 1830 and 1840. He came to the conclusion that the embryos of higher forms never actually resemble lower forms, but only the embryos of lower forms; and he further maintained that such resemblances did not hold at all, or only to a very small extent, beyond the limits of the larger groups. Thus he believed that, though the embryos of Vertebrates might agree amongst themselves, there was no resemblance between them and the embryos of any invertebrate group. We now know that these limitations of Von Baer do not hold good, but it is to be remembered that the meaning _now_ attached by embryologists to such resemblances was quite unknown to him.

These preliminary remarks will, I trust, be sufficient to demonstrate how completely modern embryological reasoning is dependent on the two laws of inheritance and variation, which constitute the keystones of the Darwinian theory.

Before the appearance of the _Origin of Species_ many very valuable embryological investigations were made, but the facts discovered were to their authors merely so many ultimate facts, which admitted of being classified, but could not be explained. No explanation could be offered of why it is that animals, instead of developing in a simple and straightforward way, undergo in the course of their growth a series of complicated changes, during which they often acquire organs which have no function, and which, after remaining visible for a short time, disappear without leaving a trace.

No explanation, for instance, could be offered of why it is that a frog in the course of its growth has a stage in which it breathes like a fish, and then why it is like a newt with a long tail, which gradually becomes absorbed, and finally disappears. To the Darwinian the explanation of such facts is obvious. The stage when the tadpole breathes by gills is a repetition of the stage when the ancestors of the frog had not advanced in the scale of development beyond a fish, while the newt-like stage implies that the ancestors of the frog were at one time organized very much like the newts of to-day. The explanation of such facts has opened out to the embryologist quite a new series of problems. These problems may be divided into two main groups, technically known as those of phylogeny and those of organogeny. The problems of phylogeny deal with the genealogy of the animal kingdom. A complete genealogy would form what is known as a natural classification. To attempt to form such a classification has long been the aim of a large number of naturalists, and it has frequently been attempted without the aid of embryology. The statements made in the earlier part of my address clearly shew how great an assistance embryology is capable of giving in phylogeny; and as a matter of fact embryology has been during the last few years very widely employed in all phylogenetic questions, and the results which have been arrived at have in many cases been very striking. To deal with these results in detail would lead me into too technical a department of my subject; but I may point out that amongst the more striking of the results obtained _entirely_ by embryological methods is the demonstration that the Vertebrata are not, as was nearly universally believed by older naturalists, separated by a wide gulf from the Invertebrata, but that there is a group of animals, known as the Ascidians, formerly united with the Invertebrata, which are now universally placed with the Vertebrata.

The discoveries recently made in organogeny, or the genesis of organs, have been quite as striking, and in many respects even more interesting, than those in phylogeny, and I propose devoting the remainder of my address to a history of results which have been arrived at with reference to the origin of the nervous system.

To render clear the nature of these results I must say a few words as to the structure of the animal body. The body is always built of certain pieces of protoplasm, which are technically known to biologists as cells. The simplest organisms are composed either of a single piece of this kind, or of several similar pieces loosely aggregated together. Each of these pieces or cells is capable of digesting and assimilating food, and of respiring; it can execute movements, and is sensitive to external stimuli, and can reproduce itself. All the functions of higher animals can, in fact, be carried on in this single cell. Such lowly organized forms are known to naturalists as the Protozoa. All other animals are also composed of cells, but these cells are no longer complete organisms in themselves. They exhibit a division of labour: some carrying on the work of digestion; some, which we call nerve-cells, receiving and conducting stimuli; some, which we call muscle-cells, altering their form--in fact, contracting in one direction--under the action of the stimuli brought to them by the nerve-cells. In most cases a number of cells with the same function are united together, and thus constitute a tissue. Thus the cells which carry on the work of digestion form a lining membrane to a tube or sack, and constitute a tissue known as a secretory epithelium. The whole of the animals with bodies composed of definite tissues of this kind are known as the Metazoa.

A considerable number of early developmental processes are common to the whole of the Metazoa.

In the first place every Metazoon commences its existence as a simple cell, in the sense above defined; this cell is known as the ovum. The first developmental process which takes place consists in the division or segmentation of the single cell into a number of smaller cells. The cells then arrange themselves into two groups or layers known to embryologists as the _primary germinal layers_. These two layers are usually placed one within the other round a central cavity. The inner of the two is called the hypoblast, the outer the epiblast. The existence of these two layers in the embryos of vertebrated animals was made out early in the present century by Pander, and his observations were greatly extended by Von Baer and Remak. But it was supposed that these layers were confined to vertebrated animals. In the year 1849, and at greater length in 1859, Huxley demonstrated that the bodies of all the polype tribe or Coelenterata--that is to say of the group to which the common polype, jelly-fish and the sea-anemone belong--were composed of two layers of cells, and stated that in his opinion these two layers were homologous with the epiblast and hypoblast of vertebrate embryos. This very brilliant discovery came before its time. It fell upon barren ground, and for a long time bore no fruit. In the year 1866 a young Russian naturalist named Kowalevsky began to study by special histological methods the development of a number of invertebrated forms of animals, and discovered that at an early stage of development the bodies of all these animals were divided into germinal layers like those in vertebrates. Biologists were not long in recognizing the importance of these discoveries, and they formed the basis of two remarkable essays, one by our own countryman, Professor Lankester, and the other by a distinguished German naturalist, Professor Haeckel, of Jena.

In these essays the attempt was made to shew that the stage in development already spoken of, in which the cells are arranged in the form of two layers enclosing a central cavity has an ancestral meaning, and that it is to be interpreted to signify that all the Metazoa are descended from an ancestor which had a more or less oval form, with a central digestive cavity provided with a single opening, serving both for the introduction of food and for the ejection of indigestible substances. The body of this ancestor was supposed to have been a double-walled sack formed of an inner layer, the hypoblast, lining the digestive cavity, and an outer layer, the epiblast. To this form Haeckel gave the name of gastræa or gastrula.

There is every reason to think that Lankester and Haeckel were quite justified in concluding that a form more or less like that just described was the ancestor of the Metazoa; but the further speculations contained in their essays as to the origin of this form from the Protozoa can only be regarded as suggestive feelers, which, however, have been of great importance in stimulating and directing embryological research. It is, moreover, very doubtful whether there are to be found in the developmental histories of most animals any traces of this gastræa ancestor, other than the fact of their passing through a stage in which the cells are divided into two germinal layers.

The key to the nature of the two germinal layers is to be found in Huxley's comparison between them, and the two layers in the fresh-water polype and the sea-anemone. The epiblast is the primitive skin, and the hypoblast is the primitive epithelial wall of the alimentary tract.

In the whole of the polype group, or Coelenterata, the body remains through life composed of the two layers, which Huxley recognized as homologous with the epiblast and hypoblast of the Vertebrata; but in all the higher Metazoa a third germinal layer, known as the mesoblast, early makes its appearance between the two primary layers. The mesoblast originates as a differentiation of one or of both the primary germinal layers; but although the different views which have been held as to its mode of origin form an important section of the history of recent embryological investigations, I must for the moment confine myself to saying that from this layer there take their origin--the whole of the muscular system, of the vascular system, and of that connective-tissue system which forms the internal skeleton, tendons, and other parts.

We have seen that the epiblast represents the skin or epidermis of the simple sack-like ancestor common to all the Metazoa. In all the higher Metazoa it gives rise, as might be expected, to the epidermis, but it gives rise at the same time to a number of other organs; and, in accordance with the principles laid down in the earlier part of my address, it is to be concluded that _the organs so derived have been formed as differentiations of the primitive epidermis_. One of the most interesting of recent embryological discoveries is the fact that the nervous system is, in all but a very few doubtful cases, derived from the epiblast. This fact was made out for vertebrate animals by the great embryologist Von Baer; and the Russian naturalist Kowalevsky, to whose researches I have already alluded, shewed that this was true for a large number of invertebrate animals. The derivation of the nervous system from the epiblast has since been made out for a sufficient number of forms satisfactorily to establish the generalization that it is all but universally derived from the epiblast.

In any animal in which there is no distinct nervous system, it is obvious that the general surface of the body must be sensitive to the action of its surroundings, or to what are technically called stimuli. We know experimentally that this is so in the case of the Protozoa, and of some very simple Metazoa, such as the freshwater Polype or Hydra, where there is no distinct nervous system. The skin or epidermis of the ancestor of the Metazoa was no doubt similarly sensitive; and the fact of the nervous system being derived from the epiblast implies that the functions of the central nervous system, which were originally taken by the whole skin, became gradually concentrated in a special part of the skin which was step by step removed from the surface, and finally became a well-defined organ in the interior of the body.

What were the steps by which this remarkable process took place? How has it come about that there are nerves passing from the central nervous system to all parts of the skin, and also to the muscles? How have the arrangements for reflex actions arisen by which stimuli received on the surface of the body are carried to the central part of the nervous system, and are thence transmitted to the appropriate muscles, and cause them to contract? All these questions require to be answered before we can be said to possess a satisfactory knowledge of the origin of the nervous system. As yet, however, the knowledge of these points derived from embryology is imperfect, although there is every hope that further investigation will render it less so. Fortunately, however, a study of comparative anatomy, especially that of the Coelenterata, fills up some of the gaps left from our study of embryology.

From embryology we learn that the ganglion-cells of the central part of the nervous system are originally derived from the simple undifferentiated epithelial cells of the surface of the body. We further learn that the nerves are out-growths of the central nervous system. It was supposed till quite recently that the nerves in Vertebrates were derived from parts of the middle germinal layer or mesoblast, and that they only became secondarily connected with the central nervous system. This is now known not to be the case, but the nerves are formed as processes growing out from the central part of the nervous system.

Another important fact shewn by embryology is that the central nervous system, and percipient portion of the organs of special sense, are often formed from the same part of the primitive epidermis. Thus, in ourselves and in other vertebrate animals the sensitive part of the eye, known as the retina, is formed from two lateral lobes of the front part of the primitive brain. The crystalline lens and cornea of the eye are, however, subsequently formed from the skin.

The same is true for the peculiar compound eyes of crabs or Crustacea. The most important part of the central nervous system of these animals is the supra-oesophageal ganglia, often known as the brain, and these are formed in the embryo from two thickened patches of the skin at the front end of the body. These thickened patches become gradually detached from the surface, remaining covered over by a layer of skin. They then constitute the supra-oesophageal ganglia; but they form not only the ganglia, but also the rhabdons or retinal elements of the eye--the parts in fact which correspond to the rods and cones in our own retina. The layer of epidermis or skin which lies immediately above the supra-oesophageal ganglia becomes gradually converted into the refractive media of the crustacean eye. A cuticle which lies on its surface forms the peculiar facets on the surface of the eye, which are known as the corneal lenses, while the cells of the epidermis give rise to lens-like bodies known as the crystalline cones.

It would be easy to quote further instances of the same kind, but I trust that the two which I have given will be sufficient to shew the kind of relation which often exists between the organs of special sense, especially those of vision, and the central nervous system. It might have been anticipated _à priori_ that organs of special sense would only appear in animals provided with a well-developed central nervous system. This, however, is not the case. Special cells, with long delicate hairs, which are undoubtedly highly sensitive structures, are present in animals in which as yet nothing has been found which could be called a central nervous system; and there is every reason to think that the organs of special sense originated _pari passu_ with the central nervous system. It is probable that in the simplest organisms the whole body is sensitive to light, but that with the appearance of pigment-cells in certain parts of the body, the sensitiveness to light became localised to the areas where the pigment-cells were present. Since, however, it was necessary that stimuli received by such organs should be communicated to other parts of the body, some of the epidermic cells in the neighbourhood of the pigment-spots, which were at first only sensitive, in the same manner as other cells of the epidermis, became gradually differentiated into special nerve-cells. As to the details of this differentiation, embryology does not as yet throw any great light; but from the study of comparative anatomy there are grounds for thinking that it was somewhat as follows:--Cells placed on the surface sent protoplasmic processes of a nervous nature inwards, which came into connection with nervous processes from similar cells placed in other parts of the body. The cells with such processes then became removed from the surface, forming a deeper layer of the epidermis below the sensitive cells of the organ of vision. With these cells they remained connected by protoplasmic filaments, and thus they came to form a thickening of the epidermis underneath the organ of vision, the cells of which received their stimuli from those of the organ of vision, and transmitted the stimuli so received to other parts of the body. Such a thickening would obviously be the rudiment of a central nervous system, and it is easy to see by what steps it might become gradually larger and more important, and might gradually travel inwards, remaining connected with the sense organ at the surface by protoplasmic filaments, which would then constitute nerves. The rudimentary eye would at first merely consist partly of cells sensitive to light, and partly of optical structures constituting the lens, which would throw an image of external objects upon it, and so convert the whole structure into a true organ of vision. It has thus come about that, in the development of the individual, the retina or sensitive part of the eye is first formed in connection with the central nervous system, while the lenses of the eye are independently evolved from the epidermis at a later period.

The general features of the origin of the nervous system which have so far been made out by means of the study of embryology are the following:--

(1) That the nervous system of the higher Metazoa has been developed in the course of a long series of generations by a gradual process of differentiation of parts of the epidermis.

(2) That part of the central nervous system of many forms arose as a local collection of nerve-cells in the epidermis, in the neighbourhood of rudimentary organs of vision.

(3) That ganglion cells have been evolved from simple epithelial cells of the epidermis.

(4) That the primitive nerves were outgrowths of the original ganglion cells; and that the nerves of the higher forms are formed as outgrowths of the central nervous system.

The points on which embryology has not yet thrown a satisfactory light are:--

(1) The steps by which the protoplasmic processes, from the primitive epidermic cells, became united together so as to form a network of nerve-fibres, placing the various parts of the body in nervous communication.

(2) The process by which nerves became connected with muscles, so that 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 these points, although there is left much that is still obscure.

In our own country Mr Romaines has conducted some interesting physiological experiments on these forms; and Professor Schäfer has made some important histological investigations upon them. In Germany a series of interesting researches have also been made on them by Professors Kleinenberg, Claus and Eimer, and more especially by the brothers Hertwig, of Jena. Careful histological investigations, especially those 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 the digestive tract, a number of specially modified epithelial cells, which are no doubt delicate organs of sense. They are provided at their free extremity with 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 characterized 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 distributed both beneath the epithelium of the skin and that of the digestive tract, but is especially concentrated in the disc-like region between the mouth and tentacles. The above observations 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 (1) 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 complicated 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 was 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 muscles; 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 differentiation parts of this network may have become specially contractile, and other parts may have lost their contractility and become solely nervous. In this way the connection between nerves and muscles might be explained, and this hypothesis fits in very well with the condition of the neuro-muscular system as we find it in the Coelenterata.

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 differentiation of the nervous system may very probably have commenced, in the first instance, from a specialization in the function of part of a network formed of neuro-muscular prolongations 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, but 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 some 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 parts 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 decompositions 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 adopted 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 colloids; while the demonstration that ganglion-cells arose as epithelial 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.

XX. ON THE DEVELOPMENT OF THE SKELETON OF THE PAIRED FINS OF
ELASMOBRANCHII, CONSIDERED IN RELATION TO ITS BEARINGS ON THE NATURE OF
THE LIMBS OF THE VERTEBRATA[479].

Footnote 479: From the _Proceedings of the Zoological Society
of London_, 1881.

(With Plate 33.)

Some years ago the study of the development of the soft parts of the fins in several Elasmobranch types, more especially in _Torpedo,_ led me to the conclusion that the vertebrate limbs were remnants of two continuous lateral fins[480]. More or less similar views (which I was not at that time acquainted with) had been previously held by Maclise, Humphrey, and other anatomists; these views had not, however, met with much acceptance, and diverge in very important points from those put forward by me. Shortly after the appearance of my paper, J. Thacker published two interesting memoirs comparing the skeletal parts of the paired and unpaired fins[481].

Footnote 480: "Monograph on the Development of Elasmobranch
Fishes," pp. 319, 320.

Footnote 481: J. K. Thacker, "Median and Paired Fins; a
Contribution to the History of the Vertebrate Limbs," _Trans.
of the Connecticut Acad._ Vol. III. 1877. "Ventral Fins of
Ganoids," _Trans. of the Connecticut Acad._ Vol. IV. 1877.

In these memoirs Thacker arrives at conclusions as to the nature of the fins in the main similar to mine, but on entirely independent grounds. He attempts to shew that the structure of the skeleton of the paired fins is essentially the same as that of the unpaired fins, and in this comparison lays special stress on the very simple skeleton of the pelvic fin in the cartilaginous Ganoids, more especially in _Acipenser_ and _Polyodon_. He points out that the skeleton of the pelvic fin of _Polyodon_ consists essentially of a series of nearly isolated rays, which have a strikingly similar arrangement to that of the rays of the skeleton in many unpaired fins. He sums up his views in the following way[482]:--

Footnote 482: _Loc. cit._ p. 298.

"As the dorsal and anal fins were specializations of the median folds
of _Amphioxus,_ so the paired fins were specializations of the two
lateral folds which are supplementary to the median in completing the
circuit of the body. These lateral folds, then, are the homologues of
Wolffian ridges, in embryos of higher forms. Here, as in the median
fins, there were formed chondroid and finally cartilaginous rods.
These became at least twice segmented. The orad ones, with more or
less concrescence proximally, were prolonged inwards. The cartilages
spreading met in the middle line; and a later extension of the
cartilages dorsad completed the limb-girdle.

"The limbs of the Protognathostomi consisted of a series of parallel
articulated cartilaginous rays. They may have coalesced somewhat
proximally and orad. In the ventral pair they had extended themselves
mesiad until they had nearly or quite met and formed the hip-girdle;
they had not here extended themselves dorsad. In the pectoral limb the
same state of things prevailed, but was carried a step further,
namely, by the dorsal extension of the cartilage constituting the
scapular portion, thus more nearly forming a ring or girdle."

The most important point in Thacker's theories which I cannot accept is the derivation of the folds, of which the paired fins of the Vertebrata are supposed to be specializations, from the lateral folds of _Amphioxus;_ and Thacker himself recognizes that this part of his theory stands on quite a different footing to the remainder.

Not long after the publication of Thacker's paper, an important memoir was published by Mivart in the _Transactions_ of this Society[483]. The object of the researches recorded in this paper was, as Mivart explains, to test how far the hard parts of the limbs and of the azygos fins may have arisen through centripetal chondrifications or calcifications, and so be genetically exoskeletal[484].

Footnote 483: St George Mivart, "On the Fins of
Elasmobranchii," _Zoological Trans._ Vol. X.

Footnote 484: Mivart used the term exoskeletal in an unusual
and (as it appears to me) inconvenient manner. The term is
usually applied to dermal skeletal structures; but the skeleton
of the limbs, with which we are here concerned, is undoubtedly
not of this nature.

Mivart's investigations and the majority of his views were independent of Thacker's memoir; but he acknowledges that he has derived from Thacker the view that pelvic and pectoral girdles, as well as the skeleton of the limbs, may have arisen independently of the axial skeleton.

The descriptive part of Mivart's paper contains an account of the structure of a great variety of interesting and undescribed types of paired and unpaired fins, mainly of Elasmobranchii. The following is the summary given by Mivart of the conclusions at which he has arrived[485]:--

Footnote 485: _Loc. cit._ p. 480.

"1. Two continuous lateral longitudinal folds were developed, similar
to dorsal and ventral median longitudinal folds.

"2. Separate narrow solid supports (radials), in longitudinal series,
and with their long axes directed more or less outwards at right
angles with the long axis of the body, were developed in varying
extents in all these four longitudinal folds.

"3. The longitudinal folds became interrupted variously, but so as to
form two prominences on each side, _i.e._ the primitive paired limbs.

"4. Each anterior paired limb increased in size more rapidly than the
posterior limb.

"5. The bases of the cartilaginous supports coalesced as was needed,
according to the respective practical needs of the different separate
portions of the longitudinal folds, _i.e._ the respective needs of the
several fins.

"6. Occasionally the dorsal radials coalesced (as in _Notidanus_, &c.)
and sought centripetally (_Pristis_, &c.) adherence to the skeletal
axis.

"7. The radials of the hinder paired limb did so more constantly, and
ultimately prolonged themselves inwards by mesiad growth from their
coalesced base, till the piscine pelvic structure arose, as, _e.g._,
in _Squatina_.

"8. The pectoral radials with increasing development also coalesced
proximally, and thence prolonging themselves inwards to seek a _point
d'appui_, shot dorsad and ventrad to obtain a firm support, and at the
same time to avoid the visceral cavity. Thus they came to abut
dorsally against the axial skeleton, and to meet ventrally together in
the middle line below.

"9. The lateral fins, as they were applied to support the body on the
ground, became elongated, segmented, and narrowed, so that probably
the line of the propterygium, or possibly that of the mesopterygium,
became the cheiropterygial axis.

"10. The distal end of the incipient cheiropterygium either preserved
and enlarged preexisting cartilages or developed fresh ones to serve
fresh needs, and so grew into the developed cheiropterygium; but there
is not yet enough evidence to determine what was the precise course of
this transformation.

"11. The pelvic limb acquired a solid connection with the axial
skeleton (a pelvic girdle) through its need of a _point d'appui_ as a
locomotive organ on land.

"12. The pelvic limb became also elongated; and when its function was
quite similar to that of the pectoral limb, its structure became also
quite similar (_e.g. Ichthyosaurus_, _Plesiosaurus_, _Chelydra_, &c.);
but for the ordinary quadrupedal mode of progression it became
segmented and inflected in a way generally parallel with, but (from
its mode of use) in part inversely to, the inflections of the pectoral
limb."

Günther[486] has propounded a theory on the primitive character of the fins, which, on the whole, fits in with the view that the paired fins are structures of the same nature as the unpaired fins. The interest of Günther's views on the nature of the skeleton of the fins more especially depends upon the fact that he attempts to evolve the fin of _Ceratodus_ from the typical Selachian type of pectoral fin. His own statement on this subject is as follows[487]:--

Footnote 486: "Description of _Ceratodus_," _Phil. Trans._
1871.

Footnote 487: _Loc. cit._ p. 534.

"On further inquiry into the more distant relations of the
_Ceratodus_-limb, we may perhaps be justified in recognizing in it a
modification of the typical form of the Selachian pectoral fin.
Leaving aside the usual treble division of the carpal cartilage
(which, indeed, is sometimes simple), we find that this shovel-like
carpal forms the base for a great number of phalanges, which are
arranged in more or less regular transverse rows (zones) and in
longitudinal rows (series). The number of phalanges of the zones and
series varies according to the species and the form of the fin; in
_Cestracion philippi_ the greater number of phalanges is found in the
proximal zones and middle series, all the phalanges decreasing in size
from the base of the fin towards the margins. In a Selachian with a
long, pointed, scythe-shaped pectoral fin, like that of _Ceratodus_,
we may, from analogy, presume that the arrangement of the cartilages
might be somewhat like that shewn in the accompanying diagram, which I
have divided into nine zones and fifteen series.

"When we now detach the outermost phalanx from each side of the first
horizontal zone, and with it the other phalanges of the same series,
when we allow the remaining phalanges of this zone to coalesce into
one piece (as, in nature, we find coalesced the carpals of _Ceratodus_
and many phalanges in Selachian fins), and when we repeat this same
process with the following zones and outer series, we arrive at an
arrangement identical with what we actually find in _Ceratodus_."

While the researches of Thacker and Mivart are strongly confirmatory of the view at which I had arrived with reference to the nature of the paired fins, other hypotheses as to the nature of the skeleton of the fins have been enunciated, both before and after the publication of my memoir, which are either directly or indirectly opposed to my view.

Huxley in his memoir on _Ceratodus_, which throws light on so many important morphological problems, has dealt with the nature of paired fins[488].

Footnote 488: T. H. Huxley, "On _Ceratodus Fosteri_, with some
Observations on the Classification of Fishes," _Proc. Zool.
Soc._ 1876.

He holds, in accordance with a view previously adopted by Gegenbaur, that the limb of _Ceratodus_ "presents us with the nearest known approximation to the fundamental form of vertebrate limb or archipterygium," and is of opinion that in a still more archaic fish than _Ceratodus_ the skeleton of the fin "would be made up of homologous segments, which might be termed pteromeres, each of which would consist of a mesomere with a preaxial and a postaxial paramere." He considers that the pectoral fins of Elasmobranchii, more especially the fin of _Notidanus_, which he holds to be the most primitive form of Elasmobranch fin, "results in the simplest possible manner from the shortening of the axis of such a fin-skeleton as that of _Ceratodus_, and the coalescence of some of its elements." Huxley does not enter into the question of the origin of the skeleton of the pelvic fin of Elasmobranchii.

It will be seen that Huxley's idea of the primitive structure of the archipterygium is not easily reconcilable with the view that the paired fins are parts of a once continuous lateral fin, in that the skeleton of such a lateral fin, if it has existed, must necessarily have consisted of a series of parallel rays.

Gegenbaur[489] has done more than any other living anatomist to elucidate the nature of the fins; and his views on this subject have undergone considerable changes in the course of his investigations. After Günther had worked out the structure of the fin of _Ceratodus_, Gegenbaur suggested that it constituted the most primitive _persisting_ type of fin, and has moreover formed a theory as to the origin of the fins founded on this view, to the effect that the fins, together with their respective girdles, are to be derived from visceral arches with their rays.

Footnote 489: C. Gegenbaur, _Untersuchungen z. vergleich.
Anat. d. Wirbelthiere_ (Leipzig 1864-5): erstes Heft, "Carpus
u. Tarsus;" zweites Heft, "Brustflosse d. Fische." "Ueb. d.
Skelet d. Gliedmaassen d. Wirbelthiere im Allgemeinen u. d.
Hintergliedmaassen d. Selachier insbesondere," _Jenaische
Zeitschrift_, Vol. V. 1870. "Ueb. d. Archipterygium,"
_Jenaische Zeitschrift_, Vol. VII. 1873. "Zur Morphologie d.
Gliedmaassen d. Wirbelthiere," _Morphologisches Jahrbuch_, Vol. II.
1876.

His views on this subject are clearly explained in the subjoined passages quoted from the English translation of his _Elements of Comparative Anatomy_, pp. 473 and 477.

"The skeleton of the free appendage is attached to the extremity of
the girdle. When simplest, this is made up of cartilaginous rods
(rays), which differ in their size, segmentation, and relation to one
another. One of these rays is larger than the rest, and has a number
of other rays attached to its sides. I have given the name of
_archipterygium_ to the ground-form of the skeleton which extends from
the limb-bearing girdle into the free appendage. The primary ray is
the stem of this archipterygium, the characters of which enable us to
follow out the lines of development of the skeleton of the appendage.
Cartilaginous arches beset with the rays form the branchial skeleton.
The form of skeleton of the appendages may be compared with them; and
we are led to the conclusion that it is possible that they may have
been derived from such forms. In the branchial skeleton of the
Selachii the cartilaginous bars are beset with simple rays. In many a
median one is developed to a greater size. As the surrounding rays
become smaller, and approach the larger one, we get an intermediate
step towards that arrangement in which the larger median ray carries a
few smaller ones. This differentiation of one ray, which is thereby
raised to a higher grade, may be connected with the primitive form of
the appendicular skeleton; and as we compare the girdle with a
branchial arch, so we may compare the median ray and its secondary
investment of rays with the skeleton of the free appendage.

"All the varied forms which the skeleton of the free appendages
exhibits may be derived from a ground-form which persists in a few
cases only, and which represents the first, and consequently the
lowest, stage of the skeleton in the fin--the _archipterygium_. This
is made up of a stem which consists of jointed pieces of cartilage,
which is articulated to the shoulder-girdle and is beset on either
side with rays which are likewise jointed. In addition to the rays of
the stem there are others which are directly attached to the
limb-girdle.

"_Ceratodus_ has a fin-skeleton of this form; in it there is a stem
beset with two rows of rays. But there are no rays in the
shoulder-girdle. This biserial investment of rays on the stem of the
fin may also undergo various kinds of modifications. Among the Dipnoi,
_Protopterus_ retains the medial row of rays only, which have the form
of fine rods of cartilage; in the Selachii, on the other hand, the
lateral rays are considerably developed. The remains of the medial row
are ordinarily quite small, but they are always sufficiently distinct
to justify us in supposing that in higher forms the two sets of rays
might be better developed. Rays are still attached to the stem and are
connected with the shoulder-girdle by means of larger plates. The
joints of the rays are sometimes broken up into polygonal plates which
may further fuse with one another; concrescence of this kind may also
affect the pieces which form the base of the fin. By regarding the
free rays, which are attached to these basal pieces, as belonging to
these basal portions, we are able to divide the entire skeleton of the
fin into three segments--pro-, meso-, and metapterygium.

"The metapterygium represents the stem of the archipterygium and the
rays on it. The propterygium and the mesopterygium are evidently
derived from the rays which still remain attached to the
shoulder-girdle."

Since the publication of the memoirs of Thacker, Mivart, and myself, a pupil of Gegenbaur's, M. v. Davidoff[490], has made a series of very valuable observations, in part directed towards demonstrating the incorrectness of our theoretical views, more especially Thacker's and Mivart's view of the genesis of the skeleton of the limbs. Gegenbaur[491] has also written a short paper in connection with Davidoff's memoir, in support of his own as against our views.

Footnote 490: M. v. Davidoff, "Beiträge z. vergleich. Anat. d.
hinteren Gliedmaassen d. Fische, I.," _Morphol. Jahrbuch_,
Vol. V. 1879.

Footnote 491: "Zur Gliedmaassenfrage. An die Untersuchungen
von Davidoff's angeknüpfte Bemerkungen," _Morphol. Jahrbuch_,
Vol. V. 1879.

It would not be possible here to give an adequate account of Davidoff's observations on the skeleton, muscular system, and nerves of the pelvic fins. His main argument against the view that the paired fins are the remains of a continuous lateral fin is based on the fact that a variable but often considerable number of the spinal nerves in front of the pelvic fin are united by a longitudinal commissure with the true plexus of the nerves supplying the fin. From this he concludes that the pelvic fin has shifted its position, and that it may once therefore have been situated close behind the visceral arches. Granting, however, that Davidoff's deduction from the character of the pelvic plexus is correct, there is, so far as I see, no reason in the nature of the lateral-fin theory why the pelvic fins should not have shifted; and, on the other hand, the longitudinal cord connecting some of the ventral roots in front of the pelvic fin may have another explanation. It may, for instance, be a remnant of the time when the pelvic fin had a more elongated form than at present, and accordingly extended further forwards.

In any case our knowledge of the nature and origin of nervous plexuses is far too imperfect to found upon their characters such conclusions as those of Davidoff.

Gegenbaur, in his paper above quoted, further urges against Thacker and Mivart's views the fact that there is no proof that the fin of _Polyodon_ is a primitive type; and also suggests that the epithelial line which I have found connecting the embryonic pelvic and pectoral fins in _Torpedo_ may be a rudiment indicating a migration backwards of the pelvic fin.

With reference to the development of the pectoral fin in the Teleostei there are some observations of 'Swirski[492], which unfortunately do not throw very much light upon the nature of the limb.

Footnote 492: G. 'Swirski, _Untersuch. üb. d. Entwick. d.
Schultergürtels u. d. Skelets d. Brustflosse d. Hechts._ Inaug.
Diss. Dorpat, 1880.

'Swirski finds that in the Pike the skeleton of the limb is formed of a plate of cartilage continuous with the pectoral girdle, which soon becomes divided into a proximal and a distal portion. The former is subsequently segmented into five basal rays, and the latter into twelve parts, the number of which subsequently becomes reduced.

* * * * *

The observations which I have to lay before the Society were made with the object of determining how far the development of the skeleton of the limbs throws light on the points on which the anatomists whose opinions have just been quoted are at variance.