Chapter IV: , the probability of the theory that the trigeminal group of (7)

Such an explanation signifies that the somatic trunk-musculature of the vertebrate was derived from the dorsal longitudinal musculature of the body of the arthropod, and not from the ventral longitudinal musculature, and that therefore in the primitive arthropod stage the equivalent of the myotome of the vertebrate did not give origin to the ventral longitudinal muscles of the invertebrate ancestor. Now, as I have said, von Kennel states that in the procoelom of Peripatus a dorsal part (III. in Fig. 157) is cut off which gives origin to the dorsal body-musculature, while the ventral part which remains (I. and II. in Fig. 157) gives origin in its appendicular portion (I.) to the muscles of the appendage, and presumably in its ventral somatic portion (II.) to the ventral longitudinal muscles of the body. This dorsal cut-off part might be called the myotome, in the same sense as the corresponding part of the procoelom in the vertebrate is called the myotome. In both cases the muscles derived from it form only a part of the voluntary musculature of the animal, and in both cases the muscles in question are the dorsal longitudinal muscles of the body, to which must be added the dorso-ventral body-muscles. Now, the whole of my theory of the origin of vertebrates arose from the investigation of the structure of the cranial nerves, which led to the conception that their grouping is not, like the spinal, a dual grouping of motor and sensory elements, but a dual grouping to supply two sets of segments, characterized especially by the different embryological origin of their musculature. The one set I called the somatic segmentation, because the muscles belonging to it were the great longitudinal body-muscles; the other I called the splanchnic segmentation, because its muscles were those connected with the branchial and visceral arches. According to my theory, this latter segmentation was due to the segmentation of the appendages in the invertebrate ancestor; and in previous chapters, dealing as they do with the cranial region, attention was especially directed to the way in which the position of the striated splanchnic musculature could be explained by a transformation of the prosomatic and mesosomatic appendages. Now, I am dealing with the metasomatic region, in which it is true the appendages take a very subordinate place, but still something corresponding to the splanchnic segments of the cranial region might fairly be expected to exist, and I therefore {412}desire to emphasize what appears to me to be the fact, that the musculature, which in the region of the trunk would correspond to that derived from the ventral segmentation of the mesoblast in the region of the head, may have arisen not only from the musculature of the appendages, but also from the ventral longitudinal musculature of the body of the invertebrate ancestor, for it seems probable that this latter musculature had nothing to do with the origin of the great longitudinal muscles of the vertebrate body, either dorsal or ventral.

The way in which I imagine the obliteration of the atrial cavity to have taken place is indicated in Fig. 160, B, which is a modification of a section across a trilobite-like animal as represented in Fig. 160, A. As is seen, the pleural folds on each side have nearly met the bulged-out ventral body-surface. A continuation of the same process would give Fig. 160, C, which is, to all intents and purposes, the same as Fig. 159, C, taken from van Wijhe, and shows how the segmental duct is left in the remains of the atrial cavity. The lining walls of the atrial cavity are represented very black, in order to indicate the presence of pigment, as indeed is seen in the corresponding position in Ammocoetes. In these diagrams I have represented the median ventral surface as a large bulged-out bag, without indicating any structures in it except the ventral extension of the procoelom to form the metacoelom. At present I will leave the space between the central nervous system and the ventral mesentery blank, as in the diagrams; in my next chapter I will discuss the possible method of formation within this blank space of the notochord and midgut. Boveri considers that the obliteration of the atrial cavity in the higher vertebrates is not complete, but that its presence is still visible in the shape of the pronephric duct. The evidence of Maas and others that the duct is formed by the fusion of the pronephric tubules is, it seems to me, conclusive against Boveri's view; but yet, as may be seen from my diagrammatic figures, the very place where one would expect to find the last remnant of the atrial cavity is exactly where the pronephric duct is situated. For my own part I should expect to find evidence of a former existence of an atrial cavity rather in the pigment round the pronephros and its duct than in the duct itself.

{413}

_Al._, alimentary canal; _N._, nervous system; _My._, myotome; _Pl._, pleuron; _App._, appendage; _Neph._, nephrocoele; _Met._, metacoele; _S.d._, segmental duct; _At._, atrial chamber; _V.Mes._, ventral mesentery; _Mes._, mesonephros. The dotted line represents the splanchnopleuric mesoblast in all figures.]

{414}The conception that Amphioxus shows us how to account for the great envelope of somatic muscles which wraps round the vertebrate body, in that the ancestor of the vertebrate possessed on each side the body a segmented pleuron, is exactly in accordance with the theory of the origin of vertebrates deduced from the study of Ammocoetes, as already set forth in previous chapters. For we see that one of the striking characteristics of such forms as Bunodes, Hemiaspis, etc., is the presence of segmented pleural flaps on each side of the main part of the body; and if we pass further back to the great group of trilobites, we find in the most manifold form, and in various degrees of extent, the most markedly segmented pleural folds. In fact, the hypothetical figure (Fig. 160, A) which I have deduced from the embryological evidence, might very well represent a cross-section of a trilobite, provided only that each appendage of the trilobite possessed an excretory coxal gland.

The earliest fishes, then, ought to have possessed segmented pleural folds, which were moved by somatic muscles, and enveloped the body after the fashion of Ammocoetes and Amphioxus, and I cannot help thinking that Cephalaspis shows, in this respect also, its relation to Ammocoetes. It is well known that some of the fossil representatives of the Cephalaspids show exceedingly clearly that these animals possessed a very well-segmented body, and it is equally recognized that this skeleton is a calcareous, not a bony skeleton, and does not represent vertebræ, etc. It is generally called an aponeurotic skeleton, meaning thereby that what is preserved represents not dermal plates alone, or a vertebrate skeleton, but the calcified septa or aponeuroses between a number of muscle-segments or myomeres, precisely of the same kind as the septa between the myomeres in Ammocoetes. The termination of such septa on the surface would give rise to the appearance of dermal plates or scutes, or the septa may even have been attached to something of the nature of dermal plates. The same kind of picture would be represented if these connective tissue dissepiments of Ammocoetes were calcified, and the animal then fossilized. In agreement with this interpretation of the spinal skeleton of Cephalaspis, it may be noted that again and again, in parts of these dissepiments, I have found in old specimens of Ammocoetes nodules of cartilage formed, and at transformation it is in this very tissue that the spinal cartilages are formed.

{415}

Now, the specimens of Cephalaspis all show, as seen in Fig. 161, that the skeletal septa cover the body regularly, and then along one line are bent away from the body to form, as it were, a fringe, or rather a free pleuron, which has been easily pushed at an angle to the body-skeleton in the process of fossilization. Patten thinks that this fringed appearance is evidence of a number of segmental appendages which were jointed to the corresponding body-segments, and in the best specimen at the South Kensington Natural History Museum he thinks such joints are clearly visible. He concludes, therefore, that the cephalaspids were arthropods, and not vertebrates. I have also carefully examined this specimen, and do not consider that what is seen resembles the joint of an arthropod appendage; the appearance is rather such as would be produced if the line of attachment of Patten's appendages to the body were the place where the pleural body folds became free from the body, and so with any pressure a {416}bending or fracture of the calcified plates would take place along this line. There is, undoubtedly, an appearance of finish at the termination of these skeletal fringes, as though they terminated in a definitely shaped spear-like point, just as is seen in the trilobite pleuræ. This, again, to my mind, is rather evidence of pleural fringes than of true appendages.

As already argued, I look upon Ammocoetes as the only living fish at all resembling the cephalaspids; it is therefore instructive to compare the arrangement of this spinal dermo-septal skeleton of Cephalaspis with that of the septa between the myomeres in the trunk-region of Ammocoetes and Amphioxus. Such a skeleton in Ammocoetes would be represented by a series of plates overlapping each other, arranged as in Fig. 162, A, and in Amphioxus as in Fig. 162, B. I have lettered the corresponding parts of the two structures by similar letters, _a_, _b_, _c_. Ammocoetes differs in configuration from Amphioxus in that it possesses an extra dorsal (_a_, _d_) and an extra ventral bend. Ammocoetes is a much rounder animal than Amphioxus, and both the dorsal and ventral bends are on the extreme ventral and dorsal surfaces--surfaces which can hardly be said to exist in Amphioxus. The part, then, of such an aponeurotic skeleton {417}in Ammocoetes which I imagine corresponds to _b_, _c_ in Amphioxus, and therefore would represent the pleural fold, is the part ventral to the bend at _b_. In both the animals this bend corresponds to the position of the notochord NC.

The skeleton of Cephalaspis compares more directly with that of Ammocoetes than that of Amphioxus, for there is the same extra dorsal bend (Fig. 161, _a_, _d_) as in Ammocoetes; the lateral part of the skeleton again gives an angle _a_, _b_, _c_; the part from _b_ to _c_ would therefore represent the pleural fold. I picture to myself the sequence of events somewhat as follows:--

First, a protostracan ancestor, which, like Peripatus, possessed appendages on every segment into which coelomic diverticula passed, forming a system of coxal glands; such glands, being derived from the segmental organs of the Chætopoda, discharged originally to the exterior by separate openings on each segment. It is, however, possible, and I think probable, that a fusion of these separate ducts had already taken place in the protostracan stage, so that there was only one external opening for the whole of these metasomatic coxal glands, just as there is only one external opening for the corresponding prosomatic coxal glands of Limulus. Then, by the ventral growth of pleural body-folds, such appendages became enclosed and useless, and the coxal glands of the post-branchial segments, with their segmental or pronephric duct, were all that remained as evidence of such appendages. This dwindling of the metasomatic appendages was accompanied by the getting-rid of free appendages generally, in the manner already set forth, with the result that a smooth fish-like body-surface was formed; then the necessity of increasing mobility brought about elongation by the addition of segments between those last formed and the cloacal region. Each of such new-formed segments was appendageless, so that its segmental organ was not a coxal gland, but entirely somatic in position, and formed, therefore, a mesonephric tubule, not a pronephric one. Such glands could no longer excrete to the exterior, owing to the enclosing shell of the pleural folds; but the pronephric duct was there, already formed, and so these nephric tubules opened into that, instead of, as in the case of the branchial slits, forcing their way through the pleural walls when the atrium became closed.

{418}THE MEANING OF THE DUCTLESS GLANDS.

If it is a right conception that the excretory organs of the protostracan group, which gave origin to the vertebrates as well as to the crustaceans and arachnids, were of the nature of coxal glands, then it follows that such coxal glands must have existed originally on every segment, because they themselves were derived from the segmental organs of the annelids; it is therefore worth while making an attempt to trace the fate of such segmental organs in the vertebrate as well as in the crustacean and arachnid.

Such an attempt is possible, it seems to me, because there exists throughout the animal kingdom striking evidence that excretory organs which no longer excrete to the exterior do not disappear, but still perform excretory functions of a different character. Their cells still take up effete or injurious substances, and instead of excreting to the exterior, excrete into the blood, forming either ductless glands of special character, or glands of the nature of lymphatic glands.

The problem presented to us is as follows:--

The excretory organs of both arthropods and vertebrates arose from those of annelids, and were therefore originally present in every segment of the body. In most arthropods and vertebrates they are present only in certain regions; in the former case, as the coxal glands of the prosomatic or head-region; in the latter, as the nephric glands of the metasomatic or trunk-region, and, in the case of Amphioxus, of the mesosomatic or branchial region.

In the original arthropod, judging from Peripatus, they were present, as in the annelid, in all the segments of the body, and formed coxal glands. Therefore, in the ancestors of the living Crustacea and Arachnida, coxal glands must have existed in all the segments of the body, and we ought to be able to find the vestiges of them in the mesosomatic or branchial and metasomatic or abdominal regions of the body.

Similarly, in the vertebrates, derived, as has been shown, not from the annelids, but from an arthropod stock, evidence of the previous existence of coxal glands ought to be manifested in the prosomatic or trigeminal region, in the mesosomatic or branchial region, as well as in the metasomatic or post-branchial region.

How does an excretory organ change its character when it ceases {419}to excrete to the exterior? What should we look for in our search after the lost coxal glands?

The answer to these questions is most plainly given in the case of the pronephros, especially in Myxine, where Maas has been able to follow out the whole process of the conversion of nephric tubules into a tissue resembling that of a lymph-gland.

He states, in the first place, that the pronephros possesses a capillary network, which extends over the pronephric duct, while the tubules of the mesonephros possess not only this capillary network, equivalent to the capillaries over the convoluted tubules in the higher vertebrates, but also a true glomerulus, in that the nephric segmental arteriole forms a coil (Knauel), and pushes in the wall of the mesonephric tubule. He describes the pronephros of large adult individuals as consisting of--

1. Tubules with funnels which open into the pericardial coelom.

2. A large capillary network (the glomus) at the distal end.

3. A peculiar tissue (the 'strittige Gewebe' of the Semon-Spengel controversy), which Spengel considers to be composed of the altered epithelium of pronephric tubules, while Semon looks on it as an amalgamation of glomeruli.

Maas is entirely on the side of Spengel, and shows that this peculiar tissue is actually formed by modified pronephric tubules, which become more and more lymphatic in character.

He says: "The pronephros consists of a number of nephric tubules, placed separately one behind the other, which were originally segmental in character, each one of which is supplied by a capillary network from a segmental branch of the aorta. The tubules begin with many mouths (dorso-lateral and medial-ventral) in the pericardial cavity; on their other blind end they have lost their original external opening, and there, in the cranial portion of the head-kidney, before they have joined together to form a collecting duct, they, together with the vascular network, are transformed into a peculiar adrenal-like tissue. The most posterior of the segmental capillary nets retain their original character, and are concentrated into the separate capillary mass known as the glomus."

Later on he says: "Further, the separate head-kidney is more and more removed in structure from an excretory organ in the ordinary sense. One cannot, however, speak of it as an organ becoming rudimentary; this is proved not only by the progressive transformation {420}of its internal tissue into a tissue of a very definite character, but also by the cilia in its canals, and the steady increase in the number of its funnels. It appears, therefore, to be the conversion of an excretory organ into an organ for the transference of fluid out of the coelom into a special tissue, _i.e._ into its blood-sinus; in other words, into an organ which must be classed as belonging to the lymph-system."

In exact correspondence with this transformation of a nephric tubule into a ductless gland of the nature of a lymphatic gland, is the formation of the head-kidney in the Teleostea. Thus, Weldon points out that, though the observations of Balfour left it highly probable that the "lymphatic" tissue described by him was really a result of the transformation of part of the embryonic kidney, he did not investigate the details of its development. This was afterwards done by Emery, with the following results: "In those Teleostea which he has studied, Professor Emery finds that at an early stage the kidney consists entirely of a single pronephric funnel, opening into the pericardium, and connected with the segmental duct, which already opens to the exterior. Behind this funnel, the segmental duct is surrounded by a blastema, derived from the intermediate cell-mass, which afterwards arranges itself more or less completely into a series of solid cords, attaching themselves to the duct. These develop a lumen, and become normal segmental tubules, but it is, if I may be allowed the expression, a matter of chance how much of the blastema becomes so transformed into kidney tubules, and how much is left as the 'lymphatic' tissue of Balfour, this 'lymphatic' tissue remaining either in the pronephros only, or in both pro- and meso-nephros."

If we turn now to the invertebrates, we see also how close a connection exists between lymphatic and phagocytic organs and excretory organs. The chief merit for this discovery is due to Kowalewsky, who, taking a hint from Heidenhain's work on the kidney, in which he showed how easy it was to find out the nature of different parts of the mammalian excretory organ by the injection of different substances, such as a solution of ammoniated carmine, or of indigo-carmine, has injected into a large number of different invertebrates various colouring matters, or litmus, or bacilli, and thus shown the existence, not only of known excretory organs, but also of others, lymphatic or lymphoid in nature, not hitherto suspected.

In all cases he finds that a phagocytic action with respect to solid {421}bodies is a property of the leucocytes, and that these leucocytes which are found in the coelomic spaces of the Annelida, etc., are apparently derived from the epithelium of such spaces. Also by the proliferation of such epithelium in places, _e.g._ the septal glands of the terrestrial Oligochæta, segmental glandular masses of such tissue are formed which take up the colouring matter, or the bacilli. In the limicolous Oligochæta such septal glands are not found, but at the commencement of the nephridial organ, immediately following upon the funnel, a remarkable modification of the nephridial wall takes place to form a large cellular cavernous mass, the so-called filter, which in Euaxes is full of leucocytes; the cells are only definable by their nuclei, and look like and act in the same way as the free leucocytes outside this nephridial appendage. As G. Schneider points out, the whole arrangement is very like that described by Kowalewsky in the leeches Clepsine and Nephelis, where, also immediately succeeding the funnel of the nephridial organ, a large accessory organ is found, which is part of the nephridium, and is called the nephridial capsule. This is the organ _par excellence_ which takes up the solid carmine-grains and bacilli, and apparently, from Kowalewsky's description, contains leucocytes in large numbers. We see, then, that in such invertebrates, just as in the vertebrate, modifications of the true excretory organ may give rise to phagocytic glands of the nature of lymphatic glands. Further, these researches of Kowalewsky suggest in the very strongest manner that whenever by such means new, hitherto unsuspected glands are discovered, such glands must belong to the excretory system, _i.e._ must be derived from coelomic epithelium, even when all evidence of any coelom has disappeared. Kowalewsky himself was evidently so impressed with the same feeling that he heads one of his papers "The Excretory Organs of the Pantopoda," although the organs in question had been discovered by him by this method, and appeared as ductless glands with no external opening.

To my mind these observations of Kowalewsky are of exceeding interest, for it is immediately clear that if the segmental organs of the annelids, which must have existed on all the segments of the forefathers of the Crustacea and Arachnida (the Protostraca), have left any sign of their existence in living crustaceans and arachnids, then such indication would most likely take the form of lymphatic glands in the places where the excretory organs ought to have been.

Now, as already pointed out in Peripatus, such segmental organs {422}were formed by the ventral part of the coelom, and dipped originally into each appendage. We know also that each segment of an arachnid embryo possesses a coelomic cavity in its ventral part which extends into the appendage on each side; this cavity afterwards disappears, and is said to leave no trace in the adult of any excretory coxal gland derived from its walls. If, however, it is found that in the very position where such organ ought to have been formed a segmentally arranged ductless gland is situated, the existence of which is shown by its taking up carmine, etc., then it seems to me that in all probability such gland is the modification of the original coxal gland.

This is what Kowalewsky has done. Thus he states that Metschnikoff had fed Mysis with carmine-grains, and found tubules at the base of the thoracic feet coloured red with carmine. He himself used an allied species, _Parapodopsis cornutum_, and found here also that the carmine was taken up by tubules situated in the basal segments of the feet. In Nebalia, feeding experiments with alizarin blue and carmine stained the antennal glands, and showed the existence of glands at the base of the eight thoracic feet. These glands resemble the foot-glands of Mysis, Parapodopsis, and Palæmon, and lie in the space through which the blood passes from the thoracic feet, _i.e._ from the gills, to the heart. In Squilla also, in addition to the shell-glands, special glands were discovered on the branchial feet on the path of the blood to the heart. These glands form continuous masses of cells which constitute large compact glands at the base of the branchial feet. Single cells of the same sort are found along the whole course of the branchial venous canal, right up to the pericardium.

These observations show that the Crustacea possess not only true excretory organs in the shape of coxal glands, _i.e._ antennary glands, shell-glands, etc., in the cephalic region, but also a series of segmental glands situated at the base of the appendages, especially of the respiratory appendages: a system, that is to say, of coxal glands which have lost their excretory function, through having lost their external opening, but have not in consequence disappeared, but still remain _in situ_, and still retain an important excretory function, having become lymphatic glands containing leucocytes. Such glands are especially found in the branchial appendages, and are called branchial glands by Cuénot, who describes them for all Decapoda.

Further, it is significant that the same method reveals the {423}existence in Pantopoda of a double set of glands of similar character, one set in the basal segments of the appendage, and the other in the adjacent part of the body.

In scorpions also, Kowalewsky has shown that the remarkable lymphatic organ situated along the whole length of the nerve-cord in the abdominal region takes up carmine grains and bacilli; an organ which in Androctonus does not form one continuous gland, but a number of separate, apparently irregularly grouped, glandular bodies.

In addition to this median lymphatic gland, Kowalewsky has discovered in the scorpion a pair of lateral glands, to which he gives the name of lymphoid glands, which communicate with the thoracic body-cavity (_i.e._ the pseudocoele), are phagocytic, and, according to him, give origin to leucocytes by the proliferation of their lining cells, thus, as he remarks, reminding us of the nephridial capsules of Clepsine. These glands are so closely related in position to the coxal glands on each side that he has often thought that the lumen of the gland communicated with that of the coxal gland; he, however, has persuaded himself that there is no true communication between the two glands. Neither of these organs appears to be segmental, and until we know how they are developed it is not possible to say whether they represent fused segmental organs or not.

The evidence, then, is very strong that in the Crustacea and Arachnida the original segmental excretory organs do not disappear, but remain as ductless glands, of the nature of lymphatic glands, which supply leucocytes to the system.

Further, the evidence shows that the nephric organs, or parts of the coelom in close connection with these organs, may be transformed into ductless glands, which do not necessarily contain free leucocytes as do lymph-glands, but yet are of such great importance as excretory organs that their removal profoundly modifies the condition of the animal. Such a gland is the so-called adrenal or suprarenal body, disease of which is a feature of Addison's disease; a gland which forms and presumably passes into the blood a substance of remarkable power in causing contraction of blood-vessels, a substance which has lately been prepared in crystalline form by Jokichi Takamine, and called by him "adrenalin"; a gland, therefore, of very distinctly peculiar properties, which cannot be regarded as rudimentary, but is of vital importance for the due maintenance of the healthy state.

In the Elasmobranchs two separate glandular organs have been {424}called suprarenal; a segmental series of paired organs, each of which possesses a branch from the aorta and a sympathetic ganglion, and an unpaired series in close connection with the kidneys, to which Balfour gave the name of interrenal glands. Of these two sets of glands, Swale Vincent has shown that the extract of the interrenals has no marked physiological effect, in this respect resembling the extract of the cortical part of the mammalian gland, while the extract of the paired segmental organs of the Elasmobranch produces the same remarkable rise of blood-pressure as the extract of the medullary portion of the mammalian gland.

The development also of these two sets of glands is asserted to be different. Balfour considered that the suprarenals were derived from sympathetic ganglion-cells, but left the origin of the interrenals doubtful. Weldon showed that the cortical part of the suprarenals in the lizard was derived from the wall of the glomerulus of a number of mesonephric tubules. In Pristiurus, he stated that the mesoblastic rudiment described by Balfour as giving origin to the interrenals is derived from a diverticulum of each segmental tubule, close to the narrowing of its funnel-shaped opening into the body-cavity. With respect to the paired suprarenals he was unable to speak positively, but doubted whether they were derived entirely from sympathetic ganglia.

Weldon sums up the results of his observations by saying: "That all vertebrates except Amphioxus have a portion of the kidney modified for some unknown purpose not connected with excretion; that in Cyclostomes the pronephros alone is so modified, in Teleostei the pro- and part of the meso-nephros; while in the Elasmobranchs and the higher vertebrates the mesonephros alone gives rise to this organ, which has also in these forms acquired a secondary connection with certain of the sympathetic ganglia."

Since Weldon's paper, a large amount of literature on the origin of the adrenals has appeared, a summary of which, up to 1891, is given by Hans Rabl in his paper, and a further summary by Aichel in his paper published in 1900. The result of the investigations up to this latter paper may be summed up by saying that the adrenals, using this term to include all these organs of whatever kind, are in all cases, partly at all events, derived from some part of the walls of either the mesonephric or pronephric excretory organs, but that in addition a separate origin from the sympathetic nervous system must {425}be ascribed to the medullary part of the organ and to the separate paired organs in the Elasmobranchs, which are equivalent to the medullary part in other cases.

The evidence, then, of the transformation of the known vertebrate excretory organs--the pronephros and the mesonephros--leads to the conclusion that in our search for the missing coxal glands of the meso- and pro-somatic regions, we must look for either lymphatic glands, or ductless glands of distinct importance to the body. I have already considered the question in the prosomatic region, and have given my reasons why the pituitary gland must be looked upon as the descendant of the arthropod coxal gland. In this case also the resulting ductless gland is still of functional importance, for disease of it is associated with acromegaly. If, as is possible, it is homologous with the Ascidian hypophysial gland, then it is confirmatory evidence that this latter is said by Julin to be an altered nephridial organ.

Finally, I come to the mesosomatic or branchial region; and here, strikingly enough, we find a perfectly segmental glandular organ of mysterious origin--the thymus gland--segmental with the branchiæ, not necessarily with the myotomes, belonging, therefore, to the appendicular system; and since the branchiæ represent, according to my theory, the basal part of the appendage, such segmental glands would be in the position of coxal glands. Here, then, in the thymus may be the missing mesosomatic coxal glands.

What, then, is the thymus?

The answer to this question has been given recently by Beard, who strongly confirms Kölliker's original view that the thymus is a gland for the manufacture of leucocytes, and that such leucocytes are directly derived from the epithelial cells of the thymus. Kölliker also further pointed out that the blood of the embryo is for a certain period destitute of leucocytes. Beard confirms this last statement, and says that up to a certain stage (varying from 10 to 16 mm. in length of the embryo) the embryos of _Raja batis_ have no leucocytes in the blood or elsewhere. Up to this period the thymus-placode is well formed, and the first leucocytes can be seen to be formed in it from its epithelial cells; then such formation takes place with great rapidity, and soon an enormous discharge of leucocytes occurs from the thymus into the tissue-spaces and blood. He therefore concludes that all lymphoid tissues in the body arise originally from the thymus gland, _i.e._ from leucocytes discharged from the thymus.

{426}The segmental branchial glands, known by the name of thymus, are, according to this view, the original lymphatic glands of the vertebrate; and it is to be noted that, in fishes and in Amphibia, lymphatic glands, such as we know them in the higher mammals, do not exist; they are characteristic of the higher stages of vertebrate evolution. In the lower vertebrates, the only glandular masses apart from the cell-lining of the body-cavity itself, which give rise to leucocyte-forming tissue, are these segmental branchial glands, or possibly also the modified post-branchial segmental glands, known as the head-kidney in Teleostea, etc.

The importance ascribed by Beard to the thymus in the formation of leucocytes in the lowest vertebrates would be considerably reduced in value if the branchial region of Ammocoetes possessed neither thymus glands nor anything equivalent to them. Such, however, is not the case. Schaffer has shown that in the young Ammocoetes masses of lymphatic glandular tissue are found segmentally arranged in the neighbourhood of each gill-slit--tissue which soon becomes converted into a swarming mass of leucocytes, and shows by its staining, etc., how different it is from a blood-space. The presence of this thymus leucocyte-forming tissue, as described by Schaffer, is confirmed by Beard, and I myself have seen the same thing in my youngest specimen of Ammocoetes.

Further, the very methods by which Kowalewsky has brought to light the segmental lymph-glands of the branchial region of the Crustacea, etc., are the same as those by which Weiss discovered the branchial nephric glands in Amphioxus--excretory organs which Boveri considers to represent the pronephros of the Craniota. In this supposition Boveri is right, in so far that both pronephros and the tubules in Amphioxus belong to the same system of excretory organs; but I entirely agree with van Wijhe that the region in Amphioxus is wrong. The tubules in Amphioxus ought to be represented in the branchial region of the Craniota, not in the post-branchial region; van Wijhe therefore suggests that further researches may homologize them with the thymus gland in the Craniota, not with the pronephros. This suggestion of van Wijhe appears to me a remarkably good one, especially in view of the position of the thymus glands in Ammocoetes and the nephric branchial glands in Amphioxus. If, as I have pointed out, the atrial cavity of Amphioxus has been closed in Ammocoetes by the apposition of {427}the pleural fold with the branchial body-surface, then the remains of the position of the atrial chamber must exist in Ammocoetes as that extraordinary space between the somatic muscles and the branchial basket-work filled with blood-spaces and modified muco-cartilage. It is in this very space, close against the gill-slits, that the thymus glands of Ammocoetes are found, in the very place where the nephric tubules of Amphioxus would be found if its atrial cavity were closed completely. Instead, therefore, of considering with Boveri that the branchial nephric tubules of Amphioxus still exist in the Craniota as the pronephros, and that the atrial chamber has narrowed down to the pronephric duct, I would agree with van Wijhe that the pronephros is post-branchial, and suggest that by the complete closure of the atrial space in the branchial region the branchial nephric tubules have lost all external opening, and consequently, as in all other cases, have changed into lymphatic tissue and become the segmental thymus glands.

As van Wijhe himself remarks, the time is hardly ripe for making any positive statement about the relationship between the thymus gland and branchial excretory organs. There is at present not sufficient consensus of opinion to enable us to speak with any certainty on the subject, yet there is so much suggestiveness in the various statements of different authors as to make it worth while to consider the question briefly.

On the one hand, thymus, tonsils, parathyroids, epithelial cell-nests, and parathymus, are all stated to be derivatives of the epithelium lining the gill-slits, and Maurer would draw a distinction between the organs derived from the dorsal side of the gill-cleft and those derived from the ventral side--the former being thymus, the latter forming the epithelial cell-nests, _i.e._ parathyroids. The thymus in Ammocoetes, according to Schaffer, lies both ventral and dorsal to the gill-cleft; Maurer thinks that only the dorsal part corresponds to the thymus, the ventral part corresponding to the parathyroids, etc. Structurally, the thymus, parathyroids, and the epithelial cell-nests are remarkably similar, so that the evidence appears to point to the conclusion that, in the neighbourhood of the gill-slits, segmentally arranged organs of a lymphatic character are situated, which give origin to the thymus, parathyroids, tonsils, etc. Now, among these organs, _i.e._ among those ventrally situated, Maurer places the carotid gland, so that, if he is right, the origin of the carotid gland {428}might be expected to help in the elucidation of the origin of the thymus.

The origin of the carotid gland has been investigated recently by Kohn, who finds that it is associated with the sympathetic nervous system in the same way as the suprarenals. He desires, in fact, to make a separate category for such nerve-glands, or paraganglia, as he calls them, and considers them all to be derivatives of the sympathetic nervous system, and to have nothing to do with excretory organs. The carotid gland is, according to him, the foremost of the suprarenal masses in the Elasmobranchs, viz. the so-called axillary heart.

In my opinion, nests of sympathetic ganglion-cells necessarily mean the supply of efferent fibres to some organ, for all such ganglia are efferent, and also, if they are found in the organ, would have been brought into it by way of the blood-vessels supplying the organ, so that Aichel's statement of the origin of the suprarenals in the Elasmobranchs seems to me much more probable than a derivation from nerve-cells. If, then, it prove that Aichel is right as to the origin of the suprarenals, and Kohn is right in classifying the carotid gland with the suprarenals, then Maurer's statements would bring the parathyroids, thymus, etc., into line with the adrenals, and suggest that they represent the segmented glandular excretory organs of the branchial region, into which, just as in the interrenals of Elasmobranchs, or the cortical part of the adrenals of the higher vertebrates, there has been no invasion of sympathetic ganglion-cells.

Wheeler makes a most suggestive remark in his paper on Petromyzon: he thinks he has obtained evidence of serial homologues of the pronephric tubules in the branchial region of Ammocoetes, but has not been able up to the present to follow them out. If what he thinks to be serial homologues of the pronephric tubules in the branchial region should prove to be the origin of the thymus glands of Schaffer, then van Wijhe's suggestion that the thymus represents the excretory organs of the branchial region would gain enormously in probability. Until some such further investigation has been undertaken, I can only say that it seems to me most likely that the thymus, etc., represent the lymphatic branchial glands of the Crustacea, and therefore represent the missing coxal glands of the branchial region.

This, however, is not all, for the appendages of the mesosomatic region, as I have shown, do not all bear branchiæ; the foremost or {429}opercular appendage carries the thyroid gland. Again, the basal part of the appendage is all that is left; the thyroid gland is in position a coxal gland. It ought, therefore, to represent the coxal gland of this appendage, just as the thymus, tonsils, etc., represent the coxal glands of the rest of the mesosomatic appendages. In the thyroid gland we again see a ductless gland of immense importance to the economy, not a useless organ, but one, like the other modified coxal glands, whose removal involves far-reaching vital consequences. Such a gland, on my theory, was in the arthropod a part of the external genital ducts which opened on the basal joint of the operculum. What, then, is the opinion of morphologists as to the meaning of these external genital ducts?

In a note to Gulland's paper on the coxal glands of Limulus, Lankester states that the conversion of an externally-opening tubular gland (coxal gland) into a ductless gland is the same kind of thing as the history of the development of the suprarenal from a modified portion of mesonephros, as given by Weldon. Further, that in other arthropods with glands of a tubular character opening to the exterior at the base of the appendages, we also have coxal nephridia, such as the shell-glands of the Entomostraca, green glands of Crustacea (antennary coxal gland); and further on he writes: "When once the notion is admitted that ducts opening at the base of limbs in the Arthropoda are possibly and even probably modified nephridia, we immediately conceive the hypothesis that the genital ducts of the Arthropoda are modified nephridia."

So, also, Korschelt and Heider, in their general summing up on the Arthropoda, say: "In Peripatus, where the nephridia appear, as in the Annelida, in all the trunk-segments, a considerable portion of the primitive segments is directly utilized for the formation of the nephridia. In the other groups, the whole question of the rise of the organs known as nephridia is still undecided, but it may be mentioned as very probable that the salivary and anal glands of Peripatus, the antennal and shell-glands of the Crustacea, the coxal glands of Limulus and the Arachnida, as well as the efferent genital ducts, are derived from nephridia, and in any case are mesodermal in origin."

The necessary corollary to this exceedingly probable argument is that glandular structures such as the uterine glands of the scorpion already described, which are found in connection with these terminal {430}genital ducts, may be classed as modified nephridial glands, and that therefore the thyroid gland of Ammocoetes, which, on the theory of this book, arose in connection with the opercular genital ducts of the palæostracan ancestor, represents the coxal glands of this fused pair of appendages. Such a gland, although its function in connection with the genital organs had long disappeared, still, in virtue of its original excretory function, persisted, and even in the higher vertebrates, after it had lost all semblance of its former structure and become a ductless gland of an apparently rudimentary nature, still, by its excretory function, demonstrates its vital importance even to the highest vertebrate.

By this simple explanation we see how these hitherto mysterious ductless glands, pituitary, thymus, tonsils, thyroid, are all accounted for, are all members of a common stock--coxal glands--which originally, as in Peripatus, excreted at the base of the prosomatic and mesosomatic appendages, and are still retained because of the importance of their excretory function, although ductless owing to the modification of their original appendages.

Finally, there is yet another organ in the vertebrate which follows the same law of the conversion of an excretory organ into a lymphatic organ when its connection with the exterior is obliterated, and that is the vertebrate body-cavity itself. According to the scheme here put forth, the body-cavity of the vertebrate arose by the fusion of a ventral prolongation of the original nephrocoele on each side; prolongations which accompanied the formation of the new ventral midgut, and by their fusion formed originally a pair of cavities along the whole length of the abdomen, being separated from each other by the ventral mesentery of the gut. Subsequently, by the ventral fusion of these two cavities, the body-cavity of the adult vertebrate was formed.

This is simply a statement of the known method of formation of the body-cavity in the embryo, and its phylogenetic explanation is that the body-cavity of the vertebrate must be looked upon as a ventral prolongation of the original ancestral body-cavity. Embryology clearly teaches that the original body-cavity or somite was confined to the region of the notochord and central nervous system, and there, just as in Peripatus, was divisible into a dorsal part, giving origin to the myocoele, and a ventral part, forming the nephrocoele. From this original nephrocoele are formed the pronephric excretory organs, the mesonephric excretory organs, and the body-cavity.

{431}That the vertebrate body-cavity was originally a nephrocoele is generally accepted, and its excretory function is shown by the fact that it communicates with the exterior in all the lower vertebrates, either through abdominal pores or by way of nephridial funnels. Bles has shown how largely these two methods of communicating with the exterior mutually exclude each other. In the higher vertebrates both channels become closed, except in the case of the Fallopian tubes, and thus, so to speak, the body-cavity becomes a ductless gland, still, however, with an excretory function, but now, as in all other cases, forming a part of the lymphatic rather than of the true excretory system.

SUMMARY.

The consideration of the formation of the vertebrate cranial region, as
set forth in previous chapters, indicates that the ancestor of the
vertebrates was not an arachnid purely or a crustacean purely, but
possessed partly crustacean and partly arachnid characters. In order to
express this conclusion, I have used the term Protostraca, invented by
Korschelt and Heider, to indicate a primitive arthropod group, from which
both arachnids and crustaceans may be supposed to have arisen, and have
therefore stated that the vertebrate did not arise directly from the
annelids, but from the Protostraca. Such an origin signifies that the
origin of the excretory organs of the vertebrate must not be looked for
in the segmental organs of the annelid, but rather in such modified
annelid organs as would naturally exist in a primitive arthropod group.
The nature of such organs may be inferred, owing to the fortunate
circumstance that so primitive an arthropod as Peripatus still exists,
and we may conclude that the protostracan ancestor possessed in every
segment a pair of appendages and a pair of coelomic cavities, which
extended into the base of these appendages. The ventral portion of each
of these coelomic cavities separated off from the dorsal and formed a
nephrocoele, giving origin to a segmental excretory organ, which, seeing
that its end-vesicle was in the base of the appendage, and seeing also
the nature of the known arachnid and crustacean excretory organs, may
fitly be termed a coxal gland. This, then, is the working hypothesis to
explain the difficulties connected with the origin of the pronephros and
mesonephros--that the original segmental organs were coxal glands, and
therefore indicated the presence of appendages. This hypothesis leads to
the following conclusions:--

1. The coxal glands belonging to the post-branchial appendages of the
invertebrate ancestor are represented by the pronephric tubules, and
existed over the whole metasomatic region.

2. Such glands discharged into a common duct--the pronephric duct--which
opened into the cloacal region, either in the protostracan stage, when
the metasomatic appendages were still in existence, just as the coxal
glands of the prosomatic region in Limulus discharge into a common duct,
or else the pronephric duct was formed when the appendages were
obliterated.

{432}3. The metasomatic appendages disappeared owing to their enclosure
by pleural folds, which, meeting in the mid-ventral line, not only caused
the obliteration of the appendages, and gave a smooth fish-like
body-surface to the animal, but also caused the formation of an atrial
cavity.

4. Into these pleural folds the dorsal longitudinal muscles of the body
extended, and ultimately reached to the ventral surface, thus forming the
somatic muscles of the vertebrate body.

5. When the pleural folds had met in the mid-ventral line the animal had
become a vertebrate, and was dependent for its locomotion on the
movements of these somatic muscles, and not on the movements of
appendages. Consequently, elongation of the trunk-region took place, for
the purpose of increasing mobility, by the formation of new metameres.

6. Each of such metameres possessed its own segmental excretory organ,
formed in the same way as the previous pronephric organs, but, as there
were no appendages in these new-formed segments, the excretory organs
took on the characters of a mesonephros, not a pronephros, and opened
into the pronephric duct, because the direct way to the exterior was
blocked by the enveloping pleural folds.

7. The group of annelids from which the protostracan ancestor of the
vertebrates arose was the highest annelidan group, viz. the Polychæta, as
shown by the nature of the excretory organs in Amphioxus.

8. The coxal glands of the protostracan ancestor existed on all the
segments, and were, therefore, divisible into three groups, prosomatic,
mesosomatic, and metasomatic; these three groups of coxal glands still
exist in the vertebrate as ductless glands.

9. The prosomatic coxal glands form the pituitary body.

10. The mesosomatic coxal glands form the thymus, thyroid, parathyroids,
tonsils, etc.

11. The metasomatic coxal glands form the adrenals.

12. The procoelom of the vertebrate is the procoelom of the protostracan
ancestor, which splits into a dorsal part, the myocoele, and a ventral
part, the nephrocoele. This latter part not only forms the pronephros and
mesonephros, but also by a ventral extension gives origin to the walls of
the vertebrate body-cavity or metacoele.

13. This ventral extension of the original nephrocoele at first excreted
to the exterior, through abdominal pores, or through peritoneal funnels.
When such paths to the exterior became closed, it also became a ductless
gland, belonging to the lymphatic system.

{433}CHAPTER XIII

_THE NOTOCHORD AND ALIMENTARY CANAL_

Relationship between notochord and gut.--Position of unsegmented tube of
notochord.--Origin of notochord from a median groove.--Its function as an
accessory digestive tube.--Formation of notochordal tissue in
invertebrates from closed portions of the digestive tube.--Digestive
power of the skin of Ammocoetes.--Formation of new gut in Ammocoetes at
transformation.--Innervation of the vertebrate gut.--The three outflows
of efferent nerves belonging to the organic system.--The original close
contiguity of the respiratory chamber to the cloaca.--The elongation of
the gut.--Conclusion.

In the previous chapters all the important organs of the arthropod have been found in the vertebrate in their appropriate place, of similar structure, and innervated from corresponding parts of the central nervous system. Such comparison is possible only as long as the ventral and dorsal surfaces of the vertebrate correspond with the respective surfaces of the arthropod, and no reversal is assumed. This method of comparative anatomy is the surest and most certain guide to the relationship between two animals, and when the facts obtained by the anatomical method are so strikingly confirmatory of the palæontological evidence, the combined evidence becomes so strong as to amount almost to a certainty that vertebrates did arise from arthropods in the manner mapped out in previous chapters, and not from a hypothetical group of animals, such as is postulated in the theory of their origin from forms like Balanoglossus.

The latter theory derives the alimentary canal of the vertebrate from that of the invertebrate, and finds in the latter the commencement of the notochord. In the comparison which I have made the alimentary canal of the invertebrate ancestor has become the tube of the central nervous system of the vertebrate, and there is no sign of a notochord whatever. All the organs of the arthropod have already been allocated; where the notochord is situated in the {434}vertebrate there is nothing but a gap in the invertebrate, but the position of that gap can be settled with great accuracy from the previous comparison of organs in the two groups. So, also, the alimentary canal of the vertebrate is from the very nature of the case a new organ, yet, as has been shown in Chapter V., the comparison of the respiratory organs in the two groups gives a strong suggestion of the manner in which such a canal was formed.

THE ORIGIN OF THE NOTOCHORD.

The time has now come to endeavour to frame a plausible theory of the method of formation of the notochord and the new alimentary canal, and thus to complete the diagram on p. 413. The comparative method is no longer available, for these structures are both unrepresented as such in the arthropod; any suggested explanation, therefore, must be more tentative, and cannot give the same feeling of certainty as is the case with all the organs already considered. Our only chance of finding out the past history of the notochord lies in the embryological method, in the hope that, according to the 'law of recapitulation,' the ancestral history may be repeated in the ontogeny with sufficient clearness to enable some conclusion to be drawn.

At the outset, one point comes out clearly--the close relationship between the notochord and the vertebrate gut; they are both derived from the same layer, both parts of the same structure. On this point all embryologists are agreed; it is expressed in such statements as, "the notochord, as well as the alimentary canal, is formed from hypoblast"; "the notochord arises as a thickening in the dorsal wall of the alimentary canal." The two structures are so closely connected together that they must be considered together. If we can conjecture the origin of the one, we may be sure that we have the clue to the origin of the other. The two together form the one new organ which distinguishes the vertebrate from the arthropod, the only thing left which requires explanation for the completion of this strange history.

What, then, is the notochord? What are its characteristics? In the highest vertebrates it is conspicuous only in the embryo; with the development of the axial skeleton it is more and more squeezed out of existence, until in the adult it is no longer visible. By the 'law of recapitulation' this developmental history implies that, as we descend the vertebrate phylum, the notochord ought to be more and {435}more conspicuous, more and more permanent during the life of the animal. Such is, indeed, found to be the case, until at last, in the lowest vertebrates, such as the lamprey, and in forms like Amphioxus, the notochord persists throughout the life of the animal as a large important axial supporting rod.