However many may have been the original number of segments belonging to the spinal region, one thing is certain—the segmental character of this region is remarkably clearly shown, not only by the presence of the segmental spinal nerves, but also by the marked segmentation of the mesoblastic structures. The question, therefore, that requires elucidation above all others is the origin of the spinal mesoblastic segments,i.e.of the cœlomic cavities of the trunk-region, and the structures derived from their walls.
Proceeding on the same lines as in the case of the cranial segments, it is necessary in the first instance to inquire of the vertebrate itself as to the scope of the problem in this region. In addition to the variability in the number of segments so characteristic of the spinal region, the complete absence in each spinal segment of a lateral root affords another marked difference between the two regions. Here, except, of course, at the junction of the spinal and cranial regions, each segmental nerve arises from two roots only, dorsal and ventral, and these roots are separately sensory and motor, and not mixed in function as was the lateral root of each cranial segment. Now, these lateral roots were originally the nerves supplying the prosomatic and mesosomatic appendages with motor as well as sensory fibres. The absence, therefore, of lateral roots in the spinal region implies that in the vertebrate none of the musculature belonging to the metasomatic appendages has remained. Consequently, as far as muscles are concerned, the clue to the origin of the spinal segments must be sought for in the segmentation of the body-muscles.
Here, in contradistinction to the cranial region, the segmentation is most marked, for the somatic spinal musculature of all vertebrates can be traced back to a simple sheet of longitudinal ventral and dorsal muscles, such as are seen in all fishes. This sheet is split into segments or myotomes by transverse connective tissue septa or myo-commata; each myotome corresponding to one spinal segment.
In addition to the evidence of segmentation afforded by the body-musculature in all the higher vertebrates, similar evidence is given by the segmental arrangement of parts of the supporting tissue to form vertebræ. Such segments have received the name of sclerotomes, and each sclerotome corresponds to one spinal segment.
Yet another marked peculiarity of this region is the segmental arrangement of the excretory organs. Just as our body-musculaturehas arisen from the uniformly segmented simple longitudinal musculature of the lowest fish, so, as we pass down the vertebrate phylum, we find more and more of a uniform segmental arrangement in the excretory organs.
The origin of all these three separate segmentations may, in accordance with the phraseology of the day, be included in the one term—the origin of the spinal mesoblastic segments—i.e.of the cœlomic cavities of the trunk-region and the structures derived from their walls.
The Origin of the Segmental Excretory Organs.
Of these three clues to the past history of the spinal region, the segmentation manifested by the presence of vertebræ is the least important, for in Ammocœtes there is no sign of vertebræ, and their indications only appear at transformation. Especially interesting is the segmentation due to the excretory organs, for the evidence distinctly shows that such excretory organs have steadily shifted more and more posteriorly during the evolution of the vertebrate.
In Limulus the excretory organs are in the prosomatic region—the coxal glands; these become in the vertebrate the pituitary body.
In Amphioxus the excretory organs are in the mesosomatic region, segmentally arranged with the gills.
In vertebrates the excretory organs are in the metasomatic region posterior to the gills, and are segmentally arranged in this region. Their investigation has demonstrated the existence of three distinct stages in these organs: 1. A series of segmental excretory organs in segments immediately following the branchial segments. This is the oldest of the three sets, and to these organs the name of thepronephrosis given. 2. A second series which extends more posteriorly than the first, overlaps them to an extent which is not yet settled, and takes their place; to them is given the name of themesonephros. 3. A third series continuous with the mesonephric is situated in segments still more posterior, supplants the mesonephros and forms the kidneys of all the higher vertebrates. This forms themetanephros.
These three sets of excretory organs are not exactly alike in their origin, in that the pronephric tubules are formed from a different portion of the cœlomic walls to that from which the meso- andmetanephric tubules are formed, and the former alone gives origin to a duct, which forms the basis for the generative and urinary ducts, and is called thesegmental duct. The mesonephric tubules, called also the Wolffian body, open into this duct.
In order to make the embryology of these excretory organs quite clear, I will make use of van Wijhe's phraseology and also of his illustrations. He terms the whole cœlomic cavity theprocœlom, which is divisible into a ventral unsegmented part, the body-cavity ormetacœlom, and a dorsal segmented part, thesomite. This latter part again is divided into a dorsal part—theepimere—and a part connecting the dorsal part with the body-cavity, to which therefore he gives the name ofmesomere.
The cavity of the epimere disappears, and its walls form the muscle and cutis plates of the body. The part which forms the muscles is known as themyotome, which separates off from the mesomere, leaving the latter as a blind sac—themesocœlom—communicating by a narrow passage with the body cavity ormetacœlom. At the same time, from the mesomere is formed thesclerotome, which gives rise to the skeletal tissues of the vertebræ, etc., so that van Wijhe's epimere and mesomere together correspond to the original term, protovertebra, or somite of Balfour; and when the myotome and sclerotome have separated off, there is still left the intermediate cell-mass of Balfour and Sedgwick,i.e.the sac-like mesocœle of van Wijhe, the walls of which give origin to the mesonephrotome ormesonephros. Further, according to van Wijhe, the dorsal part of the unsegmented metacœlom is itself segmented, but not, as in the case of the mesocœle, with respect to both splanchnopleuric and somatopleuric walls. The segmentation is manifest only on the somatopleuric side, and consists of a distinct series of hollow somatopleuric outgrowths, called by himhypomeres, which give rise to thepronephrosand the segmental duct.
Van Wijhe considers that the whole metacœlom was originally segmented, because in the lower vertebrates the segmentation reaches further ventral-wards, so that in Selachia the body-cavity is almost truly segmental. Also in the gill-region of Amphioxus the cavities which are homologous with the body-cavity arise segmentally.
Fig. 156.—Diagrams to illustrate the Development of the Vertebrate Cœlom.(Aftervan Wijhe.)N., central nervous system;Nc., notochord;Ao., aorta;Mg., midgut. A,My., myocœle;Mes., mesocœle;Met., metacœle;Hyp., hypomere (pronephric). B and C,My., myotome;Mes., mesonephros;S.d., segmental duct (pronephric);Met., body cavity.
Fig. 156.—Diagrams to illustrate the Development of the Vertebrate Cœlom.(Aftervan Wijhe.)N., central nervous system;Nc., notochord;Ao., aorta;Mg., midgut. A,My., myocœle;Mes., mesocœle;Met., metacœle;Hyp., hypomere (pronephric). B and C,My., myotome;Mes., mesonephros;S.d., segmental duct (pronephric);Met., body cavity.
Fig. 156.—Diagrams to illustrate the Development of the Vertebrate Cœlom.(Aftervan Wijhe.)
N., central nervous system;Nc., notochord;Ao., aorta;Mg., midgut. A,My., myocœle;Mes., mesocœle;Met., metacœle;Hyp., hypomere (pronephric). B and C,My., myotome;Mes., mesonephros;S.d., segmental duct (pronephric);Met., body cavity.
As is well known, Balfour and Semper were led, from their embryological researches, to compare the nephric organs of vertebrates with those of annelids, and, indeed, the nature of the vertebrate segmental excretory organs has always been the fact which has kept alive the belief in the origin of vertebrates from a segmented annelid. These segmental organs thus compared were the mesonephric tubules, and doubts arose, especially in the mind of Gegenbaur, as to the validity of such a comparison, because the mesonephric tubules did not open to the exterior, but into a duct—the segmental duct—which was an unsegmented structure opening into the cloaca; also because the segmental duct, which was the excretory duct of the pronephros, was formed first, and the mesonephric tubules only opened into it after it was fully formed. Further, the pronephros was said to arise from an outbulging of the somatopleuric mesoblast, which extended over a limited number of metameres, and was not segmental, but continuous. Gegenbaur and others therefore argued that the original prevertebrate excretory organ was the pronephros and its duct, not the mesonephros, from which they concluded that the vertebrate must have been derived from an unsegmented type of animal, and not from the segmented annelid type.
Such a view, however, has no further reason for acceptance, as it was based on wrong premises, for Rückert has shown that the pronephros does arise as a series of segmental nephric tubules, and is not unsegmented. He also has pointed out that in Torpedo the anterior part of the pronephric duct shows indications of being segmented, a statement fully borne out by the researches of Maas on Myxine, who gives the clearest evidence that in this animal the anterior part of the pronephric duct is formed by the fusion of a series of separate ducts, each of which in all probability once opened out separately to the exterior.
Rückert therefore concludes that Balfour and Semper were right in deriving the segmental organs of vertebrates from those of annelids, but that the annelid organs are represented in the vertebrate, not by the mesonephric tubules, but by the pronephric tubules and their ducts, which originally opened separately to the exterior. By the fusion of such tubules the anterior part of the segmental duct was formed, while its posterior part either arose by a later cœnogenetic lengthening, or is the only remnant of a series of pronephric tubules which originally extended the whole length of the body, as suggested also by Maas and Boveri. Rückert therefore supposed that the mesonephric tubules were a secondary set of nephric organs, which were not necessarily directly derived from the annelid nephric organs.
At present, then, Rückert's view is the one most generally accepted—the original annelid nephric organs are represented by the pronephric tubules and the pronephric duct, not by the mesonephric tubules, which are a later formation. This latter statement would hold good if the mesonephric tubules were found entirely in segments posterior to those containing the pronephric tubules; such, however, is said not to be the case, for the two sets of organs are said to overlap in some cases; even when they exist in the same segments, the former are said always to be formed from a more dorsal part of the cœlom than the pronephros, always to be a later formation, and never to give any indication of communicating with the exterior except by way of the pronephric duct.
The recent observations of Brauer on the excretory organs of the Gymnophiona throw great doubt on the existence of mesonephric and pronephric tubules in the same segment. He criticizes the observations on which such statements are based, and concludes that, as in Hypogeophis, the nephrotome which is cut off after the separation of the sclero-myotome gives origin to the pronephros in the more anterior regions, just as it gives origin to the mesonephros in the more posterior regions. In fact, the observations of van Wijhe and others do not in reality show that two excretory organs may be formed in one segment, the one mesonephric from the remains of the mesomere and the other pronephric from the hypomere, but rather that in such cases there is only one organ—the pronephros—part of which is formed from the mesomere and part from the hypomere. Brauer goes further than this, and doubts the validity of any distinction between pronephros and mesonephros, on the ground of the former arising from a more ventral part of the procœlom than the latter; for, as he says, it is only possible to speak of one part of the somite as being more ventral than another part when both parts are in the same segment; so that if pronephric and mesonephric organs are never in the same segment, we cannot say with certainty that the former arises more ventrally than the latter.
These observations of Brauer strongly confirm Sedgwick's original statement that the pronephric and mesonephric organs are homodynamous organs, in that they are both derived from the original serially situated nephric organs, the differences between them being of a subordinate nature and not sufficient to force us to believe that the mesonephros is an organ of quite different origin to thepronephros. So, also, Price, from his investigations of the excretory organs of Bdellostoma, considers that in this animal both pronephros and mesonephros are derived from a common embryonic kidney, to which he gives the nameholonephros.
Brauer also is among those who conclude that the vertebrate excretory organs were derived from those of annelids; he thinks that the original ancestor possessed a series of similar organs over the whole pronephric and mesonephric regions, and that the anterior pronephric organs, which alone form the segmental duct, became modified for a larval existence—that their peculiarities were adaptive rather than ancestral. This last view seems to me very far-fetched, without any sufficient basis for its acceptance. According to the much more probable and reasonable view, the pronephros represents the oldest and original excretory organs, while the mesonephros is a later formation. Brauer's evidence seems to me to signify that the pronephros, mesonephros, and metanephros are all serially homologous, and that the pronephros bears much the same relation to the mesonephros that the mesonephros does to the metanephros. The great distinction of the pronephros is that it, and it alone, forms the segmental duct.
We may sum up the conclusions at which we have now arrived as follows:—
1. The pronephric tubules and the pronephric duct are the oldest part of the excretory system, and are distinctly in evidence for a few segments only in the most anterior part of the trunk-region immediately following the branchial region. They differ also from the mesonephric tubules by not being so clearly segmental with the myotomes.
2. The mesonephric tubules belong to segments posterior to those of the pronephros, are strictly segmental with the myotomes, and open into the pronephric duct.
3. All observers are agreed that the two sets of excretory organs resemble each other in very many respects, as though they arose from the same series of primitive organs, and, according to Sedgwick and Brauer, no distinction of any importance does exist between the two sets of organs. Other observers, however, consider that the pronephric organs, in part at all events, arise from a part of the nephrocœle more ventral than that which gives origin to the mesonephric organs, and that this difference in position of origin, combinedwith the formation of the segmental duct, does constitute a true morphological distinction between the two sets of organs.
4. All the recent observers are in agreement that the vertebrate excretory organs strongly indicate a derivation from the segmental organs of annelids.
The very strongest support has been given to this last conclusion by the recent discoveries of Boveri and Goodrich upon the excretory organs of Amphioxus. According to Boveri, the nephric tubules of Amphioxus open into the dorsal cœlom by one or more funnels. Around each funnel are situated groups of peculiar cells, called by him 'Fadenzellen,' each of which sends a long process across the opening of the funnel. Goodrich has examined these 'Fadenzellen,' and found that they are typical pipe-cells, or solenocytes, such as he has described in the nephridial organs of various members of the annelid group Polychæta. Also, just as in the Polychæta, the ciliated nephric tubule has no internal funnel-shaped opening into the cœlom, but terminates in these groups of solenocytes. "Each solenocyte consists of a cell-body and nucleus situated at the distal free extremity of a delicate tube; the proximal end of the tube pierces the wall of the nephridial canal and opens into its lumen. A single long flagellum arising from the cells works in the tube and projects into the canal."
The exceedingly close resemblance between the organs of Amphioxus and those of Phyllodoce, as given in his paper, is most striking, and, as he says, leads to the conclusion that the excretory organs of Amphioxus are essentially identical with the nephridia of certain polychæte worms.
It is to me most interesting to find that the very group of annelids, the Polychæta, which possess solenocytes so remarkably resembling those of the excretory organs of Amphioxus, are the highest and most developed of all the Annelida. I have argued throughout that the law of evolution consists in the origination of successive forms from the dominant group then alive, dominance signifying the highest type of brain-power achieved up to that time. The highest type among Annelida is found in the Chætopoda; from them, therefore, the original arthropod type must have sprung. This original group of Arthropoda gave rise to the two groups of Crustacea and Arachnida, in my opinion also to the Vertebrata, and, as already mentioned, it is convenient to give it a generalizedname, the Protostraca, from which subsequently the Palæostraca arose.
The similarity between the excretory organs of Amphioxus and those of Phyllodoce suggests that the protostracan ancestor of the vertebrates arose from the highest group of the Chætopoda—the Polychæta. The evidence which I have already given points, however, strongly to the conclusion that the vertebrate did not arise from members of the Protostraca near to the polychæte stock, but rather from members in which the arthropod characters had already become well developed—members, therefore, which were nearer the Trilobita than the Polychæta. Such early arthropods would very probably have retained in part excretory organs of the same character as those found in the original polychæte stock, and thus account for the presence of solenocytes in the excretory organs of Amphioxus.
In connection with such a possibility, I should like to draw attention to the observations of Claus and Spangenberg on the excretory organs of Branchipus—that primitive phyllopod, which is recognized as the nearest approach to the trilobites at present living. According to Claus, an excretory apparatus exists in the neighbourhood of each nerve-ganglion, and Spangenberg finds a perfectly similar organ in the basal segment of each appendage—a system, therefore, of excretory organs as segmentally arranged as those of Peripatus. Claus considers that although these organs formed an excretory system, it is not possible to compare them with the annelid segmental organs, because he thought the cells in question arose from ectoderm. Now, the striking point in the description of the excretory cells in these organs, as described both by Claus and Spangenberg, is that they closely resemble the pipe-cells or solenocytes of Goodrich; each cell possesses a long tube-like projection, which opens on the surface. They appear distinctly to belong to the category of flame-cells, and resemble solenocytes more than anything else. According to Goodrich, the solenocyte is probably an ectodermal cell, so that even if it prove to be the case, as Claus thought, that these pipe-cells of Branchipus are ectodermal, they would still claim to be derived from the segmental organs of annelids, especially of the Polychæta, being, to use Goodrich's nomenclature, true nephridial organs, as opposed to cœlomostomes.
These observations of Claus and Spangenberg suggest not only that the primitive arthropod of the trilobite type possessed segmentalorgans in every segment directly derived from those of a polychæte ancestor, but also that such organs were partly somatic and partly appendicular in position. Such a suggestion is in strict accord with the observations of Sedgwick on the excretory organs of the most primitive arthropod known, viz. Peripatus, where also the excretory organs, which are true segmental organs, are partly somatic and partly appendicular. Further, the excretory organs of the Scorpion and Limulus group are again partly somatic and partly appendicular, receiving the name of coxal glands, because there is a ventral projection of the gland into the coxa of the corresponding appendage.
Judging from all the evidence available, it is probable that when the arthropod stock arose from the annelids, simultaneously with the formation of appendages, the segmental somatic nephric organs of the latter extended ventrally into the appendage, and thus formed a segmental set of excretory organs, which were partly somatic, partly appendicular in position, and might therefore be called coxal glands.
As already stated, all investigators of the origin of the vertebrate excretory organs are unanimous in considering them to be derived from segmental organs of the annelid type. I naturally agree with them, but, in accordance with my theory, would substitute the words "primitive arthropod" for the word "annelid," for all the evidence I have accumulated in the preceding chapters points directly to that conclusion. Further, the most primitive of the three sets of vertebrate segmental organs—the pronephros, mesonephros, and metanephros—is undoubtedly the pronephros; consequently the pronephric tubules are those which I consider to be more directly derived from the coxal glands of the primitive arthropod ancestor. Such a derivation appears to me to afford an explanation of the difficulties connected with the origin of the pronephros and mesonephros respectively, which is more satisfactory than that given by the direct derivation from the annelid.
The only living animal which we know of as at all approaching the most primitive arthropod type is, as pointed out by Korschelt and Heider, Peripatus; and Peripatus, as is well known, possesses a true cœlom and true cœlomic excretory organs in all the segments of the body. Sedgwick shows that at first a true cœlom, as typical as that of the annelids, is formed in each segment of the body, and that then this cœlom (which represents in the vertebrate van Wijhe's pro-cœlom)splits into a dorsal and a ventral part. In the anterior segments of the body the dorsal part disappears (presumably its walls give origin to the mesoblast from which the dorsal body-muscles arise), while the ventral part remains and forms a nephrocœle, giving origin to the excretory organs of the adult. According to von Kennel, the cavity becomes divided into three spaces, which for a time are in communication—a lateral (I.), a median (II.), and a dorso-median (III.). The dorso-median portion becomes partitioned off, and this, as well as the greater part of the lateral portion, which lies principally in the foot, is used up in providing elements for the formation of the body- and appendage-muscles respectively and the connective tissue.
In Fig.157I reproduce von Kennel's diagram of a section across a Peripatus embryo, in which I. represents the lateral appendicular part of the cœlom, II. the ventral somatic part, and III. the dorsal part which separates off from the ventral and lateral parts, and, as its walls give origin largely to the body-muscles, may be called the myocœle. The muscles of the appendages are formed from the ventral part of the original procœlom, just as I have argued is the case with the muscles of the splanchnic segmentation in vertebrates.
Sedgwick states that the ventral part of the cœlom extends into the base of each appendage, and there forms the end-sac of each nephric tubule, into which the nephric funnel opens, thus forming a coxal gland; this end-sac or vesicle in the appendage is called by him the internal vesicle (i.v.), because later another vesicle is formed from the ventral cœlom in the body itself, close against the nerve-cord on each side, which he calls the external vesicle (e.v.). (Cf.Fig.158, taken from Sedgwick.) This second vesicle is, according to him, formed later in the development from the nephric tubule of the internal vesicle, so that it discharges its contents to the exterior by the same opening as the original tubule. Of course, as he points out, the whole system of internal and external vesicles and nephric tubules are all simply derivatives of the original ventral part of the cœlom or nephrocœle.
Fig. 157.—Transverse Section of Peripatus Embryo.(Aftervon Kennel.)Al., alimentary canal;N., nerve-cord;App., appendage;I,II,III, the three divisions (lateral, median, and dorso-median) of the cœlom.
Fig. 157.—Transverse Section of Peripatus Embryo.(Aftervon Kennel.)Al., alimentary canal;N., nerve-cord;App., appendage;I,II,III, the three divisions (lateral, median, and dorso-median) of the cœlom.
Fig. 157.—Transverse Section of Peripatus Embryo.(Aftervon Kennel.)
Al., alimentary canal;N., nerve-cord;App., appendage;I,II,III, the three divisions (lateral, median, and dorso-median) of the cœlom.
Fig. 158.—Section of Peripatus.(AfterSedgwick.)Al., alimentary canal;N., nerve-cord;App., appendage;i.v., internal, ande.v., external vesicles of the segmented excretory tubule (coxal gland).
Fig. 158.—Section of Peripatus.(AfterSedgwick.)Al., alimentary canal;N., nerve-cord;App., appendage;i.v., internal, ande.v., external vesicles of the segmented excretory tubule (coxal gland).
Fig. 158.—Section of Peripatus.(AfterSedgwick.)
Al., alimentary canal;N., nerve-cord;App., appendage;i.v., internal, ande.v., external vesicles of the segmented excretory tubule (coxal gland).
Here, then, in Peripatus, and presumably, therefore, in members of the Protostraca, we see that the original segmental organs of the annelid have become a series of nephric organs, which extended into the base of the appendages, and may therefore be called coxal glands; also it is clear, from Sedgwick's description, that if the appendages disappeared, the nephric organs would still remain, not as coxal glands, but as purely somatic excretory glands. They would still be homologous with the annelid segmental organs, or with the coxal glands, but would arisein totofrom a part of the ventral cœlom or nephrocœle, more dorsal than the former appendicular part, because the appendages and their enclosed cœlom are always situated ventrally to the body. Again, according to Sedgwick, the nephric tubules are connected with two cœlomic vesicles, the one in the appendage the internal vesicle, and the other, the so-called bladder, or the external vesicle, in the body itself, close against the nerve-cord. Sedgwick appears to consider that either of these vesicles may form the end-sac of a nephric tubule, for he discusses the question whether the single vesicle, which in each case gives origin to the nephridia of the first three legs, corresponds to the internal or external vesicle. Hedecides, it is true, in favour of the internal vesicle, and therefore considers the excretory organ to be appendicular,i.e.a coxal gland, in these segments as well as in those more posterior. Still, the very discussion shows that in his opinion, at all events, the external vesicle might represent the end-sac of the tubule, in the absence of the internal or appendicular vesicle.
Such an arrangement as Sedgwick describes in Peripatus is the very condition required to give rise to the pronephric and mesonephric tubules, as deduced by me from the consideration of the vertebrate, and harmonizes and clears up the controversy about the mesonephros and pronephros in the most satisfactory manner. Both pronephros and mesonephros are seen to be derivatives of the original annelid segmental organs, not directly from an annelid, but by way of an arthropodan ancestor; the difference between the two is simply that the pronephric organs were coxal glands, and indicate, therefore, the presence of the original metasomatic appendages, while the mesonephric organs were homologous organs, formed in segments of later origin which had lost their appendages. For this reason the pronephros is said to be formed, in part at least, from a portion of the cœlom situated more ventrally than the purely somatic part which gives rise to the mesonephros. For this reason Sedgwick, Brauer, etc., can say that the mesonephros is strictly homodynamous with the pronephros; while equally Rückert, Semon, and van Wijhe can say it is not homodynamous, in so far that the two organs are not derived strictly from absolutely homologous parts of the cœlom. For this reason Semon can speak of the mesonephros as a dorsal derivative of the pronephros, just as Sedgwick says that the external or somatic vesicle of Peripatus is a derivative of the appendicular nephric organ. For this reason the pronephros, or rather a part of it, is always derived from the somatopleuric layer, for, as is clear from Miss Sheldon's drawing, the part of the cœlom in Peripatus which dips into the appendage is derived from the somatopleuric layer alone.
Such a cœlom as that of Peripatus, Fig.157, would represent the origin of the vertebrate cœlom, and would therefore represent the procœlom of van Wijhe. In strict accordance with this, we see that it separates into a dorsal part, the walls of which give origin to the somatic muscles, or at all events to the great longitudinal dorsal muscles of the animal, and a ventral part, which forms a nephrocœle,dips into the appendage, and gives origin to the muscles of the appendage. In the vertebrate, after the somatic dorsal part or myocœle has separated off, a ventral part is left, which forms a nephrocœle in the trunk-region, and gives origin to the splanchnic striated muscles in the cranial region,i.e.to the muscles which, according to my theory, were once appendicular muscles. This ventral nephrocœlic part is divisible in the trunk into a segmented part, which forms the excretory organs proper, and an unsegmented part, the metacœle or true body-cavity of the vertebrate.
This comparison of the procœlom of the vertebrate and arthropod signifies that the vertebrate metacœle was directly derived by ventral downgrowth from the arthropod nephrocœle, so that if, as I suppose, the vertebrate nervous system represents the conjoined nervous system and alimentary canal of the arthropod, then the vertebrate metacœle, or body-cavity, must have been originally confined to the region on each side of the central nervous system, and from this position have spread ventrally, to enclose ultimately the new-formed vertebrate gut. This means that the body-cavity (metacœle) of the vertebrate is not the same as the body-cavity of the annelid, but corresponds to a ventral extension of the nephrocœle, or ventral part of such body-cavity.
Such a phylogenetic history is most probable, because it explains most naturally and simply the facts of the development of the vertebrate body-cavity; for the mesoblast always originates in the neighbourhood of the notochord and central nervous system, and the lumen of the body-cavity always appears first in that region, and then extends laterally and ventrally on each side until it reaches the most ventral surface of the embryo, thus forming a ventral mesentery, which ultimately disappears, and the body-cavity surrounds the gut, except for the dorsal mesentery. Thus Shipley, in his description of the formation of the mesoblastic plates which line the body-cavity in Ammocœtes, describes them as commencing in two bands of mesoblast situated on each side, close against the commencing nervous system:—
"These two bands are separated dorsally by the juxtaposition of the dorsal wall of the mesenteron and the epiblast, and ventrally by the hypoblastic yolk-cells which are in contact with the epiblast over two-thirds of the embryo. Subsequently, but at a much later date, the mesoblast is completed ventrally by the downgrowth oneach side of these mesoblastic plates. The subsequent downward growth is brought about by the cells proliferating along the free ventral edge of the mesoblast, these cells then growing ventralwards, pushing their way between the yoke-cells and epiblast."
The derivation of the vertebrate pronephric segmental organs from the metasomatic coxal glands of a primitive arthropod would mean, if the segmental organs of Peripatus be taken as the type, that such glands opened to the exterior on every segment, either at the base of the appendage or on the appendage itself. It is taken for granted by most observers that the pronephric segmental organs once opened to the exterior on each segment, and then, from some cause or other, ceased to do so, and the separate ducts, by a process of fusion, came to form a single segmental duct, which opened into the cloaca. Many observers have been led to the conclusion that the pronephric duct is epiblastic in origin, although from its position in the adult, it appears far removed from all epiblastic formations. However, at no time in the developmental history is there any clear evidence of actual fusion of any part of the pronephric organ with the epidermis, and the latest observer, Brauer, is strongly of opinion that there is never sufficiently close contact with the epidermis to warrant the statement that the epiblastic cells take part in the formation of the duct. All that can be said is, that the formation of the duct takes place at a time when the pronephric diverticulum is in close propinquity to the epidermis, before the ventral downgrowth of the myotome has taken place.
The formation of the anterior portion of the pronephric duct is, according to Maas in Myxine, and Wheeler in Petromyzon, undoubtedly brought about by the fusion of a number of pronephric tubules, which, according to Maas, are clearly seen in the youngest specimens as separate segmental tubes; each of these tubules is supplied by a capillary network from a segmental branch of the aorta, as in the tubules of Amphioxus according to Boveri, and does not possess a glomerulus.
The posterior part of the duct into which the mesonephric tubules enter possesses also a capillary network, which Maas considers to represent the original capillary network of a series of pronephric tubules, the only remnant of which is the duct into which the mesonephric tubules open. He therefore argues that the pronephric duct indicates a series of pronephric tubules, which originally extendedalong the whole length of the body, and were supplanted by the mesonephric tubules, which also belonged to the same segments.
I also think that the paired appendages which have left the pronephric tubules as signs of their past existence, existed originally, in the invertebrate stage, on every segment of the body. But I do not consider that such a statement is at all equivalent to saying that such pairs of tubules must have existed upon every one of the segments existing at the present day; for it seems to me that Rückert is much more likely to be right when he says that in Selachians the duct clearly does grow back, and is not formed throughoutin situ; so that he gives a double explanation of the formation of the duct—a palingenetic anterior part formed by the fusion of the extremities of the original excretory tubules, to which a posterior cœnogenetic lengthening has been added.
It does not seem to me at all necessary that the immediate invertebrate ancestor of the vertebrate should have possessed excretory organs which opened out separately to the exterior on each segment; a fusion may already have taken place in the invertebrate stage, and so a single duct have been acquired for a number of organs. Such a suggestion has been made by Rückert, because of the fact discovered by Cunningham and E. Meyer, that the segmental organs ofLanice conchilegaare on each side connected together by a single strong longitudinal canal. I would, however, go further than this and say, that even although the nephric organs of the polychæte ancestor opened out on every segment, and although the primitive arthropodan ancestor derived from such polychæte possessed coxal glands which opened out either on to or at the base of each appendage, similarly to those of Peripatus, yet the immediate arthropodan ancestor, with its palæostracan affinities, may already have possessed metasomatic coxal glands, all of which opened into a single duct, with a single opening to the exterior.
Judging from Limulus, such was very probably the case, for Patten and Hazen have shown (1) that the coxal glands of Limulus are segmental organs belonging to the prosomatic segments; (2) that the organs belonging to the cheliceral and ectognathal segments are not developed; (3) that the four glands belonging to the endognaths become connected together by astolon, which communicates with a single nephric duct, opening to the exterior on the basal segment of the 5th prosomatic appendage (the last endognath). Atno time is there any evidence of any separate openings or any fusion with the ectoderm, such as might indicate separate openings of these prosomatic coxal segmental organs. Thus we see that in Limulus, which is presumably much nearer the annelid condition than the vertebrate, all evidence of separate nephric ducts opening to the exterior on each prosomatic segment has entirely disappeared, just as is the case in the metasomatic coxal glands (i.e.the pronephros) of the vertebrate. What is seen in the prosomatic region of Limulus, and doubtless also of the Eurypterids, may very probably have occurred in the metasomatic region of the immediate invertebrate ancestors of the vertebrate, and so account for the single pronephric duct belonging to a number of pronephric organs.
The interpretation of these various embryological investigations may be summed up as follows:—
1. The ancestor of the vertebrates possessed a pair of appendages on each segment; into the base of each of these appendages the segmental excretory organ sent a diverticulum, thus forming a coxal gland.
2. Such coxal glands, even in the invertebrate stage, may have discharged into a common duct which opened to the exterior most posteriorly.
3. Then, from some cause, the appendages were rendered useless, and dwindled away, leaving only the pronephric organs to indicate their former presence. At the end of this stage the animal possessed vertebrate characteristics.
4. For the purpose of increasing mobility, of forming an efficient swimming instead of a crawling animal, the body-segments increased in number, always, as is invariably the case, by the formation of new ones between those already formed and the cloacal region, and so of necessity caused an elongation of the pronephric duct. Into this there now opened the ducts of the segmental organs formed by recapitulation, those, therefore, belonging to the body-segments—mesonephric—having nothing to do with appendages, for the latter had already ceased to exist functionally, and would not, therefore, be repeated with each meristic repetition.
This, so to speak, passive lengthening of the pronephric duct in consequence of the lengthening of the early vertebrate body by the addition of metameres, each of which contained only mesonephric and no pronephric tubules, is, to my mind, an example of a principlewhich has played an important part in the formation of the vertebrate, viz. that the meristic variation by which the spinal region of even the lowest of existing vertebrates has been formed, has largely taken place in the vertebrate phylum itself, and that such changes must be eliminated before we can picture to ourselves the pre-vertebrate condition. As an example, I may mention the remarkable repetition of similar segments pictured by Bashford Dean in Bdellostoma. Such repetition leads to passive lengthening of such parts as are already formed but are not meristically repeated: such are the notochord, the vertebrate intestine, the canal of the spinal cord, and possibly the lateral line nerve. The fuller discussion of this point means the discussion of the formation of the vertebrate alimentary canal; I will therefore leave it until I come to that part of my subject, and only say here that the evidence seems to me to point to the conclusion that at the time when the vertebrate was formed, the respiratory and cloacal regions were very close together, the whole of the metasoma being represented by the region of the pronephros alone.
Here, as always, the evidence of Ammocœtes tends to give definiteness to our conceptions, for Wheeler points out that up to a length of 7 mm. the pronephros only is formed; there is no sign of the more posteriorly formed mesonephros. Now we know, as pointed out in Chapter VI., p.228, this is the time of Kupffer's larval stage of Ammocœtes. This is the period during which the invertebrate stage is indicated in the ontogeny, so that, in accordance with all that has gone before, this means that the metasoma of the invertebrate ancestor was confined to the region of the pronephros.
Again, take Shipley's account of the development of Petromyzon. He says—
"The alimentary canal behind the branchial region may be divided into three sections. Langerhans has termed these the stomach, midgut, and hindgut, but as the most anterior of these is the narrowest part of the whole intestine, it would, perhaps, be better to call it œsophagus. This part of the alimentary canal lies entirely in front of the yolk, and is, with the anterior region, which subsequently bears the gills, raised from the rest of the egg when the head is folded off. It is supported by a dorsal mesentery, on each side of which lies the head-kidney (pronephros)."
Further on he says—
"The hindgut is smaller than the midgut; its anterior limit is marked by the termination of the spiral valve, which does not extend into this region. The two segmental ducts open into it just where it turns ventrally to open to the exterior by a median ventral anus. Its lumen is from an early stage lined with cells which have lost their yolk, and it is in wide communication with the exterior from the first. This condition seems to be, as Scott suggests, connected with the openings of the ducts of the pronephros, for this gland is completed and seems capable of functioning long before any food could find its way through the midgut, or, indeed, before the stomodæum has opened."
Is there no significance in this statement of Shipley? Even if it be possible to find some special reason why the branchial and cloacal parts of the gut are freed from yolk and lined with serviceable epithelium a long time before the midgut, why should a bit of the midgut, which Shipley calls the œsophagus, which is connected with the region of the pronephros and not of the branchiæ, differ so markedly from the rest of the midgut? Surely the reason is that the branchial region of the gut, the pronephric region of the gut, and the cloacal region of the gut, belong to a different and earlier phase in the phylogenetic history of the Ammocœtes than does the midgut between the pronephric and cloacal regions. This observation of Shipley fits in with and emphasizes the view that the original animal from which the vertebrate arose consisted of a cephalic and branchial region, followed by a pronephric and cloacal region; the whole intermediate part of the gut, which forms the midgut, with its large lumen and spiral valve, and belongs to the mesonephric region, being a later formation brought about by the necessity of increasing the length of the body.
The Origin of the Somatic Trunk-Musculature and the Formation of an Atrial Cavity.
Next comes the question, why was the pronephros not repeated in the meristic repetition that took place during the early vertebrate stage? What, in fact, caused the disappearance of the metasomatic appendages, and the formation of the smooth body-surface of the fish?
The embryological evidence given by van Wijhe and others of the manner in which the original superficially situated pronephros isremoved from the surface and caused to assume the deeper position, as seen in the later embryo, is perfectly clear and uniform in all the vertebrate groups. The diagrams at the end of van Wijhe's paper, which I reproduce here, illustrate the process which takes place. At first the myotome (Fig.159, A) is confined to the dorsal region on each side of the spinal cord and notochord. Then (Fig.159, B) it separates from the rest of the somite and commences to extend ventrally, thus covering over the pronephros and its duct, until finally (Fig.159, C) it reaches the mid-ventral line on each side, and the foundations of the great somatic body-muscles are finally laid.
In order, therefore, to understand how the obliteration of the appendages took place, we must first find out what is the past history of the myotomes. Why are they confined at first to the dorsal region of the body, and extend afterwards to the ventral region, forcing by their growth an organ that was originally external in situation to become internal?
In the original discussion at Cambridge, I was accused of violating the important principle that in phylogeny we must look at the most elementary of the animals whose ancestors we seek, and was told that the lowest vertebrate was Amphioxus, not Ammocœtes; that therefore any argument as to the origin of vertebrates must proceed from the consideration of the former and not the latter animal. My reply was then, and is still, that I was considering the cranial region in the first place, and that therefore it was necessary to take the lowest vertebrate which possessed cranial nerves and sense-organs of a distinctly vertebrate character, a criterion evidently not possessed by Amphioxus. Such argument does not apply to the spinal region, so that, now that I have left the cranial region and am considering the spinal, I entirely agree with my critics that Amphioxus is likely to afford valuable help, and ought to be taken into consideration as well as Ammocœtes. The distinction between the value of the spinal (including respiratory) and cranial regions of Amphioxus for drawing phylogenetic conclusions is recognized by Boveri, who says that, in his opinion, "Amphioxus shows simplicity and undifferentiation rather than degeneration. If truly Amphioxus is somewhat degenerated, then it is so in its prehensile and masticatory apparatus, its sense organs, and perhaps its locomotor organs, owing to its method of living."