Illustration: Figure 185Fig. 185. Section through the trunk of a Scyllium embryo slightly younger than 28 F.sp.c.spinal canal;W.white matter of spinal cord;pr.posterior nerve-roots;ch.notochord;x.subnotochordal rod;ao.aorta;mp.muscle-plate;mp´.inner layer of muscle-plate already converted into muscles;Vr.rudiment of vertebral body;st.segmental tube;sd.segmental duct;sp.v.spiral valve;v.subintestinal vein;p.o.primitive generative cells.
Fig. 185. Section through the trunk of a Scyllium embryo slightly younger than 28 F.sp.c.spinal canal;W.white matter of spinal cord;pr.posterior nerve-roots;ch.notochord;x.subnotochordal rod;ao.aorta;mp.muscle-plate;mp´.inner layer of muscle-plate already converted into muscles;Vr.rudiment of vertebral body;st.segmental tube;sd.segmental duct;sp.v.spiral valve;v.subintestinal vein;p.o.primitive generative cells.
It is difficult to understand how the body cavity could thus extend into the muscle-plates on the supposition that it represents a primitive split in the mesoblast between the wall of the gut and the body-wall; but its extension to this part is quite intelligible, on the hypothesis that it represents the cavities of two diverticula of the alimentary tract, from the muscular walls of which the voluntary muscular system has been derived; and it may be pointed out that the derivation of part of the muscular system from what is apparently splanchnic mesoblast is easily explained on the above hypothesis, but not, so far as I see, on any other.
Such are the main features, presented by the mesoblast in Elasmobranchii, which favour the view of its having originally formed the walls of the alimentary diverticula. Against this view of its nature are the facts (1) of the mesoblast plates being at first solid, and (2) of the body cavity as a consequence of this never communicating with the alimentary canal. These points, in view of our knowledge of embryological modifications, cannot be regarded as great difficulties in my hypothesis. We have many examples of organs, which, though in most cases arising as involutions, yet appear in other cases as solid ingrowths. Such examples are afforded by the optic vesicle, auditory vesicle, and probably also by the central nervous system of Osseous Fishes. In most Vertebrates these organs are formed as hollow involutions from the exterior; in Osseous Fishes, however, as solid involutions, in which a cavity is secondarily established.
Illustration: Figure 186Fig. 186. Horizontal section through the trunk of an embryo of Scyllium considerably younger than 28 F.The section is taken at the level of the notochord, and shews the separation of the cells to form the vertebral bodies from the muscle-plates.ch.notochord;ep.epiblast;Vr.rudiment of vertebral body;mp.muscle-plate;mp´.portion of muscle-plate already differentiated into longitudinal muscles.
Fig. 186. Horizontal section through the trunk of an embryo of Scyllium considerably younger than 28 F.The section is taken at the level of the notochord, and shews the separation of the cells to form the vertebral bodies from the muscle-plates.ch.notochord;ep.epiblast;Vr.rudiment of vertebral body;mp.muscle-plate;mp´.portion of muscle-plate already differentiated into longitudinal muscles.
There are strong grounds for thinking that in all Vertebrates the mesoblast plates on each side of the notochord originate independently, much as in Elasmobranchii, and that the notochord is derived from the axial hypoblast; but there are some difficulties in the application of this general statement to all cases. In Amphibia, Ganoids, and Petromyzon, where the dorsal hypoblast is formed by a process of invagination as in Amphioxus, the dorsal mesoblast also owes its origin to this invagination, in that the indifferent invaginated layer becomes divided into hypoblast and mesoblast. Amongst these forms the mesoblast sheet, when separated from the hypoblast, is certainlynotcontinuous across the middle line in Petromyzon (Calberla) and the Newt (Scott and Osborn), and doubtfully soin the other forms. It arises, in fact, as in Elasmobranchii, as two independent plates. The fact of these plates originating from an invaginated layer can only be regarded in the light of an approximation to the primitive type found in Amphioxus.
In Petromyzon and the Newt the whole axial plate of dorsal hypoblast becomes separated off from the rest of the hypoblast as the notochord, and this mode of origin for the notochord resembles more closely that in Amphioxus than the mode of origin in Elasmobranchii.
In Teleostei, there is reason to think that the processes in the formation of the mesoblast accord closely with what has been described as typical for the Ichthyopsida, but there are still some points involved in obscurity.
Leaving the Ichthyopsida, we may pass to the consideration of the Sauropsida and Mammalia. In both of these types there is evidence to shew that a part of the mesoblast is formedin situat the same time as the hypoblast, from the lower strata of segmentation spheres. This mesoblast is absent in the front part of the area pellucida, and on the formation of the primitive streak (blastopore), an outgrowth of mesoblast arises from it as in Amphibia, etc. From this region the mesoblast spreads as a continuous sheet to the sides and posterior part of the blastoderm. In the region of the embryo, its exact behaviour has not in some cases been quite satisfactorily made out. There are reasons for thinking that it appears as two sheetsnot unitedin the axial line in both Lacertilia (fig. 126) and Mammalia (fig. 187), and this to some extent holds true for Aves (videp. 156). In Lacertilia (fig. 188) and Mammalia, the axial hypoblast becomes wholly converted into the notochord, which at the posterior end of the body is continued into the epiblast at the dorsal lip of the blastopore; while in Birds the notochord is formed by a very similar (fig. 189ch) process.
Illustration: Figure 187Fig. 187. Transverse section through an embryo Rabbit of eight days.ep.epiblast;me.mesoblast;hy.hypoblast;mg.medullary groove.
Fig. 187. Transverse section through an embryo Rabbit of eight days.ep.epiblast;me.mesoblast;hy.hypoblast;mg.medullary groove.
The above processes in the formation of the mesoblast are for the most part easily explained by a comparison with the lower types. The outgrowth of the mesoblast from the sides of the primitive streak is a rudiment of the dorsal invagination of hypoblast and mesoblast found in Amphibia; and the apparent outgrowth of the mesoblast from the epiblast in the primitive streak is no more to be taken as a proof of the epiblastic origin of the mesoblast, than the continuity of the epiblast with the invaginated hypoblast and mesoblast at the lips of the blastopore in the Frog of the derivation of these layers from the epiblast in this type.
Illustration: Figure 188Fig. 188. Diagrammatic longitudinal section through an embryo Lizard to shew the relations of the neurenteric canal(ne)and of the primitive streak(pr).am.amnion;ep.epiblast;hy.hypoblast;ch.notochord;pp.body cavity;ne.neurenteric canal;pr.primitive streak.
Fig. 188. Diagrammatic longitudinal section through an embryo Lizard to shew the relations of the neurenteric canal(ne)and of the primitive streak(pr).am.amnion;ep.epiblast;hy.hypoblast;ch.notochord;pp.body cavity;ne.neurenteric canal;pr.primitive streak.
Illustration: Figure 189Fig. 189. Transverse section through the embryonic region of the blastoderm of a Chick at the time of the formation of the notochord, but before the appearance of the medullary groove.ep.epiblast;hy.hypoblast;ch.notochord;me.mesoblast;n.nuclei in the yolk of the germinal wallyk.
Fig. 189. Transverse section through the embryonic region of the blastoderm of a Chick at the time of the formation of the notochord, but before the appearance of the medullary groove.ep.epiblast;hy.hypoblast;ch.notochord;me.mesoblast;n.nuclei in the yolk of the germinal wallyk.
The division of the mesoblast into two plates along the dorsal line of the embryo, and the formation of the notochord from the axial hypoblast, are intelligible without further explanation. The appearance of part of the mesoblast before the formation of the primitive streak is a process of the same nature as thedifferentiation of hypoblast and mesoblast in Elasmobranchii without an invagination.
In the Sauropsida, some of the mesoblast of the vascular area would appear to be formedin situout of the germinal wall, by a process of cell-formation similar to that which takes place in the yolk adjoining the blastoderm in Elasmobranchii and Teleostei. The mesoblast so formed is to be compared with that which arises on the ventral side of the embryo in the Frog, by a direct differentiation of the yolk-cells.
What was stated for the Elasmobranchii with reference to the general fate of the mesoblast holds approximately for all the other forms.
The Epiblast.
The epiblast in a large number of Chordata arises as a single row of more or less columnar cells. Since the epidermis, into which it becomes converted, is formed of two more or less distinct strata in all Chordata except Amphioxus and Ascidians, the primitive row of epiblast cells, when single, necessarily becomes divided in the course of development into two layers.
In some of the Vertebrata,viz.the Anurous Amphibia, Teleostei, Acipenser, and Lepidosteus, the epiblast is from the first formed of two distinct strata. The upper of these, formed of a single row of cells, is known as the epidermic stratum, and the lower, formed of several rows, as the nervous stratum. In these cases the two original strata of the epiblast are equivalent to those which appear at a later period in the other forms. Thus Vertebrates may be divided into groups according to the primitive condition of their epiblast,viz.a larger group with but a single stratum of cells at first; and a smaller group with two strata.
While there is no great difficulty in determining the equivalent parts of the epidermis in these two groups, it still remains an open question in which of them the epiblast retains its primitive condition.
Though it is not easy to bring conclusive proofs on the one side or the other, the balance of argument appears to me to bedecidedly in favour of regarding the condition of the epiblast in the larger group as primitive, and its condition in the smaller group as secondary, and due to the throwing back of the differentiation of the epiblast to a very early period of development.
In favour of this view may be urged (1) the fact that the simple condition is retained in Amphioxus through life. (2) The correlation in Amphibia, and the other forms belonging to this group, between a closed auditory pit and the early division of the epiblast into two strata; there being no doubt that the auditory pit was at first permanently open, a condition of the epiblast which necessitates its never having an external opening must clearly be secondary. (3) It appears more likely that a particular genetic feature should be thrown back in development, than that such an important feature, as a distinction between two primary layers, should be absolutely lost during an early period of development, and then reappear in later stages.
The fact of the epiblast of the neural canal being divided, like the remainder of the layer, into nervous and epidermic parts, cannot, I think, be used as an argument in favour of the opposite view to that here maintained. It seems probable that the central canal of the nervous system arose phylogenetically as an involution from the exterior, and that the epidermis lining it is merely part of the original epidermis, which has retained its primitive structure as a simple stratum, but is naturally distinguishable from the nervous structures adjacent to it.
Where the epiblast is divided at an early period into two strata, the nervous stratum is always the active one, and takes the main share in forming all the organs derived from the layer.
Formation of the central nervous system. In all Chordata an axial strip of the dorsal epiblast, extending from the lip of the blastopore to the anterior extremity of the head, and known as the medullary plate, becomes isolated from the remainder of the layer to give rise to the central nervous axis.
According to the manner in which this takes place, three types may, however, be distinguished. In Amphioxus the axialstrip becomes first detached from the adjoining epiblast, which then meets and forms a continuous layer above it (fig. 190A and Bnp). The sides of the medullary plate, which is thus shut off from the surface, bend over and meet so as to convert the plate into a canal (fig. 190Cnc). In the second and ordinary type the sides of the medullary plate fold over and meet so as to form a canal before the plate becomes isolated from the external epiblast.
Illustration: Figure 190Fig. 190. Sections of an Amphioxus embryo at three stages.(After Kowalevsky.)A. Section at gastrula stage.B. Section of an embryo slightly younger than that represented in fig. 169 D.C. Section through the anterior part of an embryo at the stage represented in fig. 169 E.np.neural plate;nc.neural canal;mes.archenteron in A and B, and mesenteron in C;ch.notochord;so.mesoblastic somite.
Fig. 190. Sections of an Amphioxus embryo at three stages.(After Kowalevsky.)A. Section at gastrula stage.B. Section of an embryo slightly younger than that represented in fig. 169 D.C. Section through the anterior part of an embryo at the stage represented in fig. 169 E.np.neural plate;nc.neural canal;mes.archenteron in A and B, and mesenteron in C;ch.notochord;so.mesoblastic somite.
The third type is characteristic of Lepidosteus, Teleostei, and Petromyzon. Here the axial plate becomes narrowed in such a way that it forms a solid keel-like projection towards the ventral surface (fig. 191Me). This keel subsequently becomes separated from the remainder of the epidermis, and a central canal is afterwards developed in it. Calberla and Scott hold that the epidermic layer of the skin is involuted into this keel in Petromyzon, and Calberla maintains the same view for Teleostei (fig. 32), but further observations on this subject are required. In the Teleostei a very shallow depression along the axis of the keel is the only indication of the medullary groove of other forms.
In Amphioxus (fig. 190), the Tunicata, Petromyzon (?), Elasmobranchii (fig. 182), the Urodela and Mammalia (fig. 187), the epiblast of the medullary plate is only formed of a single row of cells at the time when the formation of the central nervous system commences; but, except in Amphioxus and the Tunicata,it becomes several cells deep before the completion of the process. In other types the epiblast is several cells deep even before the differentiation of a medullary plate. In the Anura, the nervous layer of the epidermis alone is thickened in the formation of the central nervous system (fig. 72); and after the closure of the medullary canal, the epidermic layer fuses for a period with the nervous layer, though on the subsequent formation of the central epithelium of the nervous canal, there can be little doubt that it becomes again distinct.
Illustration: Figure 191Fig. 191. Section through an embryo of Lepidosteus on the fifth day after impregnation.MC.medullary cord;Ep.epiblast;Me.mesoblast;hy.hypoblast;Ch.notochord.
Fig. 191. Section through an embryo of Lepidosteus on the fifth day after impregnation.MC.medullary cord;Ep.epiblast;Me.mesoblast;hy.hypoblast;Ch.notochord.
It seems almost certain that the formation of the central nervous system from a solid keel-like thickening of the epidermis is a derived and secondary mode; and that the folding of the medullary plate into a canal is primitive. Apart from its greater frequency the latter mode of formation of the central nervous system is shewn to be the primitive type by the fact that it offers a simple explanation of the presence of the central canal of the nervous system; while the existence of such a canal cannot easily be explained on the assumption that the central nervous system was originally developed as a keel-like thickening of the epiblast.
It is remarkable that the primitive medullary plate rarely exhibits any indication of being formed of two symmetrical halves. Such indications are, however, found in the Amphibia (fig. 192andfig. 72); and, since in the adult state the nervous cord exhibits nearly as distinct traces of being formed of two united strands as does the ventral nerve-cord of many Chætopods, it isquite possible that the structure of the medullary plate in Amphibia may be more primitive than that in other types[99].
Illustration: Figure 192Fig. 192. Transverse section through the cephalic region of a young Newt embryo.(After Scott and Osborn.)In.hy.invaginated hypoblast, the dorsal part of which will form the notochord;ep.epiblast of neural plate;sp.splanchnopleure;al.alimentary tract;yk.andY.hy.yolk-cells.
Fig. 192. Transverse section through the cephalic region of a young Newt embryo.(After Scott and Osborn.)In.hy.invaginated hypoblast, the dorsal part of which will form the notochord;ep.epiblast of neural plate;sp.splanchnopleure;al.alimentary tract;yk.andY.hy.yolk-cells.
Formation of the organs of special sense. The more important parts of the organs of smell, sight, and hearing are derived from the epiblast; and it has been asserted that the olfactory pit, optic vesicles and auditory pit take their origin from a special sense plate, continuous at first with this medullary plate. In my opinion this view cannot be maintained.
In the case of the group of forms in which the epiblast is early divided into nervous and epidermic layers, the former layer alone becomes involuted in the formation of the auditory pit and the lens, the external openings of which are never developed, while it is also mainly concerned in the formation of the olfactory pit.
Summary of the more important Organs derived from the three germinal layers.
The epiblastprimarily gives origin to two very important parts of the body,viz.the central nervous system and the epidermis.
It is from the involuted epiblast of the neural tube that the whole of the grey and white matter of the brain and spinal cord appears to be developed, the simple columnar cells of the epiblast being directly transformed into the characteristic multipolar nerve cells. The whole of the sympathetic nervous systemand the peripheral nervous elements of the body, including both the spinal and the cranial nerves and ganglia, are epiblastic in origin.
The epithelium (ciliated in the young animal) lining the canalis centralis of the spinal cord, together with that lining the ventricles of the brain, is the undifferentiated remnant of the primitive epiblast.
The epiblast also forms the epidermis; not however the dermis, which is of mesoblastic origin. The line of junction between the epiblast and the mesoblast coincides with that between the epidermis and the dermis. From the epiblast are formed all such tegumentary organs or parts of organs as are epidermic in nature.
In addition to the above, the epiblast plays an important part in the formation of the organs of special sense.
According to their mode of formation, these organs may be arranged into two divisions. In the first come the organs where the sensory expansion is derived from the involuted epiblast of the medullary canal. To this class belongs the retina, including the pigment epithelium of the choroid, which is formed from the original optic vesicle budded out from the fore-brain.
To the second class belong the epithelial expansions of the membranous labyrinth of the ear, and the cavity of the nose, which are formed by an involution of the epiblast covering the external surface of the embryo. These accordingly have no primary connection with the brain. ‘Taste bulbs’ and other terminal nervous organs, such as those of the lateral line in fishes, are also structures formed from the external epiblast.
In addition to these we have the crystalline lens formed of involuted epiblast as well as the cavity of the mouth and anus, and the glands derived from them. The pituitary body is also epiblastic in origin.
From thehypoblastare derived the epithelium of the digestive canal, the epithelium of the trachea, bronchial tubes and air cells, the cylindrical epithelium of the ducts of the liver, pancreas, thyroid body, and other glands of the alimentary canal, as well as the hepatic cells constituting the parenchyma of the liver, developed from the hypoblast cylinders given off around the primary hepatic diverticula.
Homologous probably with the hepatic cells, and equally of hypoblastic origin, are the spheroidal ‘secreting cells’ of the pancreas and other glands. The epithelium of the salivary glands, though these so closely resemble the pancreas, is probably of epiblastic origin, inasmuch as the cavity of the mouth is entirely lined by epiblast.
The hypoblast also lines the allantois. To these parts must be added the notochord and subnotochordal rod. From themesoblastare formed all the remaining parts of the body. The muscles, the bones, the connective tissue and the vessels, both arteries, veins, capillaries and lymphatics with their appropriate epithelium, are entirely formed from the mesoblast.
The generative and urinary organs are entirely derived from the mesoblast. It is worthy of notice that the epithelium of the urinary glands, though resembling the hypoblastic epithelium of the alimentary canal, is undoubtedly mesoblastic.
From the mesoblast are lastly derived all the muscular, connective tissue, and vascular elements, as well of the alimentary canal and its appendages as of the skin and the tegumentary organs. Just as it is only the epidermic moiety of the latter which is derived from the epiblast, so it is only the epithelium of the former which comes from the hypoblast.
Growth in length of the Vertebrate Embryo.
With reference to the formation and growth in length of the body of the Vertebrate embryo two different views have been put forward, which can be best explained by taking the Elasmobranch embryo as our type. One of these views, generally held by embryologists and adopted in the previous pages, is that the Elasmobranch embryo arises from a differentiation of the edge of the blastoderm; which extends inwards from the edge for some little distance. This differentiation is supposed to contain within itself the rudiments of the whole of the embryo with the exception of the yolk-sack; and the hinder extremity of it, at the edge of the blastoderm, is regarded as corresponding with the hind end of the body of the adult. The growth in length takes place by a process of intussusception, and, till there are formed the full number of mesoblastic somites, it is effected, as in Chætopods, by the continual addition of fresh somites between the last-formed somite and the hind end of the body.
A second and somewhat paradoxical view has been recently brought into prominence by His and Rauber. This view has moreover since been taken up by many embryologists, and has led to strange comparisons between theformation of the mesoblastic plates of the Chætopods and the medullary folds of Vertebrata. According to this view the embryo grows in length by the coalescence of the two halves of the thickened edges of the blastoderm in the dorsal median line. The groove between the coalescing edges is the medullary groove, which increases in length by the continued coalescence of fresh portions of the edge of the blastoderm.
The following is His’ own statement of his view: “I have shewn that the embryo of Osseous Fishes grows together in length from two symmetrically-placed structures in the thickened edge of the blastoderm. Only the foremost end of the head and the hindermost end of the tail undergo no concrescence, since they are formed out of that part of the edge of the blastoderm which, together with the two lateral halves, completes the ring. The whole edge of the blastoderm is used in the formation of the embryo.”
The edges of the blastoderm which meet to form the body of the embryo are regarded as the blastopore, so that, on this view, the blastopore primitively extends for the whole length of the dorsal side of the embryo, and the groove between the coalesced lips becomes the medullary groove.
It is not possible for me to enter at any great length into the arguments used to support this position.
They may be summarised as (1) The general appearance;i.e.that the thickened edge of the blastoderm is continuous with the medullary fold.
(2) Certain measurements (His) which mainly appear to me to prove that the growth takes place by the addition of fresh somites between that last formed and the end of the body.
(3) Some of the phenomena of double monsters (Rauber).
None of these arguments appear to be very forcible, but as the view of His and Rauber, if true, would certainly be important, I shall attempt shortly to state the arguments against it, employing as my type the Elasmobranchii, by the development of which, according to His, the view which he adopts is more conclusively proved than by that of any other group.
(1) The general appearance of the thickened edge of the blastoderm becoming continuous with the medullary folds has been used as an argument for the medullary folds being merely the coalesced thickened edges of the blastoderm. Since, however, the medullary folds are merely parts of the medullary plate, and since the medullary plate is continuous with the adjoining epiblast of the embryonic rim, the latter structure must be continuous with the medullary folds however they are formed, and the mere fact of their being so continuous cannot be used as an argument either way. Moreover, were the concrescence theory true, the coalescing edges of the blastoderm might be expected to form an acute angle with each other, which they are far from doing.
(2) The medullary groove becomes closed behind earlier than in front, and the closure commences while the embryo is still quite short, andbefore the hind end has begun to project over the yolk. After the medullary canal becomes closed, and is continued behind into the alimentary canal by the neurenteric passage, it is clearly impossible for any further increase in lengthto take place by concrescence. If therefore His’ and Rauber’s view is accepted, it will have to be maintained that only a small part of the body is formed by concrescence, while the larger posterior part grows by intussusception. The difficulty involved in this supposition is much increased by the fact that long after the growth by concrescence must have ceased the yolk blastopore still remains open, and the embryo is still attached to the edge of the blastoderm; so that it cannot be maintained that the growth by concrescence has come to an end because the thickened edges of the blastoderm have completely coalesced.
The above are arguments derived simply from a consideration of the growth of the embryo; and they prove (1) that the points adduced by His and Rauber are not at all conclusive; (2) that the growth in length of the greater part of the body takes place by the addition of fresh somites behind, as in Chætopods, and it would therefore be extremely surprising that a small middle part of the body should grow in quite a different way.
Many minor arguments used by His might be replied to, but it is hardly necessary to do so, and some of them depend upon erroneous views as to the course of development, such as an argument about the notochord, which depends for its validity upon the assumption that the notochord ridge appears at the same time as the medullary plate, while, as a matter of fact, the ridge does not appear till considerably later. In addition to the arguments of the class hitherto used, there may be brought against the His-Rauber view a series of arguments from comparative embryology.
(1) Were the vertebrate blastopore to be co-extensive with the dorsal surface, as His and Rauber maintain, clear evidence of this ought to be apparent in Amphioxus. In Amphioxus, however, the blastopore is at first placed exactly at the hind end of the body, though later it passes up just on to the dorsal side (videp.4). It nearly closes before the appearance of the medullary groove or mesoblastic somites; and the medullary folds have nothing to do with its lips, except in so far as they are continuous with them behind, just as in Elasmobranchii.
(2) The food-yolk in the Vertebrata is placed on the ventral side of the body, and becomes enveloped by the blastoderm; so that in all large-yolked Vertebrates the ventral walls of the body are obviously completed by the closure of the lips of the blastopore, on the ventral side.
If His and Rauber are right the dorsal walls are also completed by the closure of the blastopore, so that the whole of the dorsal, as well as of the ventral wall of the embryo, must be formed by the concrescence of the lips of the blastopore; which is clearly areductio ad absurdumof the whole theory. To my own arguments on the subject I may add those of Kupffer, who has very justly criticised His' statements, and has shewn that growth of the blastoderm in Clupea and Gasterosteus is absolutely inconsistent with the concrescence theory.
The more the theory of His and Rauber is examined by the light of comparative embryology, the more does it appear quite untenable; and it may be laid down as a safe conclusion from a comparative study of vertebrateembryology that the blastopore of Vertebrates is primitively situated at the hind end of the body, but that, owing to the development of a large food-yolk, it also extends, in most cases, over a larger or smaller part of the ventral side.
The origin of the Allantois and Amnion.
The development and structure of the allantois and amnion have already been dealt with at sufficient length in the chapters on Aves and Mammalia; but a few words as to the origin of these parts will not be out of place here.
The Allantois. The relations of the allantois to the adjoining organs, and the conversion of its stalk into the bladder, afford ample evidence that it has taken its origin from a urinary bladder such as is found in Amphibia. We have in tracing the origin of the allantois to deal with a case of what Dohrn would call ‘change of function.’ The allantois is in fact a urinary bladder which,precociouslydeveloped and enormously extended in the embryo, has acquired respiratory (Sauropsida) and nutritive (Mammalia) functions. No form is known to have been preserved with the allantois in a transitional state between an ordinary bladder and a large vascular sack.
The advantage of secondary respiratory organs during fœtal life, in addition to the yolk-sack, is evinced by the fact that such organs are very widely developed in the Ichthyopsida. Thus in Elasmobranchii we have the external gills (cf.p.62). Amongst Amphibia we have the tail modified to be a respiratory organ in Pipa Americana; and in Notodelphis, Alytes and Cæcilia compressicanda the external gills are modified and enlarged for respiratory purposes within the egg (cf.pp.140and143).
The Amnion. The origin of the amnion is more difficult to explain than that of the allantois; and it does not seem possible to derive it from any pre-existing organ.
It appears to me, however, very probable that it was evolvedpari passuwith the allantois, as a simple fold of the somatopleure round the embryo, into which the allantois extended itself as it increased in size and became a respiratory organ. It would be obviously advantageous for such a fold, having once started, to become larger and larger in order to give more and more room for the allantois to spread into.
The continued increase of this fold would lead to its edges meeting on the dorsal side of the embryo, and it is easy to conceive that they might then coalesce.
To afford room for the allantois close to the surface of the egg, where respiration could most advantageously be carried on, it would be convenient that the two laminæ of the amnion—the true and false amnion—should then separate and leave a free space above the embryo, and thus it may have come about that a separation finally takes place between the true and false amnion.
This explanation of the origin of the amnion, though of course hypothetical, has the advantage of suiting itself in most points to the actual ontogenyof the organ. The main difficulty is the early development of the head-fold of the amnion, since, from the position of the allantois, it might have been anticipated that the tail-fold would be the first formed and most important fold of the amnion.
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[99]A parallel to the unpaired medullary plate of most Chordata is supplied by the embryologically unpaired ventral cord of most Gephyrea and some crustacea. In these forms there can be little doubt that the ventral cord has arisen from the fusion of two originally independent strands, so that it is not an extremely improbable hypothesis to suppose that the same may have been the case in the Chordata.
[99]A parallel to the unpaired medullary plate of most Chordata is supplied by the embryologically unpaired ventral cord of most Gephyrea and some crustacea. In these forms there can be little doubt that the ventral cord has arisen from the fusion of two originally independent strands, so that it is not an extremely improbable hypothesis to suppose that the same may have been the case in the Chordata.
The present section of this work would not be complete without some attempt to reconstruct, from the materials recorded in the previous chapters, and from those supplied by comparative anatomy, the characters of the ancestors of the Chordata; and to trace as far as possible from what invertebrate stock this ancestor was derived.
The second of these questions has been recently dealt with in a very suggestive manner by both Dohrn (No.250) and Semper (Nos.255and256), but it is still so obscure that I shall refrain from any detailed discussion of it.
While differing very widely in many points both Dohrn and Semper have arrived at the view, already tentatively put forward by earlier anatomists, that the nearest allies of the Chordata are to be sought for amongst the Chætopoda, and that the dorsal surface of the Chordata with the spinal cord corresponds morphologically with the ventral surface of the Chætopods with the ventral ganglion chain. In discussing this subject some time ago[100]I suggested that we must look for the ancestors of the Chordata, not in allies of the present Chætopoda, but in a stock of segmented forms descended from the same unsegmented types as the Chætopoda, but in which two lateral nerve-cords, like those of Nemertines, coalesced dorsally, instead of ventrally to form a median nervous cord. This group of forms, if my suggestion as to its existence is well founded, appears now to have perished. The recent researches of Hubrecht on the anatomy of the Nemertines[101]have, however, added somewhat to the probability of my views, in that they shew that in some existing Nemertines the nerve-cords approach each other very closely in the dorsal line.
With reference to the characters of the ancestor of the Chordata the following pages contain a few tentative suggestions rather than an attempt to deal with the whole subject; while theorigin of certain of the organs is dealt with in a more special manner in the chapters on organogeny which form the second part of this work.
Before entering upon the more special subject of this chapter, it will be convenient to clear the ground by insisting on a few morphological conclusions to be drawn from the study of Amphioxus,—a form which, although probably in some respects degenerate, is nevertheless capable of furnishing on certain points very valuable evidence.
(1) In the first place it is clear from Amphioxus that the ancestors of the Chordata were segmented, and that their mesoblast was divided into myotomes which extended even into the region in front of the mouth. The mesoblast of the greater part of what is called the head in the Vertebrata proper was therefore segmented like that of the trunk.
(2) The only internal skeleton present was the unsegmented notochord—a fact which demonstrates that the skeleton is of comparatively little importance for the solution of a large number of fundamental questions, as for example the point which has been mooted recently as to whether gill-clefts existed at one time in front of the present mouth; and for this reason:—that from the evidence of Amphioxus and the lower Vertebrata[102]it is clear that such clefts, if they ever existed,had atrophiedcompletelybefore the formation of cartilaginous branchial bars; so that any skeletal structures in front of the mouth, which have been interpreted by morphologists as branchial bars, can never have acted in supporting the walls of branchial clefts.
(3) The region which, in the Vertebrata, forms the œsophagus and stomach, was, in the ancestors of the Chordata, perforated by gill-clefts. This fact, which has been clearly pointed out by Gegenbaur, is demonstrated by the arrangement of the gill-clefts in Amphioxus, and by the distribution of the vagus nerve in the Vertebrata[103]. On the other hand the insertion of the liver, which was probably a very primitive organ, appears to indicate with approximate certainty the posterior limit of the branchial clefts.
With these few preliminary observations we may pass to the main subject of this section. A fundamental question which presents itself on the threshold of our enquiries is the differentiation of the head.
In the Chætopoda the head is formed of a præoral lobe and of the oral segment; while in Arthropods a somewhat variable number of segments are added behind to this primitive head, and form with it what may be called a secondary compound head. It is fairly clear that the section of the trunk, which, in Amphioxus, is perforated by the visceral clefts, has become the head in the Vertebrates proper, so that the latter forms are provided with a secondary head like that of Arthropods. There remain however difficult questions (1) as to the elements of which this head is composed, and (2) as to the extent of its differentiation in the ancestors of the Chordata.
In Arthropods and Chætopods there is a very distinct element in the head known as the procephalic lobe in the case of Arthropods, and the præoral lobe in that of Chætopods; and this lobe is especially characterized by the fact that the supraœsophageal ganglia and optic organs are formed as differentiationsof part of the epiblast covering it. Is such an element to be recognized in the head of the Chordata? From a superficial examination of Amphioxus the answer would undoubtedly be no; but then it has to be borne in mind that Amphioxus, in correlation with its habit of burying itself in sand, is especially degenerate in the development of its sense-organs; so that it is not difficult to believe that its præoral lobe may have become so reduced as not to be recognizable. In the true Vertebrata there is a portion of the head which has undoubtedly many features of the præoral lobe in the types already alluded to,viz.the part containing the cerebral hemispheres and the thalamencephalon. If there is any part of the brain homologous with the supraœsophageal ganglia of the Invertebrates, and it is difficult to believe there is not such a part, it must be part of, or contain, the fore-brain. The fore-brain resembles the supraœsophageal ganglia in being intimately connected in its development with the optic organs, and in supplying with nerves only organs of sense. Its connection with the olfactory organs is an argument in the same direction. Even in Amphioxus there is a small bulb at the end of the nervous tube supplying what is very probably the homologue of the olfactory organ of the Vertebrata; and it is quite possible that this bulb is the reduced rudiment of what forms the fore-brain in the Vertebrata.
The evidence at our disposal appears to me to indicate that the third nerve belongs to the cranio-spinal series of segmental nerves, while the optic and olfactory nerves appear to me equally clearly not to belong to this series[104]. The mid-brain, as giving origin to the third nerve, would appear not to have been part of the ganglion of the præoral lobe.
These considerations indicate with fair probability that the part of the head containing the fore-brain is the equivalent of the præoral lobe of many Invertebrate forms; and the primitive position of the Vertebrate mouth on the ventral side of the head affords a distinct support for this view. It must however be admitted that this part of the head is not sharply separated in development from that behind; and, though the fore-brain isusually differentiated very early as a distinct lobe of the primitive nervous tube, yet that such differentiation is hardly more marked than in the other parts of the brain. The termination of the notochord immediately behind the fore-brain is, however, an argument in favour of the morphological distinctness of the latter structure.
The evidence at our disposal appears to indicate that the posterior part of the head was not differentiated from the trunk in lower Chordata; but that, as the Chordata rose in the scale of development, more and more centralizing work became thrown on the anterior part of the nervous cord, andpari passuthis part became differentiated into the mid- and hind-brain. An analogy for such a differentiation is supplied in the compound subœsophageal ganglion of many Arthropods; and, as will be shewn in the chapter on the nervous system, there is strong embryological evidence that the mid- and hind-brains had primitively the same structure as the spinal cord. The head appears however to have suffered in the course of its differentiation a great concentration in its posterior part, which becomes progressively more marked, even within the limits of the surviving Vertebrata. This concentration is especially shewn in the structure of the vagus nerve, which, as first pointed out by Gegenbaur, bears evidence of having been originally composed of a great series of nerves, each supplying a visceral cleft. Rudiments of the posterior nerves still remain as the branches to the œsophagus and stomach[105].
The atrophy of the posterior visceral clefts seems to have taken place simultaneously with the concentration of the neural part of the head; but the former process did not proceed so rapidly as the latter, so that the visceral region of the head is longer in the lower Vertebrata than the neural region, and is dorsally overlapped by the anterior part of the spinal cord and the anterior muscle-plates (videfig. 47).
On the above view the posterior part of the head must have been originally composed of a series of somites like those of thetrunk, but in existing Vertebrata all trace of these, except in so far as they are indicated by the visceral clefts, has vanished in the adult. The cranial nerves however, especially in the embryo, still indicate the number of anterior somites; and an embryonic segmentation of the mesoblast has also been found in many lower forms in the region of the head, giving rise to a series of cavities known as head-cavities, enclosed by mesoblastic walls which afterwards break up into muscles. These cavities correspond with the nerves, and it appears that there is a præmandibular cavity corresponding with the third nerve (fig. 193,1pp) and a mandibular cavity (2pp) and a cavity in each of the succeeding visceral arches. The fifth nerve, the seventh nerve, the glossopharyngeal nerve, and the successive elements of the vagus nerve correspond with the posterior head-cavities.