The Urinogenital System.
The first traces of the urinary system become visible at about the time of the appearance of the third visceral cleft. At about this period the somatopleure and splanchnopleure become more or less fused together at the level of the dorsal aorta, and thus, as has been already mentioned, each of the original plates of mesoblast becomes divided into a vertebral plate and lateral plate (Pl.11, fig. 6). The mass of cells resulting from this fusion corresponds with Waldeyer's intermediate cell-mass in the Fowl.
At about the level of the fifth protovertebra the first trace of the urinary system appears.
From the intermediate cell-mass a solid knob grows outwards towards the epiblast (woodcut, fig. 4,pd). This knob consists at first of 20-30 cells, which agree in character with the neighbouring cells of the intermediate cell-mass, and are at this period rounded. It is mainly, if not entirely, derived from the somatic layer of the mesoblast.
From this knob there grows backwards a solid rod of cells which keeps in very close contact with the epiblast, and rapidly diminishes in size towards its posterior extremity. Its hindermost part consists in section of at most one or two cells. It keeps so close to the epiblast that it might be supposed to bederived from that layer were it not for the sections shewing its origin from the knob above mentioned. We have in this rod the commencement of what I have elsewhere[224]called the segmental duct.
Fig. 4. Two sections of a Pristiurus Embryo with three visceral clefts.
Development of the segmental ductThe sections are to shew the development of the segmental duct (pd) or primitive duct of the kidneys. InA(the anterior of the two sections) this appears as a solid knob projecting towards the epiblast. InBis seen a section of the column which has grown backwards from the knob inA.spn.rudiment of a spinal nerve;mc.medullary canal;ch.notochord;X.string of cells below the notochord;mp.muscle-plate;mp´.specially developed portion of muscle-plate;ao.dorsal aorta;pd.segmental duct;so.somatopleura;sp.splanchnopleura;pp.pleuro-peritoneal or body-cavity;ep.epiblast;al.alimentary canal.
The sections are to shew the development of the segmental duct (pd) or primitive duct of the kidneys. InA(the anterior of the two sections) this appears as a solid knob projecting towards the epiblast. InBis seen a section of the column which has grown backwards from the knob inA.
spn.rudiment of a spinal nerve;mc.medullary canal;ch.notochord;X.string of cells below the notochord;mp.muscle-plate;mp´.specially developed portion of muscle-plate;ao.dorsal aorta;pd.segmental duct;so.somatopleura;sp.splanchnopleura;pp.pleuro-peritoneal or body-cavity;ep.epiblast;al.alimentary canal.
My observations shew that the segmental duct is developed in the way just described in both Pristiurus and Torpedo. Its origin in Pristiurus is shewn in the adjoining woodcut, and in Torpedo inPl.11, fig. 7,sd.
At a stage somewhat older than I, the condition of the segmental duct has not very materially altered. It has increased considerably in length, and the knob at its front end is both absolutely smaller, and also consists of fewer cells than before (Pl.11, fig. 7,sd). These cells have become more columnar, and have begun to arrange themselves radially; thus indicating the early appearance of the lumen of the duct. The cells forming the front part of the rod, as well as those of the knob, commence to exhibit a columnar character, but in the hinder part of therod the cells are still rounded. In no part of it has a lumen appeared.
At this period also the knob, partly owing to the commencing separation of the muscle-plate from the remainder of the mesoblast, begins to pass inwards and approach the pleuro-peritoneal cavity.
At the same stage the first not very distinct traces of the remainder of the urinary system become developed. These appear in the form of solid outgrowths from the intermediate cell-mass just at the most dorsal part of the body-cavity.
The outgrowths correspond in numbers with the vertebral segments, and are at first quite disconnected with the segmental duct. At this stage they are only distinctly visible in the first few segments behind the front end of the segmental duct. A full description of them will come more conveniently in the next stage.
By a stage somewhat earlier than K important changes have taken place in the urinary system.
The segmental duct has acquired a lumen in its anterior portion, which opens at its front end into the body-cavity. (Pl.11, fig. 9,sd.) The lumen is formed by the columnar cells spoken of in the last stage, acquiring a radiating arrangement round a central point, at which a small hole appears. After the lumen has once become formed, it rapidly increases in size.
The duct has also grown considerably in length, but its hind extremity is still as thin, and lies as close to the epiblast, as at first. The segmental involutions which commenced to be formed in the last stage, have now appeared for every vertebral segment along the whole length of the segmental duct, and even for two or three segments behind this.
They are simple independent outgrowths arising from the outer and uppermost angle of the body-cavity, and are at first almost without a trace of a lumen; though their cells are arranged as two layers. They grow in such a way as to encircle the oviduct on its inner and upper side (Pl.11, fig. 8 andPl.12, fig. 14b,st). When the hindermost ones are formed, a slight trace of a lumen is perhaps visible in the front ones. At a stage slightly subsequent to this, in Scyllium canicula, I noticed 29of them; the first of them arising in the segment immediately behind the front end of the oviduct (Pl.12, fig. 17,st), and two of them being formed in segments just posterior to the hinder extremity of the oviduct.
Pl.12, figs. 16 and 18 represent two longitudinal sections shewing the segmental nature of the involutions and their relation to the segmental duct.
Many of the points which have been mentioned can be seen by referring toPl.11 and 12. Anteriorly the segmental duct opens into the pleuro-peritoneal cavity. In the sections behind this there may be seen the segmental duct with a distinct lumen, and also a pair of segmental involutions (Pl.12, fig. 14a). In the still posterior sections the segmental duct would be quite without a lumen, and would closely adjoin the epiblast.
It seems not out of place to point out that the modes of the development of the segmental duct and of the segmental involutions are strikingly similar. Both arise as solid involutions, from homologous parts of the mesoblast. The segmental duct arises in the vertebral segment immediately in front of that in which the first segmental involution appears;so that the segmental duct appears to be equivalent to a single segmental involution.
The next stage corresponds with the first appearance of the external gills. The segmental duct now communicates by a wide opening with the body-cavity (Pl.11, fig. 9,sd). It possesses a lumen along its whole length up to the extreme hind end (Pl.11, fig. 9a). It is, however, at this hinder extremity that the most important change has taken place. This end has grown downwards towards that part of the alimentary canal which still lies behind the anus. This downgrowth is beginning to shew distinct traces of a lumen, and will appear in the next stage as one of the horns by which the segmental ducts communicate with the cloaca (Pl.11, fig. 9b). All the anterior segmental involutions have now acquired a lumen. But this is still absent in the posterior ones (Pl.11, fig. 9a).
Owing to the disappearance of the body-cavity in the region behind the anus, the primitive involutions there remain as simple masses of cells still disconnected with the segmental duct (Pl.11, figs. 9b, 9cand 9d).
Primitive Ova.The true generative products make their first appearance as theprimitive ovabetween stages I and K.
In the sections of one of my embryos of this stage they are especially well shewn, and the following description is taken from those displayed in that embryo.
They are confined to the region which extends posteriorly nearly to the end of the small intestine and anteriorly to the abdominal opening of the segmental duct.
Their situation in this region is peculiar. There is no trace of a distinct genital ridge, but the ova mainly lie in the dorsal portion of the mesentery, and therefore in a part of the mesoblast which distinctly belongs to the splanchnopleure (Pl.12, fig. 14a). Some are situated external to the segmental involutions; and others again, though this is not common, in a part of the mesoblast which distinctly belongs to the body-wall (Pl.12, fig. 14b).
The portion of mesentery, in which the primitive ova are most densely aggregated, corresponds to the future position of the genital ridge, but the other positions occupied by ova are quite outside this. Some ova are in fact situated on the outside of the segmental duct and segmented tubes, and must therefore effect a considerable migration before reaching their final positions in the genital ridge on the inner side of the segmental duct (Pl.12, fig. 14b).
The condition of the tissue in which the ova appear may at once be gathered from an examination of the figures given. It consists of an irregular epithelium of cells partly belonging to the somatopleure and partly to the splanchnopleure, but passing uninterruptedly from one layer to the other. The cells which compose it are irregular in shape, but frequently columnar (Pl.12, figs. 14aand 14b).
They are formed of a nucleus which stains deeply, invested by avery delicatelayer of protoplasm. At the junction of somatopleure and splanchnopleure they are more rounded than elsewhere. Very few loose connective-tissue cells are present. The cells just described vary from .008Mm.to .01Mm.in diameter.
The primitive ova are situated amongst them and stand out with extraordinary clearness, to which justice is hardly done in my figures.
The normal full-sized ova exhibit the following structure. They consist of a mass of somewhat granular protoplasm of irregular, but more or less rounded, form. Their size varies from .016 - .036Mm.In their interior a nucleus is present, which varies from .012 - .016Mm., but its size as a rule bearsnorelation to the size of the containing cell.
This is illustrated by the subjoined list of measurements.
The numbers given refer to degrees on my micrometer scale.
Since it is the ratio alone which it is necessary to call attention to, the numbers are not reduced to decimals of a millimeter. Each degree of my scale is equal, however, with the object glass employed, to .002Mm.
This series brings out the result I have just mentioned with great clearness.
In one case we find a cell has three times the diameter of the nucleus 16 : 5½; in another case 10 : 8, the nucleus has only a slightly smaller diameter than the cell. The irrationality of the ratio is fairly shewn in some of my figures, though none of the largest cells with very small nuclei have been represented.
The nuclei are granular, and stain fairly well with hæmatoxylin. They usually contain a single deeply stained nucleolus, but in many cases, especially where large (and this independentlyof the size of the cell), they contain two nucleoli (Pl.12, figs. 14cand 14d), and are at times so lobed as to give an apparent indication of commencing division.
A multi-nucleolar condition of the nuclei, like that figured by Götte[225], does not appear till near the close of embryonic life, and is then found equally in the large ova and in those not larger than the ova which exist at this early date.
As regards the relation of the primitive ova to each other and the neighbouring cells, there are a few points which deserve attention. In the first place, the ova are, as a rule, collected in masses at particular points, and not distributed uniformly (fig. 14a). The masses in some cases appear as if they had resulted from the division of one primitive ovum, but can hardly be adduced as instances of a commencing coalescence; since if the ova thus aggregated were to coalesce, an ovum would be produced of a very much greater size than any which is found during the early stages. Though at this stage no indication is present of such a coalescence of cells to form ova as is believed to take place by Götte, still the origin of the primitive ova is not quite clear. One would naturally expect to find a great number of cells intermediate between primitive ova and ordinary columnar cells. Cells which may be intermediate are no doubt found, but not nearly so frequently as might have been anticipated. One or two cells are shewn inPl.12, fig. 14a,x, which are perhaps of an intermediate character; but in most sections it is not possible to satisfy oneself that any such intermediate cells are present.
In one case what appeared to be an intermediate cell was measured, and presented a diameter of .012Mm.while its nucleus was .008Mm.Apart from certain features of the nucleus, which at this stage are hardly very marked, the easiest method of distinguishing a primitive ovum from an adjacent cell is the presence of a large quantity of protoplasm around the nucleus. The nucleus of one of the smallest primitive ova is not larger than the nucleus of an ordinary cell (being about .008Mm.in both). It is perhaps the similarity in the size of the nuclei which renders it difficult at first to distinguish developing primitive ova from ordinary cells. Except with thevery thinnest sections a small extra quantity of protoplasm around a nucleus might easily escape detection, and the developing cell might only become visible when it had attained to the size of a small typical primitive ovum.
It deserves to be noticed that the nuclei even of some of the largest primitive ova scarcely exceed the surrounding nuclei in size. This appears to me to be an argument of some weight in shewing that the great size of primitive ova is not due to the fact of their having been formed by a coalescence of different cells (in which case the nucleus would have increased in the same proportion as the cell); but to an increase by a normal method of growth in the protoplasm around the nucleus.
It appears to me to be a point of great importance certainly to determine whether the primitive ova arise by a metamorphosis of adjoining cells, or may not be introduced from elsewhere. In some of the lower animals,e.g.Hydrozoa, there is no question that the ova are derived from the epiblast; we might therefore expect to find that they had the same origin in Vertebrates. Further than this, ova are frequently capable in a young state of executing amœboid movements, and accordingly of migrating from one layer to another. In the Elasmobranchii the primitive ova exhibit in a hardened state an irregular form which might appear to indicate that they possess a power of altering their shape, a view which is further supported by some of them being at the present stage situated in a position very different from that which they eventually occupy, and which they can only reach by migration. If it could be shewn that there were no intermediate stages between the primitive ova and the adjoining cells (their migratory powers being admitted) a strong presumption would be offered in favour of their having migrated from elsewhere to their present position. In view of this possibility I have made some special investigations, which have however led to no very satisfactory results. There are to be seen in the stages immediately preceding the present one, numerous cells in a corresponding position to that of the primitive ova, which might very well be intermediate between the primitive ova and ordinary cells, but which offer no sufficiently well marked features for a certain determination of their true nature.
In the particular embryo whose primitive ova have been described these bodies were more conspicuous than in the majority of cases, but at the same time they presented no special or peculiar characters.
In a somewhat older embryo of Scyllium the cells amongst which the primitive ova lay had become very distinctly differentiated as an epithelium (the germinal epithelium of Waldeyer) well separated by what might almost be called a basement membrane from the adjoining connective-tissue cells. Hardly any indication of a germinal ridge had appeared, but the ova were more definitely confined than in previous embryos to the restricted area which eventually forms this. The ova on the average were somewhat smaller than in the previous cases.
In several embryos intermediate in age between the embryo whose primitive ova were described at the commencement of this section and the embryo last described, the primitive ova presented some peculiarities, about the meaning of which I am not quite clear, but which may perhaps throw some light on the origin of these bodies.
Instead of the protoplasm around the nucleus being clear or slightly granular, as in the cases just described, it was filled in the most typical instances with numerous highly refracting bodies resembling yolk-spherules. In osmic acid specimens (Pl.12, fig. 15) these stain very darkly, and it is then as a rule very difficult to see the nucleus; in specimens hardened in picric acid and stained with hæmatoxylin these bodies are stained of a deep purple colour, but the nucleus can in most cases be distinctly seen. In addition to the instances in which the protoplasm of the ova is quite filled with these bodies, there are others in which they only occupy a small area adjoining the nucleus (Pl.12, fig. 15a), and finally some in which only one or two of these bodies are present. The protoplasm of the primitive ova appears in fact to present a series of gradations between a state in which it is completely filled with highly refracting spherules and one in which these are completely absent.
This state of things naturally leads to the view that the primitive ova, when they are first formed, are filled with these spherules, which are probably yolk-spherules, but that theygradually lose them in the course of development. Against this interpretation is the fact that the primitive ova in the younger embryo first described are completely without these bodies; this embryo however unquestionably presented an abnormally early development of the ova; and I am satisfied that embryos present considerable variations in this respect.
If the primitive ova are in reality in the first instance filled with yolk-spherules, the question arises as to whether, considering that they are the only mesoblast cells filled at this period with yolk-spherules, we must not suppose that they have migrated from some peripheral part of the blastoderm into their present position. To this question I can give no satisfactory answer. Against a view which would regard the spherules in the protoplasm as bodies which appear subsequently to the first formation of the ova, is the fact that hitherto no instances in which these spherules were present have been met with in the late stages of development; and they seem therefore to be confined to the first stages.
Notochord.
The changes undergone by the notochord during this period present considerable differences according to the genus examined. One type of development is characteristic of Scyllium and Pristiurus; a second type, of Torpedo.
My observations being far more complete for Scyllium and Pristiurus than for Torpedo, it is to the two former genera only that the following account applies, unless the contrary is expressly stated. Only the development of the parts of the notochord in the trunk are here dealt with; the cephalic section of the notochord is treated of in a subsequent section.
During stage G the notochord is composed of flattened cells arranged vertically, rendering the histological characters of the notochord difficult to determine in transverse sections. In longitudinal sections, however, the form and arrangement of the cells can be recognised with great ease. At the beginning of stage G each cell is composed of a nucleus invested by granular protoplasm frequently vacuolated and containing in suspension numerous yolk-spherules. It is difficult to determine whetherthere is only one vacuole for each cell, or whether in some cases there may not be more than one.
Round the exterior of the notochord there is present a distinct though delicate cuticular sheath.
The vacuoles are at first small, but during stage G rapidly increase in size, while at the same time the yolk-spherules completely vanish from the notochord.
As a result of the rapid growth of the vacuoles, the nuclei, surrounded in each case by a small amount of protoplasm, become pushed to the centre of the notochord, the remainder of the protoplasm being carried to the edge. The notochord thus becomes composed during stages H and I (Pl.11, fig. 4-6) of a central area mainly formed of nuclei with a small quantity of protoplasm around them, and of a thin peripheral layer of protoplasm without nuclei, the widish space between the two being filled with clear fluid. The exterior of the cells is indurated, so that they may be said to be invested by a membrane[226]; the cells themselves have a flattened form, and each extends from the edge to the centre of the notochord, the long axis of each being rather greater than half the diameter of the cord.
The nuclei of the notochord are elliptical vesicles, consisting of a membrane filled with granular contents, amongst which is situated a distinct nucleolus. They stain deeply with hæmatoxylin. Their long diameter in Scyllium is about 0.02Mm.
The diameter of the whole notochord in Pristiurus during stage I is about 0.1Mm.in the region of the back, and about 0.08Mm.near the posterior end of the body.
Owing to the form of its constituent cells, the notochord presents in transverse sections a dark central area surrounded by a lighter peripheral one, but its true structure cannot be unravelled without the assistance of longitudinal sections. In these (Pl.12, fig. 10) the nuclei form an irregular double row in the centre of the cord. Their outlines are very clear, but those of the individual cells cannot for certain be made out. It is, however, easy to see that the cells have a flattened and wedge-shaped form, with the narrow ends overlapping and interlocking at the centre of the notochord.
By the close of stage I the cuticular sheath of the notochord has greatly increased in thickness.
During the period intermediate between stages I and K the notochord undergoes considerable transformations. Its cells cease to be flattened, and become irregularly polygonal, and appear but slightly more compressed in longitudinal sections than in transverse ones. The vacuolation of the cells proceeds rapidly, and there is left in each cell only a very thin layer of protoplasm around the nucleus. Each cell, as in the earlier stages, is bounded by a membrane-like wall.
Accompanying these general changes special alterations take place in the distribution of the nuclei and the protoplasm. The nuclei, accompanied by protoplasm, gradually leave the centre and migrate towards the periphery of the notochord. At the same time the protoplasm of the cells forms a special layer in contact with the investing sheath.
The changes by which this takes place can easily be followed in longitudinal sections. InPl.12, fig. 11 the migration of the nuclei has commenced. They are still, however, more or less aggregated at the centre, and very little protoplasm is present at the edges of the notochord. The cells, though more or less irregularly polygonal, are still somewhat flattened. InPl.12, fig. 12 the notochord has made a further progress. The nuclei now mainly lie at the side of the notochord, where they exist in a somewhat shrivelled state, though still invested by a layer of protoplasm.
A large portion of the protoplasm of the cord forms an almost continuous layer in close contact with the sheath, which is more distinctly visible in some cases than in others.
While the changes above described are taking place the notochord increases in size. At the age of fig. 11 it is in the anterior part of the body of Pristiurus about 0.11Mm.At the age of fig. 12 it is in the same species 0.12Mm., while in Scyllium stellare it reaches about 0.17Mm.
During stage K (Pl.11, fig. 8) the vacuolation of the cells of the notochord becomes even more complete than during the earlier stages, and in the central cells hardly any protoplasm is present, though a starved nucleus surrounded by a little protoplasm may be found in an occasional corner.
The whole notochord becomes very delicate, and can with great difficulty be conserved whole in transverse sections.
The layer of protoplasm which appeared during the last stage on the inner side of the cuticular membrane of the notochord becomes during the present stage a far thicker and more definite structure. It forms a continuous layer with irregular prominences on its inner surface; and contains numerous nuclei. The layer sometimes presents in transverse sections hardly any indication of a division into a number of separate cells, but in longitudinal sections this is generally very obvious. The cells are directed very obliquely forwards, and consist of an oblong nucleus invested by protoplasm. The layer formed by them is very delicate and very easily destroyed. In one example its thickness varied from .004 to .006Mm., in another it reached .012Mm.The thickness of the cuticular membrane is about .002Mm.or rather less.
The diameter of a notochord in the anterior part of the body of a Pristiurus embryo of this stage is about 0.21Mm.Round the exterior of the notochord the mesoblast cells are commencing to arrange themselves as a special sheath.
In Torpedo the notochord at first presents the same structure as in Pristiurus,i.e.it forms a cylindrical rod of flattened cells.
The vacuolation of these cells does not however commence till a relatively very much later period than in Pristiurus, and also presents a very different character (Pl.11, fig. 7).
The vacuoles are smaller, more numerous, and more rounded than in the other genera, and there can be no question that in many cases there is more than one vacuole in a cell. The most striking point in which the notochord of Torpedo differs from that of Pristiurus consists in the fact that in Torpedo there is never any aggregation of the nuclei at the centre of the cord, but the nuclei are always distributed uniformly through it. As the vacuolation proceeds the differences between Torpedo and the other genera become less and less marked. The vacuoles become angular in form, and the cells of the cord cease to be flattened, and become polygonal.
At my final stage for Torpedo (slightly younger than K) the only important feature distinguishing the notochord from thatof Pristiurus, is the absence of any signs of nuclei or protoplasm passing to the periphery. Around the exterior of the cord there is early found in Torpedo a special investment of mesoblastic cells.
EXPLANATION OF PLATES 11 AND 12.
Complete List of Reference Letters.
al.Alimentary tract.an.Point where anus will be formed.ao.Dorsal aorta.ar.Rudiment of anterior root of spinal nerve.b.Anterior fin.c.Connective-tissue cells.cav.Cardinal vein.ch.Notochord.df.Dorsal fin.ep.Epiblast.ge.Germinal epithelium.ht.Heart.l.Liver.mp.Muscle-plate.mp´.Early formed band of muscles from the splanchnic layer of the muscle-plates.nc.Neural canal.p.Protoplasm from yolk in the alimentary tract.pc.Pericardial cavity.po.Primitive ovum.pp.body-cavity.pr.Rudiment of posterior root of spinal nerve.sd.Segmental duct.sh.Cuticular sheath of notochord.so.Somatic layer of mesoblast.sp.Splanchnic layer of mesoblast.spc.Spinal cord.sp.v.Spiral valve.sr.Interrenal body.st.Segmental tube.sv.Sinus venosus.ua.Umbilical artery.um.Umbilical cord.uv.Umbilical vein.V.Splanchnic vein.v.Blood-vessel.vc.Visceral cleft.vr.Vertebral rudiment.W.White matter of spinal cord.x.Subnotochordal rod (except in fig. 14a).y.Passage connecting the neural and alimentary canals.
Plate 11.
Fig. 1. Section from the caudal region of a Pristiurus embryo belonging to stage H. Zeiss C,ocul.1. Osmic acid specimen.
It shews (1) the constriction of the subnotochordal rod (x) from the summit of the alimentary canal. (2) The formation of the body-cavity in the muscle-plate and the ventral thickening of the parietal plate.
Fig. 1a. Portion of alimentary wall of the same embryo, shewing the formation of the subnotochord rod (x).
Fig. 2. Section through the caudal vesicle of a Pristiurus embryo belonging to stage H. Zeiss C,ocul.1.
It shews the bilobed condition of the alimentary vesicle and the fusion of the mesoblast and hypoblast at the caudal vesicle.
Fig. 3a. Sections from the caudal region of a Pristiurus embryo belonging to stage H. Zeiss C,ocul.1. Picric acid specimen.
It shews the communication which exists posteriorly between the neural and alimentary canals, and also by comparison with 3bit exhibits the dilatation undergone by the alimentary canal in the caudal vesicle.
Fig. 3b. Section from the caudal region of an embryo slightly younger than 3a. Zeiss C,ocul.1. Osmic acid specimen.
Fig. 4. Section from the cardiac region of a Pristiurus embryo belonging to stage H. Zeiss C,ocul.1. Osmic acid specimen.
It shews the formation of the heart (ht) as a cavity between the splanchnopleure and the wall of the throat.
Fig. 5. Section from the posterior dorsal region of a Scyllium embryo, belonging to stage H. Zeiss C,ocul.1. Osmic acid specimen.
It shews the general features of an embryo of stage H, more especially the relations of the body-cavity in the parietal and vertebral portions of the lateral plate, and the early-formed band of muscle (mp´) in the splanchnic layer of the vertebral plate.
Fig. 6. Section from the œsophageal region of Scyllium embryo belonging to stage I. Zeiss C,ocul.1. Chromic acid specimen.
It shews the formation of the rudiments of the posterior nerve-roots (pr) and of the vertebral rudiments (Vr).
Fig. 7. Section of a Torpedo embryo belonging to stage slightly later than I. Zeiss C,ocul.1, reduced 1/3. Osmic acid specimen.
It shews (1) the formation of the anterior and posterior nerve-roots. (2) The solid knob from which the segmental duct (sd) originates.
Fig. 8. Section from the dorsal region of a Scyllium embryo belonging to a stage intermediate between I and K. Zeiss C,ocul.1. Chromic acid specimen.
It illustrates the structure of the primitive ova, segmental tubes, notochord, etc.
Fig. 8a. Section from the caudal region of an embryo of the same age as 8. Zeiss A,ocul.1.
It shews (1) the solid œsophagus. (2) The narrow passage connecting the pericardial (pc) and body cavities (pp).
Fig. 9. Section of a Pristiurus embryo belonging to stage K. Zeiss A,ocul.1. Osmic acid specimen.
It shews the formation of the liver (l), the structure of the anterior fins (b), and the anterior opening of the segmental duct into the body-cavity (sd).
Figs. 9a, 9b, 9c, 9d. Four sections through the anterior region of the same embryo as 9. Osmic acid specimens.
The sections shew (1) the atrophy of the post-anal section of the alimentary tract (9b, 9c, 9d). (2) The existence of the segmental tubes behind the anus (9b, 9c, 9d). With reference to these it deserves to be noted that the segmental tubes behind the anus are quite disconnected, as is proved by the fact that a tube is absent on one side in 9cbut reappears in 9d. (3) The downward prolongation of the segmental duct to join the posterior or cloacal extremity of the alimentary tract (9b).
Plate 12.
Fig. 10. Longitudinal and horizontal section of a Scyllium embryo of stage H. Zeiss C,ocul.1. Reduced by 1/3. Picric acid specimen.
It shews (1) the structure of the notochord; (2) the appearance of the early formed band of muscles (mp´) in the splanchnic layer of the protovertebra.
Fig. 11. Longitudinal and horizontal sections of an embryo belonging to stage I. Zeiss C,ocul.1. Chromic acid specimen. It illustrates the same points as the previous section, but in addition shews the formation of the rudiments of the vertebral bodies (Vr) which are seen to have the same segmentation as the muscle-plates.
Fig. 12.[227]Longitudinal and horizontal section of an embryo belonging to the stage intermediate between I and K. Zeiss C,ocul.1. Osmic acid specimen illustrating the same points as the previous section.
Fig. 13. Longitudinal and horizontal section of an embryo belonging to stage K. Zeiss C,ocul.1, and illustrating same points as previous section.
Figs. 14a, 14b, 14c, 14d. Figures taken from preparations of an embryo of an age intermediate between I and K, and illustrating the structure of the primitive ova. Figs. 14aand 14bare portions of transverse sections. Zeiss C,ocul.3 reduced 1/3. Figs. 14cand 14dare individual ova, shewing the lobate form of nucleus. Zeiss F,ocul.2.
Fig. 15. Osmic acid preparation of primitive ova belonging to stage K. Zeiss immersionNo.2,ocul.1. The protoplasm of the ova is seen to be nearly filled with bodies resembling yolk-spherules: and one ovum is apparently undergoing division.
Fig. 15a. Picric acid preparation shewing a primitive ovum partially filled with bodies resembling yolk-spherules.
Fig. 16. Horizontal and longitudinal section of Scyllium embryo belonging to stage K. Zeiss A,ocul.1. Picric acid preparation. The connective-tissue cells are omitted.
The section shews that there is one segmental tube to each vertebral segment.
Fig. 17. Portion of a Scyllium embryo belonging to stage K, viewed as a transparent object.
It shews the segmental duct and the segmental involutions—two of which are seen to belong to segments behind the end of the alimentary tract.
Fig. 18. Vertical longitudinal section of a Scyllium embryo belonging to stage K. Zeiss A,ocul.1. Hardened in a mixture of osmic and chromic acid. It shews
(1) the commissures connecting together the posterior roots of the spinal nerves;
(2) the junction of the anterior and posterior roots;
(3) the relations of the segmental ducts to the segmental involutions and the alternation of calibre in the segmental tube;
(4) the germinal epithelium lining the body-cavity.
[192]Unless the contrary is stated, the facts recorded in this chapter apply only to the genera Scyllium and Pristiurus.
[193]The layers are known as epidermic (horny) and mucous layers by English writers, and as Hornschicht and Schleimschicht by the Germans. For their existence in all Vertebrates,videLeydigUeber allgemeine Bedeckungen der Amphibien, p. 20. Bonn, 1876.
[194]VideLeydig,loc. cit.
[195]VideGötte,Entwicklungsgeschichte der Unke.
[196]VideSelf,“Development of Spinal Nerves in Elasmobranchii.”Phil. Trans.1876. [This Edition,No.VIII.]
[197]For Birds,videElements of Embryology, Foster and Balfour,pp.144, 145, and for Mammals, Kölliker,Entwicklungsgeschichte, p. 283.
[198]For the nervous supply in fishes,videStannius,Peripher. Nerv. System d. Fische. In Osseous Fishes he states that the thoracic fin is supplied by branches from the first three though sometimes from the first four spinal nerves. In Acipenser there are branches from the first six nerves. In Spinax the limb is supplied by the rami anteriores of the fourth and succeeding ten spinal nerves. In the Rays not only do the sixteen anterior spinal nerves unite to supply the fin, but in all there are rami anteriores from thirty spinal nerves which pass to the thoracic limb.
[199]Philosophical Transactions, 1871.
[200]Ursprung d. Wirbelthiere and Functionswechsels.
[201]Grundriss d. Vergleichenden Anat.p. 494.
[202]Loc. cit.
[203]No attempt has been made to describe in detail the different appearances presented by the protovertebræ in the various parts of the body, but in each stage a protovertebra from the dorsal region is taken as typical.
[204]Zeitschrift f. Anat. Entwicklungsgeschichte,Vol.1.
[205]Hensenloc. cit.
[206]For the history of protovertebræ and muscle-plates in Birds,videElements of Embryology, Foster and Balfour. The statement there made that the horizontal splitting of the mesoblast does not extend to the summit of the vertebral plate, must however be regarded as doubtful.
[207]VideElements of Embryology, p. 56.
[208]Dr Götte,Entwicklungsgeschichte der Unke, p. 534, gives a different account of the development of the protovertebræ from that in the text. He states that the muscle-plates do not give rise to the main dorso-lateral muscles, but only to some superficial ventral muscles, while the dorso-lateral muscles are according to him formed from part of the kernel of the protovertebræ internal to the muscle-plates. The account given in the text is the result of my own investigations, and accords precisely with the recent statements of Professor Kölliker,Entwicklungsgeschichte, 1876.
[209]The type of development of the muscle-plates of Amphibians would become identical with that of Elasmobranchii if their first-formed mass of muscle corresponded with the early-formed muscles of Elasmobranchii, and the remaining cells of both layers of the protovertebræ became in the course of development converted into muscle-cells indistinguishable from those formed at first. Is it possible that, owing to the distinctness of the first-formed mass of muscle, Dr Götte can have overlooked the fact that its subsequent growth is carried on at the expense of the adjacent cells of the somatic layer?
[210]Ehrlich,“Ueber den peripher. Theil d. Urwirbel.”Archiv f. Mic. Anat.Vol.XI.
[211]The most important other instances in addition to that of the Cœlenterata which can be adduced in favour of the epiblastic origin of the mesoblast are the Bird and Mammal, in which according to the recent observations of Hensen for the Mammal, and Kölliker for the Mammal and Bird, the mesoblast is split off from the epiblast. If the views I have elsewhere put forward about the meaning of the primitive groove be accepted, the derivation of the mesoblast from the epiblast in these instances would be apparent rather than real, and have no deep morphological significance for the present question.
Other instances may be brought forward from various groups, but none of these are sufficiently well confirmed to be of any value in the determination of the present question.
[212]VideAnthropogenie, p. 197.
[213]VideSelf,“Development of Elasmobranch Fishes,”Journal ofAnat. and Phys.Vol.X.note onp.682, and also Review of Professor Kölliker's“Entwicklungsgeschichte des Menschen u. d. höheren Thiere,”Journal ofAnat. And Phys.Vol.X.
[214]Professor Haeckel speaks of the splitting of the mesoblast in Vertebrates into a somatic and splanchnic layer as a secondary process (Gastrula u. Eifurchung d. Thiere), but does not make it clear whether he regards this secondary splitting as taking place along the old lines. It appears to me to be fairly certain that even if the original unsplit condition of the mesoblast is to be regarded as a secondary condition, yet that the splitting of this must take place along the old lines, otherwise a change in the position of the body-cavity in the adult would have to be supposed—an unlikely change producing unnecessary complication. The succeeding argument is based on the assumption that the unsplit condition is a secondary condition, but that the split which eventually appears in this occurs along the old lines, separating the primitive splanchnopleure from the primitive somatopleure.
[215]Quart. Jl. of Micros. Science, July, 1875. [This Edition,No.VI.]
[216]Jenaische Zeitschrift,Vol.IX.
[217]Quart. Jl. of Micros.Science,Vol.XXV.1874, andPhil. Trans.1875.
[218]Archives de Zoologie,Vol.IV.
[219]Archiv f. Micr. Anat.Vol.XIII.
[220]The recent researches of Selenka,Zeitschrift f. Wiss. Zoologie,Vol.XXVII.1876, demonstrate that in Echinoderms the muscles are derived from the cells first split off from the hypoblast, and that the diverticula only form the water-vascular system and the epithelial lining of the body-cavity.
[221]Kowalevsky,“Würmer u. Arthropoden,”Mém. Acad. Pétersbourg, 1871.
[222]“Zur Entwicklungsgeschichte d. Brachiopoden”, Protokoll d. ersten Session der Versammlung Russischer Naturforscher in Kasan, 1873. Published inKaiserliche Gesellschaft Moskau, 1874 (Russian). Abstracted in Hoffmann and Schwalbe,Jahresbericht f.1873.
[223]Comparison of Early Stages,Quart. Jl. Micros. Science, July, 1875. [This Edition,No.VI.]
[224]“Urinogenital Organs of Vertebrates,”Journ. of Anat. and Phys.Vol.X.[This Edition,No.VII.]
[225]Entwicklungsgeschichte der Unke,Pl.1, fig. 8.
[226]This membrane is better looked upon, as is done by Gegenbaur and Götte, as intercellular matter.
[227]The apparent structure in the sheath of the notochord in this and the succeeding figure is merely the result of an attempt on the part of the engraver to represent the dark colour of the sheath in the original figure.
External Epiblast.
The change already alluded to in the previous chapter (p.317) by which the external epiblast or epidermis becomes divided into two layers, is completed before the close of stage L.
In the tail region at this stage three distinct strata may be recognized in the epidermis. (1) An outer stratum of flattened horny cells, which fuse together to form an almost continuous membrane. (2) A middle stratum of irregular partly rounded and partly flattened cells. (3) An internal stratum of columnar cells, bounded towards the mesoblast by a distinct basement membrane (Pl.13, fig. 8), unquestionably pertaining to the epiblast. This layer is especially thickened in the terminal parts of the paired fins (Pl.13, fig. 1). The two former of these strata together constitute the epidermic layer of the skin, and the latter the mucous layer.
In the anterior parts of the body during stage L the skin only presents two distinct strata,viz.an inner somewhat irregular layer of rounded cells, the mucous layer, and an outer layer of flattened cells (Pl.13, fig. 8).
The remaining history of the external epiblast, consisting as it does of a record of the gradual increase in thickness of the epidermic strata, and a topographical description of its variations in structure and thickness in different parts, is of no special interest and need not detain us here.
In the late embryonic periods subsequent to stage Q the layers of the skin cease to be so distinct as at an earlier period,partly owing to the innermost layer becoming less columnar, and partly to the presence of a large number of mucous cells, which have by that stage made their appearance.
I have followed with some care the development of the placoid scales, but my observations so completely accord with those of Dr O. Hertwig[228], that it is not necessary to record them. The so-called enamel layer is a simple product of the thickening and calcification of the basement membrane, and since this membrane is derived from the mucous layer of the epidermis, the enamel is clearly to be viewed as an epidermic product. There is no indication of a gradual conversion of the bases of the columnar cells forming the mucous layer of the epidermis into enamel prisms, as is frequently stated to occur in the formation of the enamel of the teeth in higher Vertebrates.
Lateral line.
The lateral line and the nervous structures appended to it have been recently studied from an embryological point of view by Götte[229]in Amphibians and by Semper[230]in Elasmobranchii.
The most important morphological result which these two distinguished investigators believe themselves to have arrived at is the direct derivation of the lateral nerve from the ectoderm. On this point there is a complete accord between them, and Semper especially explains that it is extremely easy to establish the fact.
As will appear from the sequel, I have not been so fortunate as Semper in elucidating the origin of the lateral nerve, and my observations bear an interpretation not in the least in accordance with the views of my predecessors, though not perhaps quite conclusive against them.
It must be premised that two distinct structures have to be dealt with,viz.thelateral lineformed of modified epidermis, and thelateral nervewhose origin is in question.
The lateral line is the first of the two to make its appearance, at a stage slightly subsequent to K, in the form of alinear thickening of the inner row of cells of the external epiblast, on each side, at the level of the notochord.
This thickening, in my youngest embryo in which it is found, has but a very small longitudinal extension, being present through about 10 thin sections in the last part of the head and first part of the trunk. The thickening, though short, is very broad, measuring about 0.28Mm.in transverse section, and presents no signs of a commencing differentiation of nervous structures. The large intestinal branch of the vagus can be seen in all the anterior sections in close proximity to this line, and appears to me to give off to it posteriorly a small special branch which can be traced through a few sections,videPl.13, fig. 2,n.l. But this branch is not sufficiently well marked to enable me to be certain of its real character. In any case the posterior part of the lateral lineis absolutely without any adjoining nervous structures or traces of such.
The rudiment of the epidermic part of the lateral line is formed of specially elongated cells of the mucous layer of the epiblast, but around the bases of these certain rounder cells of a somewhat curious appearance are intercalated.
There is between this and my next youngest embryo an unfortunately large gap with reference to the lateral line, although in almost every other respect the two embryos might be regarded as belonging to the same stage. The lateral line in the older embryo extends from the hind part of the head to a point well behind the anus, and is accompanied by a nerve for at least two-thirds of its length.
In the foremost section in which it appears the intestinal branch of the vagus is situated not far from it,and may be seen at intervals giving off branches to it. There is no sign that these are otherwise than perfectly normal branches of the vagus. Near the level of the last visceral cleft the intestinal branch of the vagus gives off a fair-sized branch, which from the first occupies a position close to the lateral line though well within the mesoblast (Pl.13, fig. 3a,n.l). This branch is the lateral nerve, and though somewhat larger, is otherwise much like the nerve I fancied I could see originating from the intestinal branch of the vagus during the previous stage.
It rapidly thins out posteriorly and also approaches closerand closer to the lateral line. At the front end of the trunk it is quite in contact with it, and a short way behind this region the cells of the lateral line arrange themselves in a gable-like form, in the angle of which the nerve is situated (Pl.13, figs. 3b, and 3c). In this position the nerve though small is still very distinct in all good sections, and is formed of a rod of protoplasm, with scattered nuclei, in which I could not detect a distinct indication of cell-areas. The hinder part of the nerve becomes continually smaller and smaller, without however presenting any indication of becoming fused with the epiblast, and eventually ceases to be visible some considerable distance in front of the posterior end of the lateral line.
The lateral line itself presents some points of not inconsiderable interest. In the first place, it is very narrow anteriorly and throughout the greater part of its length, but widens out at its hinder end, and is widest of all at its termination, which is perfectly abrupt. The following measurements of it were taken from an embryo belonging to stage L, which though not quite my second youngest embryo is only slightly older. At its hinder end it was 0.17Mm.broad. At a point not far from this it was 0.09Mm.broad, and anteriorly it was 0.05Mm.broad. These measurements clearly shew that the lateral line is broadest at what may be called its growing-point, a fact which explains its extraordinary breadth in the anterior part of the body at my first stage,viz.0.28Mm., a breadth which strangely contrasts with the breadth,viz.0.05Mm., which it has in the same part of the body at the present stage.
It still continues to form a linear area of modified epidermis, and has no segmental characters. Anteriorly it is formed by the cells of the mucous layer becoming more columnar (Pl.13 fig. 3a). In its middle region the cells of the mucous layer in it are still simply elongated, but, as has been said above, have a gable-like arrangement, so as partially to enclose the nerve (Pl.13, fig. 3b). Nearer the hind end of the trunk a space appears in it between its columnar cells and the flattened cells of the outermost layer of the skin (Pl.13, fig. 3c), and this space becomes posteriorly invested by a very definite layer of cells. The space (Pl.13, fig. 3d) or lumen has a slit-like section, and is not formed by the closing in of an originally open groove, but bythe formation of a cavity in the midst of the cells of the lateral line. Its walls are formed by a layer of columnar cells on the inner side, and flattened cells on the outer side, both layers however appearing to be derived from the mucous layer of the epidermis. The outer layer of cells attains its greatest thickness dorsally.
During stages M, N, O, the lateral nerve gradually passes inwards into the connective tissue between the dorso-lateral and the ventro-lateral muscles, and becomes even before the close of stage N completely isolated from the lateral line.
The growth of the lateral line itself remains for some time almost stationary; anteriorly the cells retain the gable-like arrangement which characterised them at an earlier period, but cease to enclose the nerve; posteriorly the line retains its original more complicated constitution as a closed canal. In stage O the cells of the anterior part of the line, as well as those of the posterior, commence to assume a tubular arrangement, and the lateral line takes the form of a canal. The tubular form is due to a hollowing out of the lateral line itself and a rearrangement of its cells. As the lateral line becomes converted into a canal it recedes from the surface.
In stage P the first indication of segmental apertures to the exterior make their appearance,videPl.13, fig. 4. The lateral line forms a canal situated completely below the skin, but at intervals (corresponding with segments) sends upwards and outwards prolongations towards the exterior. These prolongations do not during stage P acquire external openings. As is shewn in my figure, a special area of the inner border of the canal of the lateral line becomes distinguished by its structure from the remainder.
No account of the lateral line would be complete without some allusion to the similar sensory structures which have such a wide distribution on the heads of Elasmobranchii; and this is especially important in the present instance, owing to the light thrown by a study of their development on the origin of the nerves which supply the sense-organs of this class. The so-called mucous canals of the head originate in the same way as does the lateral line; they are products of the mucous layer of the epidermis. They eventually form either canals with numerousopenings to the exterior, or isolated tubes with terminal ampulliform dilatations.
I have not definitely determined whether the canal-system of the head arises in connection with the lateral line, or only eventually becomes so connected. The important point to be noticed is, that at first no nervous structures are to be seen in connection with it. In stage O nerves for the mucous canals make their appearance as delicate branches of the main stems. These nerve-stems are very much ramified, and their branches have, in a large number of instances, an obvious tendency towards a particular sense-organ (Pl.13, figs. 5 and 6).
I have not during stage O been able to detect a case of direct continuity between the two. This is, however, established in the succeeding stage P, in the case of the canals, and the facility with which it may be observed would probably render the embryo Elasmobranch a very favourable object for studying the connection between nerves and terminal sense-organs. The nerve (Pl.13, fig. 7) dilates somewhat before uniting with the sense-organ, and the protoplasm of the nerve and the sense-organ become completely fused. The basement membrane of the skin is not continuous across their point of junction, and appears to unite with a delicate membrane-like structure, which invests the termination of the nerve. The ampullæ would seem to receive their nervous supply somewhat later than the canals, and the terminal swellings of the nerves supplying them are larger than in the case of the canals, and the connection between the ampullæ and the nerves not so clear. In the case of the head, there can for Elasmobranchii be hardly a question that the nerves which supply the mucous canals grow centrifugally from the original cranial nerve-stems, and do not originate in a peripheral manner from the integument.
This is an important point to make certain of in settling any doubtful features in the nervous supply of the lateral line. Professor Semper[231], with whom as dealing with Elasmobranchii we are more directly concerned, makes the following statement:“At the time when at the front end the lateral nerve has already completely separated itself from the ectoderm, and is situated amongst the muscles, it still lies in the middle of the body closeto the ectoderm, and at the hind end of the body is not yet completely segmented off (abgegliedert) from the ectoderm.”Although the last sentence of this quotation may seem to be opposed to my statements, yet it appears to me probable that Professor Semper has merely seen the lateral nerve partially enclosed in the ectoderm. This position of the nerve no doubt affords apresumption, but only a presumption, in favour of a direct origin of the lateral nerve from the ectoderm; but against this interpretation of it are the following facts: