The hypoblast has undergone no changes of importance.
The layers at the posterior end of the embryo retain the characters of the last stage.
Sixth day after impregnation.—At this stage (Plate 34, fig. 8) the embryo is considerably more advanced than at the last stage. The trunk has decidedly increased in length, and the head forms a relatively smaller portion of the whole. The regions of the brain are more distinct. The optic vesicles (op.) have grown outwards so as to nearly reach the edges of the area which forms the parietal part of the body. The fore-brain projects slightly in front, and the mid-brain is seen as a distinct rounded prominence. Behind the latter is placed the hind-brain, which passes insensibly into the spinal cord. On either side of the mid- and hind-brain a small region is slightly marked off from the rest of the parietal part, and on this are seen two more or less transversely directed streaks, which, by comparison with the Sturgeon[502], we are inclined to regard as the two first visceral clefts (br.c.). We have, however, failed to make them out in sections, and owing to the insufficiency of our material, we have not even studied them in surface views as completely as we could have wished.
The body is now laterally compressed, and more decidedly raised from the yolk than in the previous stages. In the lateral regions of the trunk the two segmental or archinephric ducts (sg.) are visible in surface views: the front end of each is placed at the level of the hinder border of the head, and is marked by a flexure inwards towards the middle line. The remainder of each duct is straight, and extends backwards for about half the length of the embryo. The tail has much the same appearance as in the last stage.
The vertebral regions of the mesoblastic plates are now segmented for the greater part of the length of the trunk, and thesomites of which they are composed (Plate 36, fig. 30,pr.) are very conspicuous in surface views.
Our sections of this stage are not so complete as could be desired: they shew, however, several points of interest.
The central canal of the nervous system is large, with well-defined walls, and in hardened specimens is filled with a coagulum. It extends nearly to the region of the tail.
The optic vesicles, which are so conspicuous in surface views, appear in section (Plate 35, fig. 26,op.) as knob-like outgrowths of the fore-brain, and very closely resemble the figures given by Oellacher of these vesicles in Teleostei[503].
From the analogy of the previous stage, we are inclined to think that they have a lumen continuous with that of the fore-brain. In our only section through them, however, they are solid, but this is probably due to the section merely passing through them to one side.
The auditory pits (Plate 35, fig. 27,au.) are now well marked, and have the form of somewhat elongated grooves, the walls of which are formed of a single layer of columnar cells belonging to the nervous layer of the epidermis, and extending inwards so as nearly to touch the brain.
In an earlier stage it was pointed out that the dorsal part of the medullary keel was different in its structure from the remainder, and that it was destined to give rise to the nerves. The process of differentiation is now to a great extent completed, and may best be seen in the auditory region (Plate 35, fig. 27, VIII.). In this region there was present during the last stage a great rhomboidal mass of cells at the dorsal region of the brain (Plate 35, fig. 24, VIII.). In the present stage, this, which is the rudiment of the seventh and auditory nerves, is seen growing down on each side from the roof of the hind-brain, between the brain and the auditory involution, and abutting against the wall of the latter.
Rudiments of the spinal nerves are also seen at intervals as projections from the dorsal angles of the spinal cord (Plate 36, fig. 29,sp.n.). They extend only for a short distance outwards, gradually tapering off to a point, and situatedbetween the epiblast and the dorsal angles of the mesoblastic somites.
The process of formation of the cranial nerves and dorsal roots of the spinal nerves is, it will be seen, essentially the same as that already known in the case of Elasmobranchii, Aves,&c.The nerves arise as outgrowths of a special crest of cells, theneural crestof Marshall, which is placed along the dorsal angle of the cord. The peculiar position of the dorsal roots of the spinal nerves is also very similar to what has been met with in the early stages of these structures by Marshall in Birds[504], and by one of us in Elasmobranchii[505].
In the parietal region a cavity has now appeared in part of the trunk between the splanchnic and somatic layers of the mesoblast (Plate 36, fig. 29,b.c.), the somatic layer (so.) consisting of a single row of columnar cells on the dorsal side, while the remainder of each somite is formed of the splanchnic layer (sp.). In many of the sections the somatic layer is separated by a considerable interval from the epiblast.
We have been able to some extent to follow the development of the segmental duct. The imperfect preservation of our specimens has, as in other instances, rendered the study of the point somewhat difficult, but we believe that the figure representing the development of the duct some way behind its front end (Plate 36, fig. 29) is an accurate representation of what may be seen in a good many of our sections.
It appears from these sections that the duct (Plate 36, fig. 29,sg.) is developed as a hollow ridge-like outgrowth of the somatic layer of mesoblast, directed towards the epiblast, in which it causes a slight bulging. The cavity of the ridge freely communicates with the body-cavity. The anterior part of this ridge appears to be formed first. Very soon, in fact, in an older embryo belonging to this stage, the greater part of the groove becomes segmented off as a duct lying between the epiblast and somatic mesoblast (Plate 36, fig. 28,sg.), while the front end still remains, as we believe, in communication with the body-cavity by an anterior pore.
This mode of development corresponds in every particular with that observed in Teleostei by Rosenberg and Oellacher.
The structure of the notochord (nc.) at this stage is very similar to that observed by one of us in Elasmobranchii[506]. The cord is formed of transversely arranged flattened cells, the outer parts of which are vacuolated, while the inner parts are granular, and contain the nuclei. This structure gives rise to the appearance in transverse sections of an axial darker area and a peripheral lighter portion.
The hypoblast retains for the most part its earlier constitution, but underneath the notochord, in the trunk, it is somewhat thickened, and the cells at the two sides spread in to some extent under the thickened portion (Plate 36, fig. 29,s.nc.). This thickening, as is shewn in transverse sections at the stage when the segmental duct becomes separated from the somatic mesoblast (Plate 36, fig. 28,s.nc.), is the commencement of the subnotochordal rod.
The tail end of the embryo still retains its earlier characters.
Seventh day after impregnation.—Our series of specimens of this stage is very imperfect, and we are only able to call attention to the development of a certain number of organs.
Our sections clearly establish the fact that the optic vesicles are now hollow processes of the fore-brain. Their outer ends are dilated, and are in contact with the external skin. The formation of the optic cup has not, however, commenced. The nervous layer of the skin adjoining the outer wall of the optic cup is very slightly thickened, constituting the earliest rudiment of the lens.
In one of our embryos of this day the developing auditory vesicle still has the form of a pit, but in the other it is a closed vesicle, already constricted off from the nervous layer of the epidermis.
With reference to the development of the excretory duct we cannot add much to what we have already stated in describing the last stage.
The duct is considerably dilated anteriorly (Plate 36, fig. 31,sg.); but our sections throw no light on the nature of the abdominal pore. The posterior part of the duct has still the formof a hollow ridge united with somatic mesoblast (Plate 36, fig. 32,sg.).
During this stage, the embryo becomes to a small extent folded off from the yolk-sack both in front and behind, and in the course of this process the anterior and posterior extremities of the alimentary tract become definitely established.
We have not got as clear a view of the process of formation of these two sections of the alimentary tract as we could desire, but our observations appear to shew that the process is in many respects similar to that which takes place in the formation of the anterior part of the alimentary tract in Elasmobranchii[507]. One of us has shewn that in Elasmobranchii the ventral wall of the throat is formednotby a process of folding in of the hypoblastic sheet as in Birds, but by a growth of the ventral face of the hypoblastic sheet on each side of and at some little distance from the middle line. Each growth is directed inwards, and the two eventually meet and unite, thus forming a complete ventral wall for the gut. Exactly the same process would seem to take place inLepidosteus, and after the lumen of the gut is in this way established, a process of mesoblast on each side also makes its appearance, forming a mesoblastic investment on the ventral side of the alimentary tract. Some time after the alimentary tract has been thus formed, the epiblast becomes folded in, in exactly the same manner as in the Chick, the embryo becoming thereby partially constricted off from the yolk (Plate 36, figs. 33, 34).
The form of the lumen of the alimentary tract differs somewhat in front and behind. In front, the hypoblastic sheet remains perfectly flat during the formation of the throat, and thus the lumen of the latter has merely the form of a slit. The lumen of the posterior end of the alimentary tract is, however, narrower and deeper (Plate 36, figs. 33, 34,al.). Both in front and behind, the lateral parts of the hypoblastic sheet become separated from the true alimentary tract as soon as the lumen of the latter is established.
It is quite possible that at the extreme posterior end of the embryo a modification of the above process may take place, forin this region the hypoblast appears to us to have the form of a solid cord.
We could detect no true neurenteric canal, although a more or less complete fusion of the germinal layers at the tail end of the embryo may still be traced.
During this stage the protoplasm of the notochordal cells, which in the last stage formed a kind of axial rod in the centre of the notochord, begins to spread outwards toward the sheath of the notochord.
Eighth day after impregnation.—The external form of the embryo (Plate 34, fig. 9) shews a great advance upon the stage last figured. Both head and body are much more compressed laterally and raised from the yolk, and the head end is folded off for some distance. The optic vesicles are much less prominent externally. A commencing opercular fold is distinctly seen. Our figure of this stage is not, however, so satisfactory as we could wish.
A thickening of the nervous layer of the external epiblast which will form the lens (Plate 36, fig. 35,l.) is more marked than in the last stage, and presses against the slightly concave exterior wall of the optic vesicle (op.). The latter has now a large cavity, and its stalk is considerably narrowed.
The auditory vesicles (Plate 36, fig. 36,au.) are closed, appearing as hollow sacks one on each side of the brain, and are no longer attached to the epiblast.
The anterior opening of the segmental duct can be plainly seen close behind the head. The lumen of the duct is considerably larger.
The two vertebral portions of the mesoblast are now separated by a considerable space from the epiblast on one side and from the notochord on the other, and the cells composing them have become considerably elongated from side to side (Plate 36, fig. 37,ms.).
In some sections the aorta can be seen (Plate 36, fig. 37,ao.) lying close under the subnotochordal rod, between it and the hypoblast, and on either side of it a slightly larger cardinal vein (cd.v.).
The protoplasm of the notochord has now again retreated towards the centre, shewing a clear space all round. This ismost marked in the region of the trunk (Plate 36, fig. 37). The subnotochordal rod (s.nc.) lies close under it.
A completely closed fore-gut, lined by thickened hypoblast, extends about as far back as the auditory sacks (Plate 36, figs. 35 and 36,al.). In the trunk the hypoblast, which will form the walls of the alimentary tract, is separated from the notochord by a considerable interval.
Ninth day after impregnation: External characters.—Very considerable changes have taken place in the external characters of the embryo. It is about 8millims.in length, and has assumed a completely piscine form. The tail especially has grown in length, and is greatly flattened from side to side: it is wholly detached from the yolk, and bends round towards the head, usually with its left side in contact with the yolk. It is provided with well-developed dorsal and ventral fin-folds, which meet each other round the end of the tail, the tail fin so formed being nearly symmetrical. The head is not nearly so much folded off from the yolk as the tail. At its front end is placed a disc with numerous papillæ, of which we shall say more hereafter. This disc is somewhat bifid, and is marked in the centre by a deep depression.
Dorsal to it, on the top of the head, are two widely separated nasal pits. On the surface of the yolk, in front of the head, is to be seen the heart, just as in Sturgeon embryos. Immediately below the suctorial disc is a slit-like space, forming the mouth. It is bounded below by the two mandibular arches, which meet ventrally in the median line. A shallow but well-marked depression on each side of the head indicates the posterior boundary of the mandibular arch. Behind this is placed the very conspicuous hyoid arch with its rudimentary opercular flap; and in the depression, partly covered over by the latter, may be seen a ridge, the external indication of the first branchial arch.
Eleventh day after impregnation: External characters.—The embryo (Plate 34, fig. 10) is now about 10millims.in length, and in several features exhibits an advance upon the embryo of the previous stage.
The tail fin is now obviously not quite symmetrical, and the dorsal fin-fold is continued for nearly the whole length of the trunk. The suctorial disc (Plate 34, fig. 11,s.d.) is much moreprominent, and the papillæ (about 30 in number) covering it are more conspicuous from the surface. It is not obviously composed of two symmetrical halves. The opercular flap is larger, and the branchial arches behind it (two of which may be made out without dissection) are more prominent.
The anterior pair of limbs is now visible in the form of twolongitudinalfolds projecting in a vertical direction from the surface of the yolk-sack at the sides of the body.
The stages subsequent to hatching have been investigated with reference to the external features and to the habits by Agassiz, and we shall enrich our own account by copious quotations from his memoir.
He states that the first batch were hatched on the eighth[508]day after being laid.“The young Fish possessed a gigantic yolk-bag, and the posterior part of the body presented nothing specially different from the general appearance of a Teleostean embryo, with the exception of the great size of the chorda. The anterior part, however, was most remarkable; and at first, on seeing the head of this youngLepidosteus, with its huge mouth-cavity extending nearly to the gill-opening, and surmounted by a hoof-shaped depression edged with a row of protuberances acting as suckers, I could not help comparing this remarkable structure, so utterly unlike anything in Fishes or Ganoids, to the Cyclostomes, with which it has a striking analogy. This organ is also used byLepidosteusas a sucker, and the moment the young Fish is hatched he attaches himself to the sides of the disc, and there remains hanging immovable; so firmly attached, indeed, that it requires considerable commotion in the water to make him loose his hold. Aërating the water by pouring it from a height did not always produce sufficient disturbance to loosen the young Fishes. The eye, in this stage, is rather less advanced than in corresponding stages in bony Fishes; the brain is also comparatively smaller, the otolith ellipsoidal, placed obliquely in the rear above the gill-opening.... Usually the gill-cover is pressed closely against the sides of the body, but in breathing an opening is seen through which water is constantly passing, astrong current being made by the rapid movement of the pectorals, against the base of which the extremity of the gill-cover is closely pressed. The large yolk-bag is opaque, of a bluish-gray colour. The body of the youngLepidosteusis quite colourless and transparent. The embryonic fin is narrow, the dorsal part commencing above the posterior end of the yolk-bag; the tail is slightly rounded, the anal opening nearer the extremity of the tail than the bag. The intestine is narrow, and the embryonic fin extending from the vent to the yolk-bag is quite narrow. In a somewhat more advanced stage,—hatched a few hours earlier,—the upper edge of the yolk-bag is covered with black pigment cells, and minute black pigment cells appear on the surface of the alimentary canal. There are no traces of embryonic fin-rays either in this stage or the one preceding; the structure of the embryonic fin is as in bony Fishes—previous to the appearance of these embryonic fin-rays—finely granular. Seen in profile, the yolk-bag is ovoid; as seen from above, it is flattened, rectangular in front, with rounded corners, tapering to a rounded point towards the posterior extremity, with re-entering sides.”
We have figured an embryo of 11millims.in length, shortly after hatching (Plate 34, fig. 12), the most important characters of which are as follows:—The yolk-sack, which has now become much reduced, forms an appendage attached to the ventral surface of the body, and has a very elongated form as compared with its shape just before hatching. The mouth, as also noticed by Agassiz, has a very open form. It is (Plate 34, fig. 13,m.) more or less rhomboidal, and is bounded behind by the mandibular arch (mn.) and laterally by the superior maxillary processes (s.mx). In front of the mouth is placed the suctorial disc (s.d.), the central papillæ of which are arranged in groups. The opercular fold (h.op.) is very large, covering the arches behind. A well-marked groove is present between the mandibular and opercular arches, but so far as we can make out it is not a remnant of the hyomandibular cleft.
The pectoral fins (Plate 34, fig. 12,pc.f.) are very prominent longitudinal ridges, which, owing to their being placed on the surface of the yolk-sack, project in a nearly vertical direction: a feature which is also found in many Teleostean embryos with large yolk-sacks.
No traces of the pelvic fins have yet become developed.
The positions of the permanent dorsal, anal, and caudal fins, as pointed out by Agassiz, are now indicated by a deposit of pigment in the embryonic fin.
In an embryo on the sixth day after hatching, of about 15millims.in length, of which we have also given a figure (Plate 34, fig. 14), the following fresh features deserve special notice.
In the region of the head there is a considerable elongation of the pre-oral part, forming a short snout, at the end of which is placed the suctorial disc. At the sides of the snout are placed the nasal pits, which have become somewhat elongated anteriorly.
The mouth has lost its open rhomboidal shape, and has become greatly narrowed in an antero-posterior direction, so that its opening is reduced to a slit. The mandibles and maxillary processes are nearly parallel, though both of them are very much shorter than in the adult. The operculum is now a very large flap, and has extended so far backwards as to cover the insertion of the pectoral fin. The two opercular folds nearly meet ventrally.
The yolk-sack is still more reduced in size, one important consequence of which is that the pectoral fins (pc.f.) appear to spring out more or less horizontally from the sides of the body, and at the same time their primitive line of attachment to the body becomes transformed from a longitudinal to a more or less transverse one.
The first traces of the pelvic fins are now visible as slight longitudinal projections near the hinder end of the yolk-sack (pl.f.).
The pigmentation marking the regions of the permanent fins has become more pronounced, and it is to be specially noted that the ventral part of the caudal fin (the permanent caudal) is considerably more prominent than the dorsal fin opposite to it.
The next changes, as Agassiz points out,“are mainly in the lengthening of the snout; the increase in length both of the lower and upper jaw; the concentration of the sucker of the sucking disc; and the adoption of the general colouring of somewhat older Fish. The lobe of the pectoral has become specially prominent, and the outline of the fins is now indicated by a fine milky granulation. Seen from above, the gill-cover isseen to leave a large circular opening leading to the gill-arches, into which a current of water is constantly passing, by the lateral expansion and contraction of the gill-cover; the outer extremity of the gill-cover covers the base of the pectorals. In a somewhat older stage the snout has become more elongated, the sucker more concentrated, and the disproportionate size of the terminal sucking-disc is reduced; the head, when seen from above, becoming slightly elongated and pointed.”
In a larva of about 18 days old and 21millims.in length, of which we have not given a figure, the snout has grown greatly in length, carrying with it the nasal organs, the openings of which now appear to be divided into two parts. The suctorial disc is still a prominent structure at the end of the snout. The lower jaw has elongated correspondingly with the upper, so that the gape is very considerable, though still very much less than in the adult.
The opercular flaps overlap ventrally, the left being superficial. They still cover the bases of the pectoral fins. The latter are described by Agassiz as being“kept in constant rapid motion, so that the fleshy edge is invisible, and the vibration seems almost involuntary, producing a constant current round the opening leading into the cavity of the gills.”
The pelvic fins are somewhat more prominent.
The yolk-sack, as pointed out by Agassiz, has now disappeared as an external appendage.
After the stage last described the young Fish rapidly approaches the adult form. To shew the changes effected we have figured the head of a larva of about a month old and 23millims.in length (Plate 34, fig. 15). The suctorial disc, though much reduced, is still prominent at the end of the snout. Eventually, as shewn by Agassiz, it forms the fleshy globular termination of the upper jaw.
The most notable feature in which the larva now differs in its external form from the adult is in the presence of an externally heterocercal tail, caused by the persistence of the primitive caudal fin as an elongated filament projecting beyond the permanent caudal (Plate 41, fig. 68).
Delicate dermal fin-rays are now conspicuous in the peripheral parts of all the permanent fins. These rays closelyresemble the horny fin-rays in the fins of embryo Elasmobranchii in their development and structure. They appear gradually to enlarge to form the permanent rays, and we have followed out some of the stages of their growth, which is in many respects interesting. Our observations are not, however, complete enough to publish, and we can only say here that their early development and structure proves their homology with the horny fibres or rays in fins of Elasmobranchii. The skin is still, however, entirely naked, and without a trace of its future armour of enamelled scales.
The tail of a much older larva, 11centims.in length, in which the scales have begun to be formed, is shewn in Plate 34, fig. 16.
We complete this section of our memoir by quoting the following passages from Agassiz as to the habits of the young fish at the stages last described:—
“In the stages intervening between plate iii, fig. 19, and plate iii, fig. 30, the youngLepidosteusfrequently swim about, and become readily separated from their point of attachment. In the stage of plate iii, fig. 30, they remain often perfectly quiet close to the surface of the water; but, when disturbed, move very rapidly about through the water.... The young already have also the peculiar habit of the adult of coming to the surface to swallow air. When they go through the process under water of discharging air again they open their jaws wide, and spread their gill-covers, and swallow as if they were choking, making violent efforts, until a minute bubble of air has become liberated, when they remain quiet again. The resemblance to a Sturgeon in the general appearance of this stage of the youngLepidosteusis quite marked.”
[499]The numbers refer to the list of memoirs of the anatomy and development given at the end of this memoir.
[500]We have examined the structure of the ovarian ova in order to throw light on the nature of these peculiar pyriform bodies. Unfortunately, the ovaries of our adult examples ofLepidosteuswere so badly preserved, that we could not ascertain anything on this subject. The ripe ova in the ovary have an investment of pyriform bodies similar to those of the just laid ova. With reference to the structure of the ovarian ova we may state that the germinal vesicles are provided with numerous nucleoli arranged in close proximity with the membrane of the vesicle.
[501]“Ueb. d. Entwick. d. Central Nerven Systems d. Teleostier,”Archiv für mikr. Anat.Vol.XV.1878.
[502]Salensky,“Recherches s. le Développement du Sterlet.”Archives de Biol.Vol.II.1881, pl.XVII.fig. 27.
[503]“Beiträge zur Entwick. d. Knochenfische,”Zeit. f. wiss. Zool.Vol.XXIII.1873,taf.III.fig.IX.2.
[504]Journal ofAnat. and Physiol.Vol.XI.p. 491, platesXX.andXXI.
[505]“Elasmobranch Fishes,”p. 156, plates 10 and 13. [This edition, p.378, pl. 11, 14.]
[506]“Elasmobranch Fishes,”p. 136, plate 11, fig. 10. [This edition, p.354, pl. 12.]
[507]F. M. Balfour,“Monograph on the Development of Elasmobranch Fishes,”p. 87, plate 9, fig. 2. [This edition, p.303, pl. 10.]
[508]This statement of Agassiz does not correspond with the dates on the specimens sent to us—a fact no doubt due to the hatching not taking place at the same time for all the larvæ.
I.Anatomy.
The brain ofLepidosteushas been figured by Busch (whose figure has been copied by Miklucho-Maclay, and apparently by Huxley), by Owen and by Wilder (No.15). The figure of the latter author, representing a longitudinal section through thebrain, is the most satisfactory, the other figures being in many respects inaccurate; but even Wilder's figure and description, though taken from the fresh object, appear to us in some respects inadequate. He offers, moreover, fresh interpretations of certain parts of the brain which we shall discuss in the sequel.
We have examined two brains which, though extremely soft, were, nevertheless, sufficiently well preserved to enable us to study the external form. We have, moreover, made a complete series of transverse sections through one of the brains, and our sections, though utterly valueless from a histological point of view, have thrown some light on the topographical anatomy of the brain.
Plate 38, figs. 47A, B, and C, represent three views of the brain,viz.: from the side, from above, and from below. We will follow in our description the usual division of the brain into fore-brain, mid-brain, and hind-brain.
The fore-brain consists of an anterior portion forming the cerebrum, and a posterior portion constituting the thalamencephalon.
The cerebrum at first sight appears to be composed of (a) a pair of posterior and somewhat dorsal lobes, forming what have usually been regarded as the true cerebral hemispheres, but called by Wilder the prothalami, and (b) a pair of anterior and ventral lobes, usually regarded as the olfactory lobes, from which the olfactory nerves spring. Mainly from a comparison with our embryonic brains described in the sequel, we are inclined to think that the usual interpretations are not wholly correct, but that the true olfactory lobes are to be sought for in small enlargements (Plate 38. figs. 47A, B, and C,olf.) at the front end of the brain[509]from which the olfactory nerves spring. The cerebrum proper would then consist of a pair of anterior and ventral lobes (ce.), and of a pair of posterior lobes (ce´.), both pairs uniting to form a basal portion behind.
The two pairs of lobes probably correspond with the two parts of the cerebrum of the Frog, the anterior of which, like that ofLepidosteus, was held to be the olfactory lobe, till Götte's researches shewed that this view was not tenable.
The anterior lobes of the cerebrum have a conical form, tapering anteriorly, and are completely separated from each other. The posterior lobes, as is best shewn in side views, have a semicircular form. Viewed from above they appear as rounded prominences, and their dorsal surface is marked by two conspicuous furrows (Plate 38, fig. 47B,ce´.), which have been noticed by Wilder, and are similar to those present in many Teleostei. Their front ends overhang the base of the anterior cerebral lobes. The basal portion of the cerebrum is an undivided lobe, the anterior wall of which forms the lamina terminalis.
What we have above described as the posterior cerebral lobes have been described by Wilder as constituting the everted dorsal border of the basal portion of the cerebrum.
The portion of the cerebro-spinal canal within the cerebrum presents certain primitive characters, which are in some respects dissimilar to those of higher types, and have led Wilder to hold the posterior cerebral lobes, together with what we have called the basal portion of the cerebrum, to be structures peculiar to Fishes, for which he has proposed the name“prothalami.”
In the basal portion of the cerebrum there is an unpaired slit-shaped ventricle, the outer walls of which are very thick. It is provided with a floor formed of nervous matter, in part of which, judging from Wilder's description, a well-marked commissure is placed. We have found in the larva a large commissure in this situation (Plate 37, figs. 44 and 45,a.c.); and it may be regarded as the homologue of the anterior commissure of higher types. This part of the ventricle is stated by Wilder to be without a roof. This appears to us highly improbable. We could not, however, determine the nature of the roof from our badly preserved specimens, but if present, there is no doubt that it is extremely thin, as indeed it is in the larva (Plate 37, fig. 46B). In a dorsal direction the unpaired ventricle extends so as to separate the two posterior cerebral lobes. Anteriorly the ventricle is prolonged into two horns, which penetrate for a short distance, asthe lateral ventricles, into the base of the anterior cerebral lobes. The front part of each anterior cerebral lobe, as well as of the whole of the posterior lobes, appears solid in our sections; but Wilder describes the anterior horns of theventricle as being prolonged for the whole length of the anterior lobes.
In the embryos of all Vertebrates the cerebrum is not at first divided into two lobes, so that the fact of the posterior part of the cerebrum inLepidosteusand probably other Ganoids remaining permanently in the undivided condition does not appear to us a sufficient ground for giving to the lobes of this part of the cerebrum the special name of prothalami, as proposed by Wilder, or for regarding them as a section of the brain peculiar to Fishes.
The thalamencephalon (th.) contains the usual parts, but is in[TN17]some respects peculiar. Its lateral walls, forming the optic thalami, are thick, and are not sharply separated in front from the basal part of the cerebrum; between them is placed the third ventricle. The thalami are of considerable extent, though partially covered by the optic lobes and the posterior lobes of the cerebrum. They are not, however, relatively so large as in other Ganoid forms, more especially the Chondrostei andPolypterus.
On the roof of the thalamencephalon is placed a large thin-walled vesicle (Plate 38, figs. 47A and B,v.th.), which undoubtedly forms the most characteristic structure connected with this part of the brain. Owing to the wretched state of preservation of the specimens, we have found it impossible to determine the exact relations of this body to the remainder of the thalamencephalon; but it appears to be attached to the roof of the thalamencephalon by a narrow stalk only. It extends forwards so as to overlap part of the cerebrum in front, and is closely invested by a highly vascular layer of the pia mater.
No mention is made by Wilder of this body; nor is it represented in his figures or in those of the other anatomists who have given drawings of the brain ofLepidosteus. It might at first be interpreted as a highly-developed pineal gland, but a comparison with the brain of the larva (videp.764) shews that this is not the case, but that the body in question is represented in the larva by a special outgrowth of the roof of the thalamencephalon. The vesicle of the roof of the thalamencephalon is therefore to be regarded as a peculiar development of the tela choroidea of the third ventricle.
How far this vesicle has a homologue in the brains of other Ganoids is not certain, since negative evidence on this subject is all but valueless. It is possible that a vesicular sack covering over the third ventricle of the Sturgeon described by Stannius[510], and stated by him to be wholly formed of the membranes of the brain, is really the homologue of our vesicle.
Wiedersheim[511]has recently described inProtopterusa body which is undoubtedly homologous with our vesicle, which he describes in the following way:—
“Dorsalwärts ist das Zwischenhirn durch ein tiefes, von Hirnschlitz eingenommenes Thal von Vorderhirn abgesetzt; dasselbe ist jedoch durch eine häutige, mit der Pia mater zusammenhängende Kuppel oder Kapsel überbrückt.”
This“Kuppel”has precisely the same relations and a very similar appearance to our vesicle. The true pineal gland is placed behind it. It appears to us possible that the body found by Huxley[512]inCeratodus, which he holds to be the pineal gland, is in reality this vesicle. It is moreover possible that what has usually been regarded as the pineal gland inPetromyzonmay in reality be the homologue of the vesicle we have found inLepidosteus.
We have no observations on the pineal gland of the adult, but must refer the reader for the structure and relations of this body to the embryological section.
The infundibulum (Plate 38, fig. 47A,in.) is very elongated. Immediately in front of it is placed the optic chiasma (Plate 38, figs. 47A and C,op.ch.) from which the optic fibres can be traced passing along the sides of the optic thalami and to the optic lobes, very much as in Müller's figure of the brain ofPolypterus.
On the sides of the infundibulum are placed two prominent bodies, the lobi inferiores (l.in.), each of which contains a cavity continuous with the prolongation of the third ventricleinto the infundibulum. The apex of the infundibulum is enlarged, and to it is attached a pituitary body (pt.).
The mid-brain is of considerable size, and consists of a basal portion connecting the optic thalami with the medulla, and a pair of large optic lobes (op.l.). The iter a tertio ad quartum ventriculum, which forms the ventricle of this part of the brain, is prolonged into each optic lobe, and the floor of each prolongation is taken up by a dome-shaped projection, the homologue of the torus semicircularis of Teleostei.
The hind-brain consists of the usual parts, the medulla oblongata and the cerebellum. The medulla presents no peculiar features. The sides of the fourth ventricle are thickened and everted, and marked with peculiar folds (Plate 38, figs. 47A and B,m.o.).
The cerebellum is much larger than in the majority of Ganoids, and resembles in all essential features the cerebellum of Teleostei. In side views it has a somewhat S-shaped form, from the presence of a peculiar lateral sulcus (Plate 38, fig. 47A,cb.). As shewn by Wilder, its wall actually has in longitudinal section this form of curvature, owing to its anterior part projecting forwards into the cavity of the iter[513]. This forward projection is not, however, so conspicuous as in most Teleostei. The cerebellum contains a large unpaired prolongation of the fourth ventricle.
II.Development.
The early development of the brain has already been described; and, although we do not propose to give any detailed account of the later stages of its growth, we have thought it worth while calling attention to certain developmental features which may probably be regarded as to some extent characteristic of the Ganoids. With this view we have figured (Plate 37, figs. 44, 45) longitudinal sections of the brain at two stages,viz.: of larvæ of 15 and 26millims., and transverse sections (Plate 37, figs. 46A-G) of the brain of a larva at about the latter stage (25millims.).
The original embryonic fore-brain is divided in both embryos into a cerebrum (ce.) in front and a thalamencephalon (th.) behind. In the younger embryo the cerebrum is a single lobe, as it is in the brains of all Vertebrate embryos; but in the older larva it is anteriorly (Plate 37, fig. 46A) completely divided into two hemispheres. The roof of the undivided posterior part of the cerebrum is extremely thin (Plate 37, fig. 46B). Near the posterior border of the base of the cerebrum there is a great development of nervous fibres, which may probably be regarded as in part equivalent to the anterior commissure (Plate 37, figs. 44, 45a.c.).
Even in the oldest of the two brains the olfactory lobes are very slightly developed, constituting, however, small lateral and ventral prominences of the front end of the hemispheres. From each of them there springs a long olfactory nerve, extending for the whole length of the rostrum to the olfactory sack.
The thalamencephalon presents a very curious structure, and is relatively a more important part of the brain than in the embryo of any other form which we know of. Its roof, instead of being, as usual, compressed antero-posteriorly[514], so as to be almost concealed between the cerebral hemispheres and the optic lobes (mid-brain), projects on the surface for a length quite equal to that of the cerebral hemispheres (Plate 37, figs. 44 and 45,th.). In the median line the roof of the thalamencephalon is thin and folded; at its posterior border is placed the opening of the small pineal gland. This body is a papilliform process of the nervous matter of the roof of this part of the brain, and instead of being directed forwards, as in most Vertebrate types, tends somewhat backwards, and rests on the mid-brain behind (Plate 37, figs. 44, 45, and 46C and D,pn.). The roof of the thalamencephalon immediately in front of the pineal gland forms a sort of vesicle, the sides of which extend laterally as a pair of lobes, shewn in transverse sections in Plate 37, figs. 46C and D, asth.l.This vesicle becomes, we cannot doubt, the vesicle on the roof of the thalamencephalon which we have described in the adult brain. Immediately in front of the pineal gland the roof of the thalamencephalon contains a transverse commissure(Plate 37, fig. 46C,z.), which is the homologue of a similarly situated commissure present in the Elasmobranch brain[515], while behind the pineal gland is placed the posterior commissure. The sides of the thalamencephalon are greatly thickened, forming the optic thalami (Plate 37, figs. 46C and D,op.th.), which are continuous in front with the thickened outer walls of the hemispheres. Below, the thalamencephalon is produced into a very elongated infundibulum (Plate 37, figs. 44, 45, 46E,in.), the apex of which is trilobed as in Elasmobranchii and Teleostei. The sides of the infundibulum exhibit two lobes, the lobi inferiores (Plate 37, fig. 46E,l.in.), which are continued posteriorly into the crura cerebri.
The pituitary body[516](Plate 37, figs. 44, 45, 46E,pt.) is small, not divided into lobes, and provided with a very minute lumen.
In front of the infundibulum is the optic chiasma (Plate 37, fig. 46D,op.ch.), which is developed very early. It is, as stated by Müller, a true chiasma.
The mid-brain (Plate 37, figs. 44 and 45,m.b.) is large, and consists in both stages of (1) a thickened floor forming the crura cerebri, the central canal of which constitutes the iter a tertio ad quartum ventriculum; and (2) the optic lobes (Plate 37, figs. 46E, F, G,op.l.) above, each of which is provided with a cavity continuous with the median iter. The optic lobes are separated dorsally and in front by a well-marked median longitudinal groove. Posteriorly they largely overlap the cerebellum. In the anterior part of the optic lobes, at the point where the iter joins the third ventricle, there may be seen slight projections of the floor into the lumen of the optic lobes (Plate 37, fig. 46E). These masses probably become in the adult the more conspicuousprominences of the floor of the ventricles of the optic lobes, which we regard as homologous with the tori semicirculares of the brain of the Teleostei.
The hind-brain is formed of the usual divisions,viz.: cerebellum and medulla oblongata (Plate 37, figs. 44 and 45,cb.,md.). The former constitutes a bilobed projection of the roof of the hind-brain. Only a small portion of it is during these stages left uncovered by the optic lobes, but the major part extends forwards for a considerable distance under the optic lobes, as shewn in the transverse sections (Plate 37, figs. 46F and G,cb.); and its two lobes, each with a prolongation of its cavity, are continued forwards beyond the opening of the iter into the fourth ventricle.
It is probable that the anterior horns of the cerebellum are equivalent to the prolongations of the cerebellum into the central cavity of the optic lobes of Teleostei, which are continuous with the so-called fornix of Göttsche.
III.Comparison of the larval and adult brain ofLepidosteus,together with some observations on the systematic value of the characters of the Ganoid brain.
The brain of the older of the two larvæ, which we have described, sufficiently resembles in most of its features that of the adult to render material assistance in the interpretation of certain of the parts of the latter. It will be remembered that in the adult brain the parts usually held to be olfactory lobes were described as the anterior cerebral lobes. The grounds for this will be apparent by a comparison of the cerebrum of the larva and adult. In the larva the cerebrum is formed of (1) an unpaired basal portion with a thin roof, and (2) of a pair of anterior lobes, with small olfactory bulbs at their free extremities.
The basal portion in the larva clearly corresponds in the adult with the basal portion, together with the two posterior cerebral lobes, which are merely special outgrowths of the dorsal edge of the primitive basal portion. The pair of anterior lobes have exactly the same relations in the larva as in the adult, except that in the former the ventricles are prolonged for theirwhole length instead of being confined to their proximal portions. If, therefore, our identifications of the larval parts of the brain are correct, there can hardly be a question as to our identifications of the parts in the adult. As concerns these identifications, the comparison of the brain of our two larvæ appears conclusive in favour of regarding the anterior lobes as parts of the cerebrum, as distinguished from the olfactory lobes, in that they are clearly derived from the undivided anterior portion of the cerebrum of the younger larva.
The comparison of the larval brain with that of the adult again appears to us to leave no doubt that the vesicle attached to the roof of the thalamencephalon in the adult is the same structure as the bilobed outgrowth of this roof in the larva; and since there is in addition a well-developed pineal gland in the larva with the usual relations, there can be no ground for identifying the vesicle in the adult with the pineal gland.
Müller, in his often quoted memoir (No.13), states that the brains of Ganoids are peculiar and distinct from those both of Teleostei and Elasmobranchii; but in addition to pointing out that the optic nerves form a chiasma he does not particularly mention the features, to which he alludes in general terms. More recently Wilder (No.15) has returned to this subject; and though, as we have already had occasion to point out, we cannot accept all his identifications of the parts of the Ganoid brain, yet he has called attention to certain characteristic features of the cerebrum which have an undoubted systematic value.
The distinctive characters of the Ganoid brain are, in our opinion, (1) the great elongation of the region of the thalamencephalon; and (2) the unpaired condition of the posterior part of the cerebrum, and the presence of so thin a roof to the ventricle of this part as to cause it to appear open above.
The immense length of the region of the thalamencephalon is a feature in the Ganoid brain which must at once strike any one who examines figures of the brains of Chondrostei,Polypterus, orAmia. It is less striking in the adultLepidosteus, though here also we have shewn that the thalamencephalon is really very greatly developed; but in the larva ofLepidosteusthis feature is still better marked, so that the brain of the larva may be described as being more characteristically Ganoid than that of the adult.
The presence of a largely developed thalamencephalon at once distinguishes a Ganoid brain from that of a Teleostean Fish, in which the optic thalami are very much reduced; butLepidosteusshews its Teleostean affinities by a commencing reduction of this part of the brain.
The large size of the thalamencephalon is also characteristic of the Ganoid brain in comparison with the brain of the Dipnoi; but is not however so very much more marked in the Ganoids than it is in some Elasmobranchii.
On the whole, we may consider the retention of a large thalamencephalon as a primitive character.
The second feature which we have given as characteristic of the Ganoid brain is essentially that which has been insisted upon by Wilder, though somewhat differently expressed by him.
The simplest condition of the cerebrum is that found in the larva ofLepidosteus, where there is an anterior pair of lobes, and an undivided posterior portion with a simple prolongation of the third ventricle, and a very thin roof. The dorsal edges of the posterior portion, adjoining the thin roof, usually become somewhat everted (cf.Wilder), and inLepidosteusthese edges have in the adult a very great development, and form (videPlate 38, fig. 47A-C,ce´.) two prominent lobes, which we have spoken of as the posterior cerebral lobes.
These characters of the cerebrum are perhaps even more distinctive than those of the thalamencephalon.
In Teleostei the cerebrum appears to be completely divided into two hemispheres, which are, however, all but solid, the lateral ventricles being only prolonged into their bases. In Dipnoi again there is either (Protopterus, Wiedersheim[517]) a completely separated pair of oval hemispheres, not unlike those of the lower Amphibia, or the oval hemispheres are not completely separated from each other (Ceratodus, Huxley[518],Lepidosiren, Hyrtl[519]); in either case the hemispheres are traversed for the whole length by lateral ventricles which are either completely or nearly completely separated from each other.
In Elasmobranchii the cerebrum is an unpaired though bilobed body, but traversed by two completely separated lateral ventricles, and without a trace of the peculiar membranous roof found in Ganoids.
Not less interesting than the distinguishing characters of the Ganoid brain are those cerebral characters which indicate affinities betweenLepidosteusand other groups. The most striking of these are, as might have been anticipated, in the direction of the Teleostei.
Although the foremost division of the brain is very dissimilar in the two groups, yet the hind-brain in many Ganoids and the mid-brain also inLepidosteusapproaches closely to the Teleostean type. The most essential feature of the cerebellum in Teleostei is its prolongation forwards into the ventricles of the optic vesicles as the valvula cerebelli. We have already seen that there is a homologous part of the cerebellum inLepidosteus; Stannius also describes this part in the Sturgeon, but no such part is represented in Müller's figure of the brain ofPolypterus, or described by him in the text.
The cerebellum is in most Ganoids relatively smaller, and this is even the case withAmia; but the cerebellum ofLepidosteusis hardly less bulky than that of most Teleostei.
The presence of tori semicirculares on the floor of the mid-brain ofLepidosteusagain undoubtedly indicates its affinities with the Teleostei, and such processes are stated by Stannius to be absent in the Sturgeon, and have not, so far as we are aware, been described in other Ganoids. Lastly we may point to the presence of well-developed lobi inferiores in the brain ofLepidosteusas an undoubted Teleostean character.
On the whole, the brain ofLepidosteus, though preserving its true Ganoid characters, approaches more closely to the brain of the Teleostei than that of any other Ganoid, including evenAmia.
It is not easy to point elsewhere to such marked resemblances of the Ganoid brain, as to the brain of the Teleostei.
The division of the cerebrum into anterior and posterior lobes, which is found inLepidosteus, probably reappears again, as already indicated, in the higher Amphibia. The presence of the peculiar vesicle attached to the roof of the thalamencephalonhas its parallel in the brain ofProtopterus, and as pointing in the same direction a general similarity in the appearance of the brain ofPolypterusto that of the Dipnoi may be mentioned.
There appears to us to be in no points a close resemblance between the brain of Ganoids and that of Elasmobranchii.
[509]The homologies of the olfactory lobes throughout the group of Fishes require further investigation.
[510]“Ueb. d. Gehirn des Störs,”Müller'sArchiv, 1843, andLehrbuch d. vergl. Anat. d. Wirbelthiere. Cattie,Archives de Biologie,Vol.III.1882, has recently described inAcipenser sturioa vesicle on the roof of the thalamencephalon, whose cavity is continuous with the third ventricle. This vesicle is clearly homologous with that inLepidosteus. (June 28, 1882.)
[511]R. Wiedersheim,Morphol. Studien, 1880, p. 71.
[512]“OnCeratodus Forsteri,”&c.,Proc. Zool. Soc.1876.
[513]In Wilder's figure the walls of the cerebellum are represented as much too thin.
[514]VideF. M. Balfour,Comparative Embryology,Vol.II.figs. 248 and 250.
[515]VideF. M. Balfour,Comparative Embryology,Vol.II.pp.355-6 [the original edition], where it is suggested that this commissure is the homologue of the grey commissure of higher types.
[516]We have not been able to work out the early development of the pituitary body as satisfactorily as we could have wished. In Plate 37, fig. 40, there is shewn an invagination of the oral epithelium to form it; in Plate 37, figs. 41 and 42, it is represented in transverse section in two consecutive sections. Anteriorly it is still connected with the oral epithelium (fig. 41), while posteriorly it is free. It is possible that an earlier stage of it is shewn in Plate 36, fig. 35. Were it not for the evidence in other types of its being derived from the epiblast we should be inclined to regard it as hypoblastic in origin.
[517]Morphol. Studien,III.Jena, 1880.
[518]“OnCeratodus Forsteri,”Proc. Zool. Soc.1876.
[519]Lepidosiren paradoxa.Prag.1845.
Olfactory organ.
Development.—The nasal sacks first arise during the late embryonic period in the form of a pair of thickened patches of the nervous layer of the epiblast on the dorsal surface of the front end of the head (Plate 37, fig. 39,ol.). The patches very soon become partially invaginated; and a small cavity is developed between them and the epidermic layer of the epiblast (Plate 37, figs. 42 and 43,ol.). Subsequently, the roof of this space, formed by the epidermic layer of the epiblast, is either broken through or absorbed; and thus open pits,lined entirely by the nervous layer of the epidermis, are formed.
We are not acquainted with any description of an exactly similar mode of origin of the olfactory pits, though the process is almost identical with that of the other sense organs.
We have not worked out in detail the mode of formation of the double openings of the olfactory pits, but there can be but little doubt that it is caused by the division of the single opening into two.
The olfactory nerve is formed very early (Plate 37, fig. 39, I), and, as Marshall has found in Aves and Elasmobranchii, it arises at a stage prior to the first differentiation of an olfactory bulb as a special lobe of the brain.
The Eye.
Anatomy.—We have not made a careful histological examination of the eye ofLepidosteus, which in our specimens was not sufficiently well preserved for such a purpose; but we havefound a vascular membrane enveloping the vitreous humour on its retinal aspect, which, so far as we know, is unlike anything which has so far been met with in the eye of any other adult Vertebrate.
The membrane itself is placed immediately outside the hyaloid membrane,i.e.on the side of the hyaloid membrane bounding the vitreous humour. It is easily removed from the retina, to which it is only adherent at the entrance of the optic nerve. In both the eyes we examined it also adhered, at one point, to the capsule of the lens, but we could not make out whether this adhesion was natural, or artificially produced by the coagulation of a thin layer of albuminous matter. In one instance, at any rate, the adhesion appeared firmer than could easily be produced artificially.
The arrangement of the vessels in the membrane is shewn diagrammatically in Plate 38, fig. 49, while the characteristic form of the capillary plexus is represented in Plate 38, fig. 50.
The arterial supply appears to be derived from a vessel perforating the retina close to the optic nerve, and obviously homologous with the artery of the processus falciformis and pecten of Teleostei and Birds, and with the arteria centralis retinæ of Mammals. From this vessel branches diverge and pursue a course towards the periphery. They give off numerous branches, the blood from which enters a capillary plexus (Plate 38, figs. 49 and 50) and is collected again by veins, which pass outwards and finally bend over and fall into (Plate 38, fig. 49) a circular vein (cr.v.) placed at the outer edge of the retina along the insertion of the iris (ir). The terminal branches of some of the main arteries appear also to fall directly into this vein.
The membrane supporting the vessels just described is composed of a transparent matrix, in which numerous cells are embedded (Plate 38, fig. 50).
Development.—In the account of the first stages of development ofLepidosteus, the mode of formation of the optic cup, the lens,&c., have been described (videPlates 35 and 36, figs. 23, 26, 35). With reference to the later stages in the development of the eye, the only subject with which we propose to deal is the growth of the mesoblastic processes which enter the cavity of the vitreous humour through the choroid slit.
Lepidosteusis very remarkable for the great number of mesoblast cells which thus enter the cavity of the vitreous humour, and for the fact that these cells areat first unaccompanied by any vascular structures(Plate 37, fig. 43,v.h). The mesoblast cells are scattered through the vitreous humour, and there can be no doubt that during early larval life, at a period however when the larva is certainly able to see, every histologist would consider the vitreous humour to be a tissue formed of scattered cells, with a large amount of intercellular substance; and the fact that it is so appears to us to demonstrate that Kessler's view of the vitreous humour being a mere transudation is not tenable.
In the larva five or six days after hatching, and about 15millims.in length, the choroid slit is open for its whole length. The edges of the slit near the lens are folded, so as to form a ridge projecting into the cavity of the vitreous humour, while nearer the insertion of the optic nerve they cease to exhibit any such structure. The mesoblast, though it projects between the lips of the ridge near the lens, only extends through the choroid slit into the cavity of the vitreous humour in the neighbourhood of the optic nerve. Here it forms a lamina with a thickened edge, from which scattered cells in the cavity of the vitreous humour seem to radiate.
At a slightly later stage than that just described, blood-vessels become developed within the cavity of the vitreous humour, and form the vascular membrane already described in the adult, placed close to the layer of nerve-fibres of the retina, but separated from this layer by the hyaloid membrane (Plate 38, fig. 48,v.sh.). The artery bringing the blood to the above vascular membrane is bound up in the same sheath as the optic nerve, and passes through the choroid slit very close to the optic nerve. Its entrance into the cavity of the vitreous humour is shewn in Plate 38, fig. 48 (vs.); its relation to the optic nerve in Plate 37, fig. 46, C and D (vs.).
The above sheath has, so far as we know, its nearest analogue in the eye ofAlytes, where, however, it is only found in the larva.
The reader who will take the trouble to refer to the account of the imperfectly-developed processus falciformis of the Elasmobrancheye in the treatiseOn Comparative Embryology, by one of us[520], will not fail to recognize that the folds of the retina at the sides of the choroid slit, and the mesoblastic process passing through this slit, are strikingly similar inLepidosteusand Elasmobranchii; and that, if we are justified in holding them to be an imperfectly-developed processus falciformis in the one case, we are equally so in the other.
Johannes Müller mentions the absence of a processus falciformis as one of the features distinguishing Ganoids and Teleostei. So far as the systematic separation of the two groups is concerned, he is probably perfectly justified in this course; but it is interesting to notice that both in Ganoids and Elasmobranchii we have traces of a structure which undergoes a very special development in the Teleostei, and that the processus falciformis of Teleostei is therefore to be regarded, not as an organ peculiar to them, but as the peculiar modification within the group of a primitive Vertebrate organ.