Fig. 2. Section through the hinder part of the medulla oblongata, stage between K and L. Zeiss A,ocul.2.
It shews (1) the vagus commissure with branches on one side from the medulla: (2) the intestinal branch of the vagus giving off a nerve to the lateral line.
Fig. 3. Longitudinal and vertical section through the head of a Scyllium embryo of stage L. Zeiss a,ocul.2.
It shews the course of the anterior branch of the seventh nerve (vii.); especially with relation to the ophthalmic branch of the fifth nerve (v.oth).
Figs. 4aand 4b. Two horizontal and longitudinal sections through the head of a Scyllium embryo belonging to stage O. Zeiss a,ocul.2.
4ais the most dorsal of the two sections, and shews the course of the anterior branch of the seventh nerve above the eye.
4bis a slightly more ventral section, and shews the course of the fifth nerve.
Fig. 5. Longitudinal and horizontal section through the hind-brain at stage O, shewing the roots of the vagus and glossopharyngeal nerves in the brain. Zeiss B,ocul.2.
There appears to be one root in the brain for the glossopharyngeal, and at least six for the vagus. The fibres from the roots divide in many cases into two bundles before leaving the brain. Swellings of the brain towards the interior of the fourth ventricle are in connection with the first five roots of the vagus, and the glossopharyngeal root; and a swelling is also intercalated between the first vagus root and the glossopharyngeal root.
Fig. 6. Horizontal section through a part of the choroid slit at stage P. Zeiss B,ocul.2.
The figure shews (1) the rudimentary processus falciformis (pfal) giving origin to the vitreous humour; and (2) the hyaloid membrane (hym) which is seen to adhere to the retina, and not to the vitreous humour or processus falciformis.
[269]Ent. d. Unke, p. 304.
[270]Das Gehirn d. Selachier, Leipzig, 1870.
[271]Proceedings of the Zoological Society, 1876,Pt.1.pp.30 and 31.
[272]“Anterior brain-mass with Sharks and Skates,”American Journal of Science and Arts,Vol.XII.1876.
[273]Entwicklungsgeschichte d. Unke.
[274]The engraver has not been very successful in rendering these membranes.
[275]“Embryologie d. Coloboms,”Sitz. d. k. Akad. Wien, Bd.LXXI.1875.
[276]Quarterly Journal of Microscopic Science,Oct.1874.
[277]Entwicklungsgeschichte der Unke.Götte was the first to draw attention to this fact. His observations were then shewn to hold true for Elasmobranchii by myself, and subsequently for Birds by Mihalkowics.
[278]Arch. f. micr. Anat.Vol.XI.
[279]W. Müller,“Ueber Entwicklung and Bau d. Hypophysis u. d. Processus infundibuli cerebri,”Jenaische Zeitschrift, Bd.VI.
[280]In the presence of this continuous outgrowth of the brain from which spring the separate nerve stems of the vagus, may perhaps be found a reconciliation of the apparently conflicting statements of Götte and myself with reference to the vagus nerve. Götte regards the vagus as a single nerve, from its originating as an undivided rudiment; but it is clear from my researches that, for Elasmobranchii at least, this method of arguing will not hold good, since it would lead to the conclusion that all the spinal nerves were branches of one single nerve, since they too spring as processes from a continuous outgrowth from the brain!
[281]The conclusion here arrived at with reference to the anterior roots, is opposed to the observations of both Gegenbaur on Hexanchus,Jenaische Zeitschrift,Vol.VI., and of Jackson and Clarke on Echinorhinus,Journal of Anatomy and Physiology, Vol.X.These morphologists identify certain roots springing from the medulla below and behind the main roots of the vagus as true anterior roots of this nerve. The existence of these roots is not open to question, but without asserting that it is impossible for me to have failed to detect such roots had they been present in the embryo, I think I may maintain if these anterior roots are not present in the embryo, their identification as vagus roots must be abandoned; and they must be regarded as belonging to spinal nerves. This point is more fully spoken of at p.428.
[282]Journal of Anatomy and Physiology,Vol.X.[This Edition,No.IX.]
[283]Nervensystem d. Fische, Rostock, 1849.
[284]Jenaische Zeitschrift,Vol.VI.
[285]Journal of Anatomy and Physiology,Vol.X.
[286]My results with reference to these roots accord exactly, so far as they go, with the more carefully worked out conclusions of Stannius,loc. cit.pp.29 and 30.
[287]The root of the seventh nerve cannot properly be distinguished from this root.
[288]Loc. cit.
[289]Hexanchus, Gegenbaur,Jenaische Zeitschrift,Vol.VI.
[290]Loc. cit.
[291]In the diagram there are only five strands represented. This is due to the fact that I have not certainly made out their true number.
[292]Loc. cit.
[293]VideJackson and Clarke,loc. cit.The authors take a different view to that here advocated, and regard the ventral roots described by them as having originally belonged to the vagus.
[294]VideVetter,“Die Kiemen und Kiefermusculatur d. Fische.”Jenaische Zeitschrift, Vol.VII.
[295]A report of the lectures appeared inNature.
[296]VidePlate 8.
[297]The description of stage K and L,pp.292and293, is a little inaccurate with reference to the number of the visceral clefts, though the number visible in the hardened embryos is correctly described.
[298]Videon the development of the gills, Schenk,Sitz. d. k. Akad. Wien,Vol.LXXI, 1875.
[299]VideDohrn,Ursprung d. Wirbelthiere.
[300]Semper, in his most recent work, maintains, if I understand him rightly, that the head is in no sense a modified part of the trunk, but admits that it is segmented in a similar fashion to the trunk.
[301]Preliminary note upon the brain and skull of Amphioxus,Proc.of the Royal Society,Vol.XXII.
[302]Ursprung d. Wirbelthiere.
The present Chapter completes the history of the primitive alimentary canal, whose formation has already been described. In order to economise space, no attempt has been made to give a full account of the alimentary canal and its appendages, but only those points have been dealt with which present any features of special interest.
The development of the following organs is described in order.
(1) The solid œsophagus.
(2) The postanal section of the alimentary tract.
(3) The cloaca and anus.
(4) The thyroid body.
(5) The pancreas.
(6) The liver.
(7) The subnotochordal rod.
The solid œsophagus.
A curious point which has turned up in the course of my investigations is the fact that for a considerable period of embryonic life a part of the œsophagus remains quite solid and without a lumen. The part of the œsophagus to undergo this peculiar change is that which overlies the heart, and extends from the front end of the stomach to the branchial region. At first, this part of the œsophagus has the form of a tube with a well-developed lumen like the remainder of the alimentarytract, but at a stage slightly younger than K its lumen becomes smaller, and finally vanishes, and the original tube is replaced by a solid rod of uniform and somewhat polygonal cells. A section of it in this condition is represented inPl.11, fig. 8a.
At a slightly later stage its outermost cells become more columnar than the remainder, and between stages K and L it loses its cylindrical form and becomes much more flattened. By stage L the external layer of columnar cells is more definitely established, and the central rounded cells are no longer so numerous (Pl.18, fig. 4,sœs).
In the succeeding stages the solid part of the œsophagus immediately adjoining the stomach is carried farther back relatively to the heart and overlies the front end of the liver. A lumen is not however formed in it by the close of stage Q, and beyond that period I have not carried my investigations, and cannot therefore state the exact period at which the lumen reappears. The limits of the solid part of the œsophagus are very satisfactorily shewn in longitudinal and vertical sections.
The solidification of the œsophagus belongs to a class of embryological phenomena which are curious rather than interesting, and are mainly worth recording from the possibility of their turning out to have some unsuspected morphological bearings.
Up to stage Q there are no signs of a rudimentary air-bladder.
The postanal section of the alimentary tract.
An account has already been given (p.307) of the posterior continuity of the neural and alimentary canals, and it was there stated that Kowalevsky was the discoverer of this peculiar arrangement. Since that account was published, Kowalevsky has given further details of his investigations on this point, and more especially describes the later history of the hindermost section of the alimentary tract. He says[303]:
The two germinal layers, epiblast and hypoblast, are continuous with each other at the border of the germinal disc. The primitive groove orfurrow appears at the border of the germinal disc and is continued from the upper to the lower side. By the closing of the groove there is formed the medullary canal above, while the part of the groove on the under surface directed below is chiefly converted into the hind end of the alimentary tract. The connection of the two tubes in Acanthias persists till the formation of the anus, and the part of the nervous tube which lies under the chorda passes gradually upwards to the dorsal side of the chorda, and persists there for a long time in the form of a large thin-walled vesicle.
The last part of the description beginning at“The connection of”does not hold good for any of the genera which I have had an opportunity of investigating, as will appear from the sequel.
In a previous section[304]the history of the alimentary tract was completed up to stage G.
In stage H the point where the anus will (at a very much later period) appear, becomes marked out by the alimentary tract sending down a papilliform process towards the skin. This is shewn inPl.8, figs.HandI,an.
That part of the alimentary tract which is situated behind this point may, for convenience, be calledthe postanal section. During stage H the postanal section begins to develop a terminal dilatation or vesicle, connected with the remainder of the canal by a narrower stalk. The relation in diameter between the vesicle and the stalk may be gathered by a comparison of figs. 3aand 3b,Pl.11. The diameter of the vesicle represented in section inPl.11, fig. 3, is 0.328Mm.
The walls both of the vesicle and stalk are formed of a fairly columnar epithelium. The vesicle communicates in front by a narrow passage (Pl.11, fig. 3a) with the neural canal, and behind is continued into two horns (Pl.11, fig. 2,al.) corresponding with the two caudal swellings spoken of above (p.288). Where the canal is continued into these two horns, its walls lose their distinctness of outline, and become continuous with the adjacent mesoblast.
In the succeeding stages up to K the tail grows longer and longer, and with it grows the postanal section of the alimentary tract, without however altering in any of its essential characters.
Its features at stage K are illustrated by an optical section of the tail of an embryo (Pl.18, fig. 5) and by a series of transverse sections through the tail of another embryo inPl.18, figs. 6a, 6b, 6c, 6d. In the optical section there is seen a terminal vesicle (alv.) opening into the neural canal, and connected with the remainder of the alimentary tract. The terminal vesicle causes the end of the tail to be dilated, as is shewn inPl.8, fig.K. The length of the postanal section extending from the abdominal paired fins to the end of the tail (equal to rather less than one-third of the whole length of the embryo), may be gathered from the same figure.
The most accurate method of studying this part of the alimentary canal is by means of transverse sections. Four sections have been selected for illustration (Pl.18, figs. 6a, 6b, 6c, and 6d) out of a fairly-complete series of about one hundred and twenty.
Posteriorly (fig. 6a) there is present a terminal vesicle .25Mm.in diameter, and therefore rather smaller than in the earlier stage, whose walls are formed of columnar epithelium, and which communicates dorsally by a narrow opening with the neural canal; to this is attached a stalk in the form of a tube, also lined by columnar epithelium, and extending through about thirty sections (Pl.18, fig. 6b). Its average diameter is about .084Mm.Overlying its front end is the subnotochordal rod (fig. 6b,x.), but this does not extend as far back as the terminal vesicle.
The thick-walled stalk of the vesicle is connected with the cloacal section of the alimentary tract by a very narrow thin-walled tube (Pl.18, 6c,al.). This for the most part has a fairly uniform calibre, and a diameter of not more than .035Mm.Its walls are formed of a flattened epithelium. At a point not far from the cloaca it becomes smaller, and its diameter falls to .03Mm.In front of this point it rapidly dilates again, and, after becoming fairly wide, opens on the dorsal side of the cloacal section of the alimentary canal just behind the anus (fig. 6d).
Near the close of stage K at a point shortly behind the anus, where the postanal section of the canal was thinnest in the early part of the stage, the alimentary canal becomes solid(Pl.11, fig. 9d), and a rupture here occurs in it at a slightly later period.
In stage L the posterior part of the postanal section of the canal is represented by a small rudiment near the end of the tail. The rudiment no longer has a terminal vesicle,nor does it communicate with the neural canal. It was visible in one series for about 40 sections, and was continued forwards by a few granular cells, lying between the aorta and the caudal vein. The portion of the postanal section of the alimentary tract just behind the cloaca, was in the same embryo represented by a still smaller rudiment of the dilated part which at an earlier period opened into the cloaca.
Later than stage L no trace of the postanal section of the alimentary canal has come under my notice, and I conclude that it vanishes without becoming converted into any organ in the adult. Since my preliminary account of the development of Elasmobranch Fishes was written, no fresh light appears to have been thrown on the question of the postanal section of the alimentary canal being represented in higher Vertebrata by the allantois.
The cloaca and anus.
Elasmobranchii agree closely with other Vertebrates in the formation of the cloaca and anus, and in the relations of the cloaca to the urinogenital ducts.
The point where the anus, or more precisely the external opening of the cloaca, will be formed, becomes very early marked out by the approximation of the wall of the alimentary tract and external skin. This is shewn for stages H and I inPl.8an.
Between stages I and K the alimentary canal on either side of this point, which we may for brevity speak of as the anus, is far removed from the external skin, but at the anus itself the lining of the alimentary canal and the skin are in absolute contact. There is, however, no involution from the exterior, but, on the contrary, the position of the anus is marked by a distinct prominence. Opposite the anus the alimentary canal dilates and forms the cloaca.
During stage K, just in front of the prominence of the anus, a groove is formed between two downgrowths of the body-wall. This is shewn inPl.11, fig. 9a. During the same stage the segmental ducts grow downwards to the cloaca, and open into it in the succeeding stage (Pl.11, fig. 9b). Up to stage K the cloaca is connected with the præanal section of the alimentary canal in front, and the postanal section behind; the latter, however, by stage L, as has been stated above, atrophies, with the exception of a very small rudiment. In stage L the posterior part of the cloaca is on a level with the hind end of the kidneys, and is situated behind the posterior horns of the body-cavity, which are continued backwards to about the point where the segmental ducts open into the cloaca, and though very small at their termination rapidly increase in size anteriorly.
Nothing very worthy of note takes place in connection with the cloaca till stage O. By this stage we have three important structures developed. (1) An involution from the exterior to form the mouth of the cloaca or anus. (2) A perforation leading into the cloaca at the hind end of this. (3) The rudiments of the abdominal pockets. All of these structures are shewn inPl.19, figs. 1a, 1b, 1c.
The mouth of the cloaca is formed by an involution of the skin, which is deepest in front and becomes very shallow behind (Pl.19, figs. 1a, 1b). At first only the mucous layer of the skin takes part in it, but when the involution forms a true groove, both layers of the skin serve to line it. At its posterior part, where it is shallowest, there is present, at stage O, a slit-like longitudinal perforation, leading into the posterior part of the cloaca (Pl.19, fig. 1c) and forming its external opening. Elsewhere the wall of the cloaca and cloacal groove are merely in contact but do not communicate. On each side of the external opening of the cloaca there is present an involution (Pl.19, fig. 1c,ab.p.) of the skin, which resembles the median cloacal involution, and forms the rudiment of an abdominal pocket. These two rudiments must not be confused with two similar ones, which are present in all the three sections represented, and mark out the line which separates the limbs from the trunk. These latter are not present in the succeeding stages. The abdominal pockets are only found in sections through the opening intothe cloaca, and are only visible in the hindermost of my three sections.
All the structures of the adult cloaca appear to be already constituted by stage O, and the subsequent changes, so far as I have investigated them, may be dealt with in very few words. The perforation of the cloacal involution is carried slowly forwards, so that the opening into the cloaca, though retaining its slit-like character, becomes continuously longer; by stage Q its size is very considerable. The cloacal involution, relatively to the cloaca, recedes backwards. In stage O its anterior end is situated some distance in front of the opening of the segmental duct into the cloaca; by stage P the front end of the cloacal involution is nearly opposite this opening, and by stage Q is situated behind it.
As I have shewn elsewhere[305], the so-called abdominal pores of Scyllium are simple pockets open to the exterior, but without any communication with the body-cavity. By stage Q they are considerably deeper than in stage O, and retain their original position near the hind end of the opening into the cloaca. The opening of the urinogenital ducts into the cloaca will be described in the section devoted to the urinogenital system.
In Elasmobranchii, as in other Vertebrata, that part of the cloaca which receives the urinogenital ducts, is in reality the hindermost section of the gut and not the involution of epiblast which eventually meets this. Thus the urinogenital ducts at first open into the alimentary canal and not to the exterior. This fact is certainly surprising, and its meaning is not quite clear to me.
The very late appearance of the anus may be noticed as a point in which Elasmobranchii agree with other Vertebrata, notably the Fowl[306]. The abdominal pockets, as might be anticipated from their structure in the adult, are simple involutions of the epiblast.
The thyroid body.
The earliest trace of the thyroid body has come under my notice in a Torpedo embryo slightly older than I. In thisembryo it appeared as a diverticulum from the ventral surface of the throat in the region of themandibular arch, and extended from the border of the mouth to the point where the ventral aorta divided into the two aortic branches of the mandibular arch. In front it bounded a groove (Pl.15, fig. 5a,Th.), directly continuous with the narrow posterior pointed end of the mouth and open to the throat, while behind it became a solid rod attached to the ventral wall of the œsophagus (Pl.15, fig. 5b,Th.). In a Scyllium embryo belonging to the early part of stage K, the thyroid gland presented the same arrangement as in the Torpedo embryo just described, with the exception that no solid posterior section of it was present.
Towards the close of stage K the thyroid body begins to elongate and become solid, though it still retains its attachment to the wall of the œsophagus. The solidification is effected by the columnar cells which line the groove elongating and meeting in the centre. As soon as the lumen is by these means obliterated, small cells make their appearance in the interior of the body, probably budded off from the original columnar cells.
The gland continues to grow in length, and by stage L assumes a long sack-like form with a layer of columnar cells bounding it externally, and a core of rounded cells filling up its interior. Anteriorly it is still attached to the throat, and its posterior extremity lies immediately below the end of the ventral aorta. The cells of the gland contain numerous yellowish concretionary pigment bodies, which are also present in the later stages.
Up to stage P the thyroid gland retains its original position. Its form and situation are shewn inPl.19, fig. 3,th., in longitudinal and vertical section for a stage between O and P. The external layer of columnar cells has now vanished, and the gland is divided up by the ingrowth of connective-tissue septa into a number of areas or lobules—the rudiments of the future follicles. These lobules are perfectly solid without any trace of a lumen. A capillary network following the septa is present.
By stage Q the rudimentary follicles are more distinctly marked, but still without a lumen, and a connective-tissue sheath indistinctly separated from the surrounding tissue has been formed. My sections do not shew a junction between the glandand the epithelium of the throat; but the two are so close together, that I am inclined to think that such a junction still exists. It is certainly present up to stage P.
Dr Müller[307], in his exhaustive memoir on the thyroid body, gives an account of its condition in two Acanthias embryos. In his earliest embryo (which, judging from the size, is perhaps about the same age as my latest) the thyroid body is disconnected from the throat, yet contains a lumen, and is not divided up into lobules. It is clear from this account, that there must be considerable differences of detail in the development of the thyroid body in Acanthias and Scyllium.
In the Bird Dr Müller's figures shew that the thyroid body develops in the region of the hyoid arch, whereas, in Elasmobranchii, it develops in the region of the mandibular arch. Dr Götte's[308]account of this body in Bombinator accords very completely with my own, both with reference to the region in which it develops, and its mode of development.
The pancreas.
The pancreas arises towards the close of stage K as a somewhat rounded hollow outgrowth from the dorsal side of that part of the gut which from its homologies may be called the duodenum. In the region where the pancreas is being formed the appearances presented in a series of transverse sections are somewhat complicated (Pl.18, fig. 1), owing to the several parts of the gut and its appendages which may appear in a single section, but I have detected no trace of other than a single outgrowth to form the pancreas.
By stage L the original outgrowth from the gut has become elongated longitudinally, but transversely compressed: at the same time its opening into the duodenum has become somewhat narrowed.
Owing to these changes the pancreas presents in longitudinal and vertical section a funnel-shaped appearance (Pl.19, fig. 4). From the expanded dorsal part of the funnel, especially from its anterior end, numerous small tubular diverticula grow outinto the mesoblast. The apex of the funnel leads into the duodenum. From this arrangement it results that at this period the original outgrowth from the duodenum serves as a receptacle into which each ductule of the embryonic gland opens separately. I have not followed in detail the further growth of the gland. It is, however, easy to note that while the ductules grow longer and become branched, vascular processes grow in between them, and the whole forms a compact glandular body in the mesentery on the dorsal side of the alimentary tract, and nearly on a level with the front end of the spiral valve. The funnel-shaped receptacle loses its original form, and elongating, assumes the character of a duct.
From the above account it follows that the glandular part of the pancreas, and not merely its duct, is derived from the original hypoblastic outgrowth from the gut. This point is extremely clear in my preparations, and does not, in spite of Schenk's observations to the contrary[309], appear to me seriously open to doubt.
The liver.
The liver arises during stage I as a ventral outgrowth from the duodenum immediately in front of the opening of the umbilical canal (duct of the yolk-sack) into the intestine. Almost as soon as it is formed this outgrowth develops two lateral diverticula opening into a median canal.
The two diverticula are the rudimentary lobes of the liver, and the median duct is the rudiment of the common bile-duct (ductus choledochus) and gall-bladder (Pl.11, fig. 9).
By stage K the hepatic diverticula have begun to bud out a number of small hollow knobs. These rapidly increase in length and number, and form the so-called hepatic cylinders. They anastomose and unite together, so that by stage L there is constructed a regular network. As the cylinders increase in length their lumen becomes very small, but appears never to vanish (Pl.19, fig. 5).
The mode of formation of the liver parenchyma by hollow and not solid outgrowths agrees with the suggestion made intheElements of Embryology, p. 133, and also with the results of Götte on the Amphibian liver. Schenk has thrown doubts upon the hypoblastic nature of the secreting tissue of the liver, but it does not appear to me, from my own investigations, that this point is open to question.
Coincidently with the formation of the hepatic network, the umbilical vein (Pl.11, fig. 9,u.v.) which unites with the subintestinal or splanchnic vein (Pl.11, fig. 8,V.) breaks up into a series of channels, which form a second network in the spaces of the hepatic network. These vascular channels of the liver appear to me to have from the first distinct walls of delicate spindle-shaped cells, and I have failed to find a stage similar to that described by Götte for Amphibians in which the blood-channels are simply lacunar spaces in the hepatic parenchyma.
The changes of the median duct of the liver are of rather a passive nature. By stage O its anterior end has dilated into a distinct gall-bladder, whose duct receives in succession the hepatic ducts, and so forms the ductus choledochus. The ductus choledochus opens on the ventral side of the intestine immediately in front of the commencement of the spiral valve.
It may be noted that the liver and pancreas are corresponding ventral and dorsal appendages of the part of the alimentary tract immediately in front of its junction with the yolk-sack.
The subnotochordal rod.
The existence of this remarkable body in Vertebrata was first made known by Dr Götte[310], who not only demonstrated its existence, but also gave a correct account of its development. Its presence in Elasmobranchii and mode of development were mentioned by myself in my preliminary account of the development of these fishes[311], and it has been independently observed and described by Professor Semper[312]. No plausible suggestion as to its function has hitherto been made, and it is therefore a matter of some difficulty to settle with what groupof organs it ought to be treated. In the presence of this difficulty it seemed best to deal with it in this chapter, since it is unquestionably developed from the wall of the alimentary canal.
At its full growth this body forms a rod underlying the notochord, and has nearly the same longitudinal extension as this. It is indicated in most of my sections by the letterx. We may distinguish two sections of it, the one situated in the head, the other in the trunk. The junction between the two occurs at the hind border of the visceral clefts.
The section in the trunk is the first to develop. It arises during stage H in the manner illustrated inPl.11, figs. 1 and 1a. The wall of the alimentary canal becomes thickened (Pl.11, fig. 1) along the median dorsal line, or else produced into a ridge into which there penetrates a narrow prolongation of the lumen of the alimentary canal. In either case the cells at the extreme summit of the thickening become gradually constricted off as a rod, which lies immediately dorsal to the alimentary tract, and ventral to the notochord. The shape of the rod varies in the different regions of the body, but it is always more or less elliptical in section. Owing to its small size and soft structure it is easily distorted in the process of preparing sections.
In the hindermost part of the body its mode of formation differs somewhat from that above described. In this part the alimentary wall is very thick and undergoes no special growth prior to the formation of the subnotochordal rod; on the contrary, a small linear portion of the wall becomes scooped out along the median dorsal line, and eventually separates from the remainder as the rod in question. In the trunk the splitting off of the rod takes place from before backwards, so that the anterior part of it is formed before the posterior.
The section of the subnotochordal rod in the head would appear from my observations on Pristiurus to develop in the same way as in the trunk, and the splitting off from the throat proceeds from before backwards (Pl.15, fig. 4a,x).
In Torpedo, this rod develops very much later in the head than in the trunk; and indeed my conclusion that it develops in the head at all is only based on grounds of analogy, since inmy oldest Torpedo embryo (just younger than K) there is no trace of it present. In a Torpedo embryo of stage I the subnotochordal rod of the trunk terminated anteriorly by uniting with the wall of the throat. The junction was effected by a narrow pedicle, so that the rod appeared mushroom-shaped in section, the stalk representing the pedicle of attachment.
On the formation of the dorsal aorta, the subnotochordal rod becomes separated from the wall of the gut and the aorta interposed between the two.
The subnotochordal rod attains its fullest development during stage K. Anteriorly it terminates at a point well in front of the ear, though a little behind the end of the notochord; posteriorly it extends very nearly to the extremity of the tail and is almost co-extensive with the postanal section of the alimentary tract, though it does not quite reach so far back as the caudal vesicle (Pl.18, fig. 6b,x). In stage L it is still fairly large in the tail, though it has begun to atrophy anteriorly. We may therefore conclude that its atrophy, like its development, takes place from before backwards. In the succeeding stages I have failed to find any trace of it, and conclude, as does Professor Semper, that it disappears completely.
Götte[313]is of opinion that the subnotochordal rod is converted into the dorsal lymphatic trunk, and regards it as the anterior continuation of the postanal gut, which he believes to be also converted into a lymphatic trunk. My observations afford no support to these views, and the fact already mentioned, that the subnotochordal rod is nearly co-extensive with the postanal section of the gut, renders it improbable that both these structures are connected with the lymphatic system.
EXPLANATION OF PLATE 18.
Complete List of Reference Letters.
Nervous System.
ar.Anterior root of spinal nerve.nc.Neural canal.pr.Posterior root of spinal nerve.spn.Spinal nerve.syg.Sympathetic ganglion.
Alimentary Canal.
al.Alimentary canal.alv.Caudal vesicle of the postanal gut.clal.Cloacal section of alimentary canal.du.Duodenum.hpd.Ductus choledochus.pan.pancreas.sœs.Solid œsophagus.spv.Intestine with rudiment of spiral valve.umc.Umbilical canal.
General.
ao.Dorsal aorta.aur.Auricle of heart.cav.Cardinal vein.ch.Notochord.eppp.Epithelial lining of the body-cavity.ir.Interrenal body.me.Mesentery.mp.Muscle-plate.mpl.Muscle-plate sending a prolongation into the limb.po.Primitive ovum.pp.Body-cavity.sd.Segmental duct.st.Segmental tube.ts.Tail swelling.vcau.Caudal vein.x.Subnotochordal rod.
Fig. 1. Transverse section through the anterior abdominal region of an embryo of a stage between K and L. Zeiss B,ocul.2. Reduced one-third.
The section illustrates the junction of a sympathetic ganglion with a spinal nerve and the sprouting of the muscle-plates into the limbs (mpl).
Fig. 2. Transverse section through the abdominal region of an embryo belonging to stage L. Zeiss B,ocul.2. Reduced one-third.
The section illustrates the junction of a sympathetic ganglion with a spinal nerve, and also the commencing formation of a branch from the aorta (still solid) which will pass through the sympathetic ganglion, and forms the first sign of the conversion of part of a sympathetic ganglion into one of the suprarenal bodies.
Fig. 3. Longitudinal and vertical section of an embryo of a stage between L and M, shewing the successive junctions of the spinal nerves and sympathetic ganglia.
Fig. 4. Section through the solid œsophagus during stage L. Zeiss A,ocul.1. The section is taken through the region of the heart, so that the cavity of the auricle (aur) lies immediately below the œsophagus.
Fig. 5. Optical section of the tail of an embryo between stages I and K, shewing the junction between the neural and alimentary canals.
Fig. 6. Four sections through the caudal region of an embryo belonging to stage K, shewing the condition of the postanal section of the alimentary tract. Zeiss A,ocul.2. An explanation of these figures is given on p.449.
Fig. 7. Section through the interrenal body of a Scyllium embryo belonging to stage Q. Zeiss C,ocul.2.
Fig. 8. Portion of a section of the interrenal body of an adult Scyllium. Zeiss C,ocul.2.
[303]Archiv f. Mic. Anat.Vol.XIII.pp.194, 195.
[304]p.303et seq.
[305]This Edition,No.VII.p.152.
[306]VideGasser,Entwicklungsgeschichte der Allantois, etc.
[307]Jenaische Zeitschrift,Vol.VI.
[308]Entwicklungsgeschichte d. Unke.
[309]Lehrbuch d. vergleichenden Embryologie.
[310]Archiv für Micros. Anatomie, Bd.V., andEntwicklungsgeschichte d. Unke.
[311]Quarterly Journal of Microscopic Science,Oct., 1874. [This Edition,No.V.]
[312]“Stammverwandtschaft d. Wirbelthiere u. Wirbellosen”and“Das Urogenitalsystem d. Plagiostomen,”Arb. Zool.-Zoot. Institut. z. Würzburg, Bd.II.
[313]Entwicklungsgeschichte d. Unke, p. 775.
The present chapter deals with the early development of the heart, the development of the general circulatory system, especially the venous part of it, and the circulation of the yolk-sack. It also contains an account of two bodies which I shall call the suprarenal and interrenal bodies, which are generally described as vascular glands.
The heart.
The first trace of the heart becomes apparent during stage G, as a cavity between the splanchnic mesoblast and the wall of the gut immediately behind the region of the visceral clefts (Pl.11, fig. 4,ht.).
The body-cavity in the region of the heart is at first double, owing to the two divisions of it not having coalesced; but even in the earliest condition of the heart the layers of splanchnic mesoblast of the two sides have united so as to form a complete wall below. The cavity of the heart is circumscribed by a more or less complete epithelioid (endothelial) layer of flattened cells, connected with the splanchnic wall of the heart by protoplasmic processes. The origin of this lining layer I could not certainly determine, but its connection with the splanchnic mesoblast suggests that it is probably a derivative of this[314]. Infront the cavity of the heart is bounded by the approximation of the splanchnic mesoblast to the wall of the throat, and behind by the stalk connecting the alimentary canal with the yolk-sack.
As development proceeds the ventral wall of the heart becomes bent inwards on each side on a level with the wall of the gut (Plate 11, fig. 4), and eventually becomes so folded in as to form for the heart a complete muscular wall of splanchnic mesoblast. The growth inwards of the mesoblast to form the dorsal wall of the heart does not, as might be expected, begin in front and proceed backwards, but commences behind and is gradually carried forwards.
From the above account it is clear that I have failed to find in Elasmobranchii any traces of two distinct cavities coalescing to form the heart, such as have been recently described in Mammals and Birds; and this, as well as the other features of the formation of the heart in Elasmobranchii, are in very close accordance with the careful description given by Götte[315]of the formation of the heart in Bombinator. The divergence which appears to be indicated in the formation of so important an organ as the heart between Pisces and Amphibians on the one hand, and Aves and Mammalia on the other, is certainly startling, and demands a careful scrutiny. The most complete observations on the double formation of the heart in Mammalia have been made by Hensen, Götte and Kölliker. These observations lead to the conclusion (1) that the heart arises as two independent splits between the splanchnic mesoblast and the hypoblast, each with an epithelioid (endothelial) lining. (2)That the heart is first formed at a period when the folding in of the splanchnopleure to form the throat hasnot commenced, and when therefore it would be impossible for it to be formed as a single tube.
In Birds almost every investigator since von Baer has detected more or less clearly the coalescence of two halves to form the unpaired heart[316]. Most investigators have however believed that there was from the first an unpaired anterior section of the heart, and that only the posterior part was formed by the coalescence of two lateral halves. Professor Darlste His, and more recently Kölliker, have stated that there is no such unpaired anterior section of the heart. My own recent observations confirm their conclusions as to the double formation of the heart, though I find that the heart has from the first a Λ-shaped form. At the apex of the Λ the two limbs are only separated by a median partition and are not continuous with the aortic arches, which do not arise till a later period[317]. In the Bird the heart arises justbehindthe completed throat, and a double formation of the heart appears, in fact, in all instances to bemost distinctly correlated with the non-closure of the throat, a non-closure which it must be noted would render it impossible for the heart to arise otherwise than as a double cavity.
In the instances in which the heart arises as a double cavityit is formed before the complete closure of the throat, and in those in which it arises as a single cavityit is formed subsequently to the complete formation of the throat. There is thus a double coincidence which renders the conclusion almost certain,that the formation of the heart as two cavities is a secondary change which has been brought about by variations in the period of the closing in of the wall of the throat.
If the closing in of the throat were deferred and yet the primitive time of formation of the heart retained, it is clear that such a condition as may be observed in Birds and Mammals must occur, and that the two halves of the heart must be formed widely apart, and only eventually united on the folding in ofthe wall of the throat. We may then safely conclude that the double formation of the heart has no morphological significance, and does not, as might at first sight be supposed, imply that the ancestral Vertebrate had two tubes in the place of the present unpaired heart. I have spoken of this point at considerable length, on account of the morphological importance which has been attached to the double formation of the heart. But the views above enunciated are not expressed for the first time. In theElements of Embryologywe say, p. 64,“The exact mode of development (of the heart) appears according to our present knowledge to be very different in different cases; and it seems probable that the differences are in fact the result of variations in the mode of formation and time of closure of the alimentary canal.”Götte again in his great work[318]appears to maintain similar views, though I do not perfectly understand all his statements. In my review of Kölliker's Embryology[319]this point is still more distinctly enunciated in the following passage:“The primitive wide separation and complete independence of the two halves of the heart is certainly surprising; but we are inclined, provisionally at least, to regard it as a secondary condition due to the late period at which the closing of the throat takes place in Mammals.”
The general circulation.
The chief points of interest in connection with the general circulation centre round the venous system. The arterial arches present no peculiarities: the dorsal aorta, as in all other Vertebrates, is at first double (Pl.11, fig. 6,ao), and, generally speaking, the arrangement of the arteries accords with what is already known in other forms. The evolution of the venous system deserves more attention.
The cardinal veins are comparatively late developments. There is at first one single primitive vein continuous in front with the heart and underlying the alimentary canal through its præanal and postanal sections. This vein is shewn in section inPl.11, fig. 8,V. It may be called either the subintestinal orsplanchnic vein. At the cloaca, where the gut enlarges and comes in contact with the skin, this vein is compelled to bifurcate (Pl.18, fig. 6,d,v.cau.), and usually the two branches into which it divides are unequal in size. The two branches meet again behind the cloaca and take their course ventral to the postanal section of the gut, and terminate close to the end of the tail,Pl.18, fig. 6,c,v.cau. In the tail they form what is usually known as the caudal vein. The venous system of Scyllium or Pristiurus, during the early parts of stage K, presents the simple constitution just described.
Before proceeding to describe the subsequent changes which take place in it, it appears to me worth pointing out the remarkable resemblance which the vascular system of an Elasmobranch presents at this stage to that of an ordinary Annelid and Amphioxus. It consists, as does the circulatory system, in Annelids, of a neural vessel (the aorta) and an intestinal vessel, the blood flowing backwards in the latter and forwards in the former. The two in Elasmobranchii communicate posteriorly by a capillary system, and in front by the arterial arches, connected like the similar vessels in Annelids with the branchiæ. Striking as is this resemblance, there is a still closer resemblance between the circulation of the Scyllium embryo at stage K and that of Amphioxus. The two systems are in fact identical except in very small details. The subintestinal vessel, absent or only represented by the caudal vein and in part by the ductus venosus in higher Vertebrates and adult Fish, forms the main and only posterior venous trunk of Amphioxus and the embryo Scyllium. The only noteworthy point of difference between Amphioxus and the embryo Scyllium is the presence of a portal circulation in the former, absent at this stage in the latter; but even this is acquired in Scyllium before the close of stage K, and does not therefore represent a real difference between the two types.
The cardinal veins make their appearance before the close of stage K, and very soon unite behind with the unpaired section of the caudal vein (Pl.11, fig. 9b,p.cav.andv.). On this junction being effected retrogressive changes take place in the original subintestinal vessel. It breaks up in front into a number of smaller vessels; the lesser of the two branches connectingit round the cloaca with the caudal vein first vanishes (Pl.11, fig. 9a,v), and then the larger; and the two cardinals are left as the sole forward continuations of the caudal vein. This latter then becomes prolonged forwards, and the two posterior cardinals open into it some little distance in front of the hind end of the kidneys. By these changes and by the disappearance of the postanal section of the gut the caudal vein is made to appear as a superintestinal and not a subintestinal vessel, and as the direct posterior continuation of the cardinal veins. Embryology proves however that the caudal vein is a true subintestinal vessel[320], and that its connection with the cardinals is entirely secondary.
The invariably late appearance of the cardinal veins in the embryo and their absence in Amphioxus leads me to regard them as additions to the circulatory system which appeared in the Vertebrata themselves, and were not inherited from their ancestors. It would no doubt be easy to point to vessels in existing Annelids which might be regarded as their equivalent, but to do so would be in my opinion to follow an entirely false morphological scent.
The circulation of the yolk-sack.
The observations recorded on this subject are so far as I am acquainted with them very imperfect, and in most cases the arteries and veins appear to have been transposed.
Professor Wyman[321], however, gives a short description of the circulation in Raja Batis, in which he rightly identifies the arteries, though he regards the arterial ring which surrounds the vascular area as equivalent to the venous sinus terminalis of the Bird.
The general features of the circulation are clearly portrayed in the somewhat diagrammatic figures onPl.9, in which the arteries are represented red, and the veins blue[322].
I shall follow the figures on this plate in my descriptions.
Fig. 1 represents my earliest stage of the circulation of the yolk-sack. At this stage there is visible a single aortic trunk passing forwards from the embryo and dividing into two branches. No venous trunk could be detected with the simple microscope, but probably venous channels were present in the thickened edge of the blastoderm.
In fig. 2 the circulation was greatly advanced[323]. The blastoderm has now nearly completely enveloped the yolk, and there remains only a small circular space (yk) not enclosed by it. The arterial trunk is present as before, and divides in front of the embryo into two branches which turn backwards and nearly form a complete ring round the embryo. In general appearance it resembles the sinus terminalis of the area vasculosa of the Bird, but in reality bears quite a different relation to the circulation. It gives off branches only on its inner side.
A venous system of returning vessels is now fully developed, and its relations are very remarkable. There is a main venous ring round the thickened edge of the blastoderm, which is connected with the embryo by a single stem which runs along the seam where the edges of the blastoderm have coalesced. Since the venous trunks are only developed behind the embryo, it is only the posterior part of the arterial ring which gives off branches.
The succeeding stage, fig. 3, is also one of considerable interest. The arterial ring has greatly extended, and now embraces nearly half the yolk, and sends off trunks on its inner side along its whole circumference.
More important changes have taken place in the venous system. The blastoderm has now completely enveloped the yolk, and as a result of this, the venous ring no longer exists, but at the point where it vanished there may be observed a number of smaller veins diverging in a brush-like fashion from the termination of the unpaired trunk which originally connected the venous ring with the heart. This point is indicated in the figure by the lettery. The brush-like divergence of the veins isa still more marked feature in a blastoderm of a succeeding stage (fig. 4).
The circulation in the succeeding stage (fig. 4) (projected in my figure) only differs in details from that of the previous stage. The arterial ring has become much larger, and the portion of the yolk not embraced (x) by it is quite small. Instead of all the branches from the ring being of nearly equal size, two of them are especially developed. The venous system has undergone no important changes.
In fig. 5 the circulation is represented at a still later stage. The arterial ring has come to embrace the whole yolk, and as a result of this, has in its turn vanished as did the venous ring before it. At this stage of the circulation there is present a single arterial and a single venous trunk. The arterial trunk is a branch of the dorsal aorta, and the venous trunk originally falls into the heart together with the subintestinal or splanchnic vein, but on the formation of the liver enters this and breaks up into capillaries in it. The venous trunk leaves the body on the right side, and the arterial on the left.
The most interesting point to be noticed in connection with the yolk-sack circulation of Scyllium is the fact of its being formed on a completely different type to that of the Amniotic Vertebrates.
The Vascular Glands.
There are in Scyllium two structures which have gone under the name of the suprarenal body. The one of these is an unpaired rod-like body lying between the dorsal aorta and the caudal vein in the region of the posterior end of the kidneys. This body I propose to callthe interrenal body. The other is formed by a series of paired bodies situated dorsal to the cardinal veins on branches of the aorta, and arranged segmentally. These bodies I shall callthe suprarenal bodies. I propose treating the literature of these bodies together, since they have usually been dealt with in this way, and indeed regarded as parts of the same system. As I hope to shew in the sequel, the origin of these bodies is very different. The interrenal body appears to bedeveloped from the mesoblast; while my researches on the suprarenal bodies confirm the brilliant investigations of Leydig, shewing that they are formed out of the sympathetic ganglia.
The most important investigations on these bodies have been made by Leydig[324]. In his first researches,Rochen u. Haie,pp.71, 72, he gives an account of the position and histology of what is probably my interrenal body[325].
The position and relations of the interrenal body vary somewhat according to Leydig in different cases. He makes the following statement about its histology.“Fat molecules form the chief mass of the body, which causes its white, or ochre-yellow colour, and one finds freely embedded in them clear vesicular nuclei.”He then proceeds to state that this structure is totally dissimilar to that of the Mammalian suprarenal body, and gives it as his opinion that it is not the same body as this. In his later researches[326]he abandons this opinion, and adopts the view that the interrenal body is part of the same system as the suprarenal bodies to be subsequently spoken of. Leydig describes the suprarenal bodies as paired bodies segmentally arranged along the ventral side of the spinal column situated on the successive arteriæ axillares, and in close connection with one or more sympathetic ganglia. He finds them formed of lobes, consisting of closed vesicles full of nuclei and cells. Numerous nerve-fibres are also described as present. With reference to the real meaning of these bodies he expresses a distinct view. He says[327],“As the pituitary body is an integral part of the brain, so are the suprarenal bodies part of the sympathetic system.”He re-affirms with still greater emphasis the same view in hisFische u. Reptilien. Though these views have not obtained muchacceptance, and the accuracy of the histological data on which they are grounded has been questioned, yet I hope to shew in the sequel not only that Leydig's statements are in the main true, but that development proves his conclusions to have been well founded.