CHAPTER X.

THE ALIMENTARY CANAL.

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 oesophagus. (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 oesophagus._

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 oesophagus remains quite solid and without a lumen. The part of the oesophagus 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 oesophagus has the form of a tube with a well-developed lumen like the remainder of the alimentary tract, 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 in Pl. 11, fig. 8_a_.

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, _soes_).

In the succeeding stages the solid part of the oesophagus 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 oesophagus are very satisfactorily shewn in longitudinal and vertical sections.

The solidification of the oesophagus 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 or furrow 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.

Footnote 303: _Archiv f. Mic. Anat._ Vol. XIII. pp. 194, 195.

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.

Footnote 304: p. 303 et seq.

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 in Pl. 8, figs. _H_ and _I, an_.

That part of the alimentary tract which is situated behind this point may, for convenience, be called _the 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. 3_a_ and 3_b_, Pl. 11. The diameter of the vesicle represented in section in Pl. 11, fig. 3, is 0.328 Mm.

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. 3_a_) 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 in Pl. 18, figs. 6_a_, 6_b_, 6_c_, 6_d_. 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 in Pl. 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. 6_a_, 6_b_, 6_c_, and 6_d_) out of a fairly-complete series of about one hundred and twenty.

Posteriorly (fig. 6_a_) there is present a terminal vesicle .25 Mm. 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. 6_b_). Its average diameter is about .084 Mm. Overlying its front end is the subnotochordal rod (fig. 6_b_, _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, 6_c_, _al._). This for the most part has a fairly uniform calibre, and a diameter of not more than .035 Mm. Its walls are formed of a flattened epithelium. At a point not far from the cloaca it becomes smaller, and its diameter falls to .03 Mm. 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. 6_d_).

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. 9_d_), 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 in Pl. 8 _an_.

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 in Pl. 11, fig. 9_a_. During the same stage the segmental ducts grow downwards to the cloaca, and open into it in the succeeding stage (Pl. 11, fig. 9_b_). 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 in Pl. 19, figs. 1_a_, 1_b_, 1_c_.

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. 1_a_, 1_b_). 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. 1_c_) 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. 1_c_, _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 into the 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.

Footnote 305: This Edition, No. VII. p. 152.

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.

Footnote 306: Vide Gasser, _Entwicklungsgeschichte der Allantois, etc._

_The thyroid body._

The earliest trace of the thyroid body has come under my notice in a Torpedo embryo slightly older than I. In this embryo it appeared as a diverticulum from the ventral surface of the throat in the region of the _mandibular 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. 5_a_, _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 oesophagus (Pl. 15, fig. 5_b_, _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 oesophagus. 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 in Pl. 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 gland and 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.

Footnote 307: _Jenaische Zeitschrift_, Vol. VI.

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.

Footnote 308: _Entwicklungsgeschichte d. Unke._

_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 out into 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.

Footnote 309: _Lehrbuch d. vergleichenden Embryologie._

_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 in the _Elements 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 group of 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.

Footnote 310: _Archiv für Micros. Anatomie_, Bd. V., and _Entwicklungsgeschichte d. Unke_.

Footnote 311: _Quarterly Journal of Microscopic Science_, Oct., 1874. [This Edition, No. V.]

Footnote 312: "Stammverwandtschaft d. Wirbelthiere u. Wirbellosen" and "Das Urogenitalsystem d. Plagiostomen," _Arb. Zool.-Zoot. Institut. z. Würzburg_, Bd. II.

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 letter _x_. 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 in Pl. 11, figs. 1 and 1_a_. 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. 4_a_, _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 in my 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. 6_b_, _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.

Footnote 313: _Entwicklungsgeschichte d. Unke_, p. 775.

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. _soes._ Solid oesophagus. _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 oesophagus 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 oesophagus.

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.