Illustration: Figure 423Fig. 423. Transverse optical section of the tail of an embryo of Phallusia mammillata.(After Kowalevsky.)The section is from an embryo of the same age as fig. 8IV.ch.notochord;n.c.neural canal;me.mesoblast;al´.hypoblast of tail.
Fig. 423. Transverse optical section of the tail of an embryo of Phallusia mammillata.(After Kowalevsky.)The section is from an embryo of the same age as fig. 8IV.ch.notochord;n.c.neural canal;me.mesoblast;al´.hypoblast of tail.
In Amphioxus the postanal gut, though distinctly developed, is not very long, and atrophies at a comparatively early period.
In Elasmobranchii this section of the alimentary tract is very well developed, and persists for a considerable period of embryonic life. The following is a history of its development in the genus Scyllium.
Shortly after the stage when the anus has become marked out by the alimentary tract sending down a papilliform process towards the skin, the postanal gut begins to develop a terminal dilatation or vesicle, connected with the remainder of the canal by a narrower stalk.
The walls both of the vesicle and stalk are formed of a fairly columnar epithelium. The vesicle communicates in front by a narrow passage with the neural canal, and behind is continued into two horns corresponding with the two caudal swellings previously spoken of (p. 55). 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, as the tail grows longer and longer, the postanal section of the alimentary tract grows with it, without however undergoing alteration in any of its essential characters. At the period of the maximum development, it has a length of about1⁄3of that of the whole alimentary tract.
Its features at a stage shortly before the external gills have become prominent are illustrated by a series of transverse sections through the tail (fig. 424). The four sections have been selected for illustration out of a fairly-complete series of about one hundred and twenty.
Posteriorly (A) there is present a terminal vesicle (alv) .25mm.in diameter, which communicates dorsally by a narrow opening with the neural canal (nc); to this is attached a stalk in the form of a tube, also lined by columnar epithelium, and extending through about thirty sections (Bal). Its average diameter is about .084mm., and its walls are very thick. Overlying its front end is the subnotochordal rod (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 sectionof the alimentary tract by a very narrow thin-walled tube (Cal). This for the most part has a fairly uniform calibre, and a diameter of not more than .035mm.Its walls are formed of flattened epithelial cells. 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 (Dal).
Illustration: Figure 424Fig. 424. Four sections through the postanal part of the tail of an embryo of the same age as fig. 28 F.A. is the posterior section.nc.neural canal;al.postanal gut;alv.caudal vesicle of postanal gut;x.subnotochordal rod;mp.muscle-plate;ch.notochord;cl.al.cloaca;ao.aorta;v.cau.caudal vein.
Fig. 424. Four sections through the postanal part of the tail of an embryo of the same age as fig. 28 F.A. is the posterior section.nc.neural canal;al.postanal gut;alv.caudal vesicle of postanal gut;x.subnotochordal rod;mp.muscle-plate;ch.notochord;cl.al.cloaca;ao.aorta;v.cau.caudal vein.
Very shortly after the stage to which the above figures belong, at a point a little behind the anus, where the postanal section of the canal was thinnest in the previous stage, it becomes solid, and a rupture here occurs in it at a slightly later period.
The atrophy of this part of the alimentary tract having once commenced proceeds rapidly. The posterior part first becomes reduced to a small rudiment near the end of the tail. There is no longer a terminal vesicle, nor a neurenteric canal. The portion of the postanal section of the alimentary tract, just behind the cloaca, is for a short time represented by a small rudiment of the dilated part which at an earlier period opened into the cloaca.
In Teleostei the vesicle at the end of the tail, discovered by Kupffer,(fig. 34,hyv) is probably the equivalent of the vesicle at the end of the postanal gut in Elasmobranchii.
In Petromyzon and in Amphibia there is a well-developed postanal gut connected with a neurenteric canal which gradually atrophies. It is shewn in the embryo of Bombinator infig. 420.
Illustration: Figure 425Fig. 425. Diagrammatic longitudinal section through the posterior end of an embryo Bird at the time of the formation of the Allantois.ep.epiblast;Sp.c.spinal canal;ch.notochord;n.e.neurenteric canal;hy.hypoblast;p.a.g.postanal gut;pr.remains of primitive streak folded in on the ventral side;al.allantois;me.splanchnic mesoblast;an.point where anus will be formed;p.c.perivisceral cavity;am.amnion;so.somatopleure;sp.splanchnopleure.
Fig. 425. Diagrammatic longitudinal section through the posterior end of an embryo Bird at the time of the formation of the Allantois.ep.epiblast;Sp.c.spinal canal;ch.notochord;n.e.neurenteric canal;hy.hypoblast;p.a.g.postanal gut;pr.remains of primitive streak folded in on the ventral side;al.allantois;me.splanchnic mesoblast;an.point where anus will be formed;p.c.perivisceral cavity;am.amnion;so.somatopleure;sp.splanchnopleure.
Amongst the amniotic Vertebrata the postanal gut is less developed than in the Ichthyopsida. A neurenteric canal is present for a short period in various Birds (Gasser, etc.) and in the Lizard, but disappears very early. There is however, as has been pointed out by Kölliker, a well-marked postanal gut continued as a narrow tube from behind the cloaca into the tail both in the Bird (fig. 425,p.a.g.) and Mammals (the Rabbit), but especially in the latter. It atrophies early as in lower forms.
The morphological significance of the postanal gut and of the neurenteric canal has already been spoken of in ChapterXII., p.323.
The Stomodæum.
The anterior section of the permanent alimentary tract is formed by an invagination of epiblast, constituting a more or less considerable pit, with its inner wall in contact with the blind anterior extremity of the alimentary tract.
In Ascidians this pit is placed on the dorsal surface (fig. 9,o), and becomes the permanent oral cavity of these forms. In the larva of Amphioxus it is stated to be formed unsymmetrically(videp.5), but further observations on its development are required.
Illustration: Figure 426Fig. 426. Longitudinal section through the brain of a young Pristiurus embryo.cer.unpaired rudiment of the cerebral hemispheres;pn.pineal gland;In.infundibulum;pt.ingrowth from mouth to form the pituitary body;mb.mid-brain;cb.cerebellum;ch.notochord;al.alimentary tract;Iaa.artery of mandibular arch.
Fig. 426. Longitudinal section through the brain of a young Pristiurus embryo.cer.unpaired rudiment of the cerebral hemispheres;pn.pineal gland;In.infundibulum;pt.ingrowth from mouth to form the pituitary body;mb.mid-brain;cb.cerebellum;ch.notochord;al.alimentary tract;Iaa.artery of mandibular arch.
In the true Vertebrata it is always formed on the ventral surface of the head, immediately behind the level of the fore-brain (fig. 426), and is deeper in Petromyzon (fig. 416,m) than in any other known form.
From the primary buccal cavity or stomodæum there grows out the pituitary pit (fig. 426,pt), the development of which has already been described (p.435).
The wall separating the stomodæum from the mesenteron always becomes perforated, usually at an early stage of development, and though in Petromyzon the boundary between the two cavities remains indicated by the velum, yet in the higher Vertebrata all trace of this boundary is lost, and the original limits of the primitive buccal cavity become obliterated; while a secondary buccal cavity, partly lined by hypoblast and partly by epiblast, becomes established.
This cavity, apart from the organs which belong to it, presents important variations in structure. In most Pisces it retains a fairly simple character, but in the Dipnoi its outer boundary becomes extended so as to enclose the ventral opening of the nasal sack, which thenceforward constitutes the posterior nares.
In Amphibia and Amniota the posterior nares also open well within the boundary of the buccal cavity.
In the Amniota further important changes take place.
In the first place a plate grows inwards from each of the superior maxillary processes (fig. 427,p), and the two plates, meeting in the middle line, form a horizontal septum dividing the front part of the primitive buccal cavity into a dorsal respiratory section (n), containing the opening of the posterior nares, and a ventral cavity, forming the permanent mouth. Thetwo divisions thus formed open into a common cavity behind. The horizontal septum, on the development within it of an osseous plate, constitutes the hard palate.
Illustration: Figure 427Fig. 427. Diagram shewing the division of the primitive buccal cavity into the respiratory section above and the true mouth below.(From Gegenbaur.)p.palatine plate of superior maxillary process;m.permanent mouth;n.posterior part of nasal passage;e.internasal septum.
Fig. 427. Diagram shewing the division of the primitive buccal cavity into the respiratory section above and the true mouth below.(From Gegenbaur.)p.palatine plate of superior maxillary process;m.permanent mouth;n.posterior part of nasal passage;e.internasal septum.
An internasal septum (fig. 427,e) may more or less completely divide the dorsal cavity into two canals, continuous respectively with the two nasal cavities.
In Mammalia a posterior prolongation of the palate, in which an osseous plate is not formed, constitutes the soft palate.
The second change in the Amniota, which also takes place in some Amphibia, is caused by the section of the mesenteron into which the branchial pouches open, becoming, on the atrophy of these structures, converted into the posterior part of the buccal cavity.
The organs derived from the buccal cavity are the tongue, the various salivary glands, and the teeth; but the latter alone will engage our attention here.
The teeth. The teeth are to be regarded as a special product of the oral mucous membrane. It has been shewn by Gegenbaur and Hertwig that in their mode of development they essentially resemble the placoid scales of Elasmobranchii, and that the latter structures extend in Elasmobranchii for a certain distance into the cavity of the mouth.
As pointed out by Gegenbaur, the teeth are therefore to be regarded as more or less specialised placoid scales, whose presence in the mouth is to be explained by the fact that the latter structure is lined by an invagination of the epidermis; The most important developmental point of difference between teeth and placoid scales consists in the fact, that in the case of the former there is a special ingrowth of epiblast to meet a connective tissue papilla which is not found in the latter.
Although the teeth are to be regarded as primitively epiblastic structures, they are nevertheless found in Teleostei and Ganoidei on the hyoidand branchial arches; and very possibly the teeth on some other parts of the mouth are developed in a true hypoblastic region.
The teeth are formed from two distinct organs,viz.an epithelial cap and a connective tissue papilla.
The general mode of development, as has been more especially shewn by the extended researches of Tomes, is practically the same for all Vertebrata, and it will be convenient to describe it as it takes place in Mammalia.
Along the line where the teeth are about to develop, there is formed an epithelial ridge projecting into the subjacent connective tissue, and derived from the innermost columnar layer of the oral epithelium. At the points where a tooth is about to be formed this ridge undergoes special changes. It becomes in the first place somewhat thickened by the development of a number of rounded cells in its interior; so that it becomes constituted of (1) an external layer of columnar cells, and (2) a central core of rounded cells; both of an epithelial nature. In the second place the organ gradually assumes a dome-shaped form (fig. 428,e), and covers over a papilla of the subepithelial connective tissue (p) which has in the meantime been developed.
Illustration: Figure 428Fig. 428. Diagram shewing the development of the teeth.(From Gegenbaur.)p.dental papilla;e.enamel organ.
Fig. 428. Diagram shewing the development of the teeth.(From Gegenbaur.)p.dental papilla;e.enamel organ.
From the above epithelial structure, which may be called theenamel organ, and from the papilla it covers, which may be spoken of as thedental papilla, the whole tooth is developed. After these parts have become established there is formed round the rudiment of each tooth a special connective tissue capsule; known as thedental capsule.
Before the dental capsule has become definitely formed the enamel organ and the dental papilla undergo important changes. The rounded epithelial cells forming the core of the enamel organ undergo a peculiar transformation into a tissue closely resembling ordinary embryonic connective tissue, while at the same time the epithelium adjoining the dental papilla and covering the inner surface of the enamel organ, acquires a somewhat different structure to the epithelium on the outer side of the organ. Its cells become very markedly columnar, and form a very regular cylindrical epithelium. This layer alone is concerned in forming the enamel. The cells of the outer epithelial layer of the enamel organ become somewhat flattened, and the surface of the layer is raised into a series of short papillæ which project into the highly vascular tissue of the dental sheath. Betweenthe epithelium of the enamel organ and the adjoining connective tissue there is everywhere present a delicate membrane known as the membrana præformativa.
The dental papilla is formed of a highly vascular core and a non-vascular superficial layer adjoining the inner epithelium of the enamel organ. The cells of the superficial layer are arranged so as almost to resemble an epithelium.
The first formation of the hard structures of the tooth commences at the apex of the dental papilla. A calcification of the outermost layer of the papilla sets in, and results in the formation of a thin layer of dentine. Nearly simultaneously a thin layer of enamel is deposited over this, from the inner epithelial layer of the enamel organ (fig. 428). Both enamel and dentine continue to be deposited till the crown of the tooth has reached its final form, and in the course of this process the enamel organ is reduced to a thin layer, and the whole of the outer layer of the dental papilla is transformed into dentine—while the inner portion remains as the pulp.
The root of the tooth is formed later than the crown, but the enamel organ is not prolonged over this part, so that it is only formed of dentine.
By the formation of the root the crown of the tooth becomes pushed outwards, and breaking through its sack projects freely on the surface.
The part of the sack which surrounds the root of the tooth gives rise to the cement, and becomes itself converted into the periosteum of the dental alveolus.
The general development of the enamel organs and dental papillæ is shewn in the diagram (fig. 428). From the epithelial ridge three enamel organs are represented as being developed. Such an arrangement may occur when teeth are successively replaced. The lowest and youngest enamel organ (e) has assumed a cap-like form enveloping a dental papilla, but no calcification has yet taken place.
In the next stage a cap of dentine has become formed, while in the still older tooth this has become covered by a layer of enamel. As may be gathered from this diagram, the primitive epithelial ridge from which the enamel organ is formed is not necessarily absorbed on the formation of a tooth, but is capable of giving rise to fresh enamel organs. When the enamel organ has reached a certain stage of development, its connection with the epithelial ridge is ruptured (fig. 428).
The arrangement represented infig. 428, in which successive enamel organs are formed from the same epithelial ridge, is found in most Vertebrata except the Teleostei. In the Teleostei, however (Tomes), a fresh enamel organ grows inwards from the epithelium for each successively formed tooth.
The Proctodæum.
In all Vertebrata the cloacal section of the alimentary tract which receives the urinogenital ducts is placed in communicationwith the exterior by means of an epiblastic invagination, constituting a proctodæum.
This invagination is not usually very deep, and in most instances the boundary wall between it and the hypoblastic cloaca is not perforated till considerably after the perforation of the stomodæum; in Petromyzon, however, its perforation is effected before the mouth and pharynx are placed in communication.
The mode of formation of the proctodæum, which is in general extremely simple, is illustrated byfig. 420an.
In most forms the original boundary between the epiblast of the proctodæum and the hypoblast of the primitive cloaca becomes obliterated after the two have become placed in free communication.
Illustration: Figure 429Fig. 429. Diagrammatic longitudinal section through the posterior end of an embryo Bird at the time of the formation of the Allantois.ep.epiblast;Sp.c.spinal canal;ch.notochord;n.e.neurenteric canal;hy.hypoblast;p.a.g.postanal gut;pr.remains of primitive streak folded in on the ventral side;al.allantois;me.mesoblast;an.point where anus will be formed;p.c.perivisceral cavity;am.amnion;so.somatopleure;sp.splanchnopleure.
Fig. 429. Diagrammatic longitudinal section through the posterior end of an embryo Bird at the time of the formation of the Allantois.ep.epiblast;Sp.c.spinal canal;ch.notochord;n.e.neurenteric canal;hy.hypoblast;p.a.g.postanal gut;pr.remains of primitive streak folded in on the ventral side;al.allantois;me.mesoblast;an.point where anus will be formed;p.c.perivisceral cavity;am.amnion;so.somatopleure;sp.splanchnopleure.
In Birds the formation of the proctodæum is somewhat more complicated than in other types, owing to the outgrowth from it of the bursa Fabricii.
The proctodæum first appears when the folding off of the tail end of the embryo commences (fig. 429,an) and is placed near the front (originally the apparent hind) end of the primitive streak. Its position marks out the front border of the postanal section of the gut.
The bursa Fabricii first appears on the seventh day (in the chick), as a dorsal outgrowth of the proctodæum. The actual perforation of the septum between the proctodæum and the cloacal section of the alimentary tract is not effected till about the fifteenth day of fœtal life, and the approximationof the epithelial layers of the two organs, preparatory to their absorption, is partly effected by the tunneling of the mesoblastic tissue between them by numerous spaces.
The hypoblastic section of the cloaca of birds, which receives the openings of the urinogenital ducts, is permanently marked off by a fold from the epiblastic section or true proctodæum, with which the bursa Fabricii communicates.
Bibliography.
Alimentary Canal and its appendages.
(561)B. Afanassiew. “Ueber Bau u. Entwicklung d. Thymus d. Säugeth.”Archiv f. mikr. Anat.Bd.XIV. 1877.(562)Fr. Boll.Das Princip d. Wachsthums.Berlin, 1876.(563)E. Gasser. “Die Entstehung d. Cloakenöffnung bei Hühnerembryonen.”Archiv f. Anat. u. Physiol., Anat. Abth.1880.(564)A. Götte.Beiträge zur Entwicklungsgeschichte d. Darmkanals im Hühnchen.1867.(565)W. Müller. “Ueber die Entwickelung der Schilddrüse.”Jenaische Zeitschrift,Vol.VI. 1871.(566)W. Müller. “Die Hypobranchialrinne d. Tunicaten.”Jenaische Zeitschrift,Vol.VII. 1872.(567)S. L. Schenk. “Die Bauchspeicheldrüse d. Embryo.”Anatomischphysiologische Untersuchungen.1872.(568)E. Selenka. “Beitrag zur Entwicklungsgeschichte d. Luftsäcke d. Huhns.”Zeit. f. wiss. Zool.1866.(569)L. Stieda.Untersuch. üb. d. Entwick. d. Glandula Thymus, Glandula thyroidea, u. Glandula carotica.Leipzig, 1881.(570)C. Fr. Wolff.“De formatione intestinorum.”Nov. Comment. Akad. Petrop.1766.(571)H. Wölfler.Ueb. d. Entwick. u. d. Bau d. Schilddrüse.Berlin, 1880.Videalso Kölliker (298), Götte (296), His (232and297), Foster and Balfour (295), Balfour (292), Remak (302), Schenk (303), etc.
Teeth.
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[278]Wölfler (No.571) states that in the Pig and Calf the thyroid body is formed as a pair of epithelial vesicles, which are developed as outgrowths of the walls of the first pair of visceral clefts. He attempts to explain the contradictory observations of other embryologists by supposing that they have mistaken the ventral ends of visceral pouches for an unpaired outgrowth of the throat. Stieda (No.569) also states that in the Pig and Sheep the thyroid arises as a paired body from the epithelium of a pair of visceral clefts, at a much later period than would appear from the observations of His and Kölliker. In view of the comparative development of this organ it is difficult to accept either Wölfler’s or Stieda’s account. Wölfler’s attempt to explain the supposed errors of his predecessors is certainly not capable of being applied in the case of Elasmobranch Fishes, or of Petromyzon; and I am inclined to think that the method of investigation by transverse sections, which has been usually employed, is less liable to error than that by longitudinal sections which he has adopted.[279]For details on these organsvideKölliker,Entwicklungsgeschichte,p.881.[280]H. Eisig, “Ueb. d. Vorkommen eines schwimmblasenähnlichen Organs bei Anneliden.”Mittheil. a. d. zool. Station z. Neapel,Vol.II.1881.
[278]Wölfler (No.571) states that in the Pig and Calf the thyroid body is formed as a pair of epithelial vesicles, which are developed as outgrowths of the walls of the first pair of visceral clefts. He attempts to explain the contradictory observations of other embryologists by supposing that they have mistaken the ventral ends of visceral pouches for an unpaired outgrowth of the throat. Stieda (No.569) also states that in the Pig and Sheep the thyroid arises as a paired body from the epithelium of a pair of visceral clefts, at a much later period than would appear from the observations of His and Kölliker. In view of the comparative development of this organ it is difficult to accept either Wölfler’s or Stieda’s account. Wölfler’s attempt to explain the supposed errors of his predecessors is certainly not capable of being applied in the case of Elasmobranch Fishes, or of Petromyzon; and I am inclined to think that the method of investigation by transverse sections, which has been usually employed, is less liable to error than that by longitudinal sections which he has adopted.
[279]For details on these organsvideKölliker,Entwicklungsgeschichte,p.881.
[280]H. Eisig, “Ueb. d. Vorkommen eines schwimmblasenähnlichen Organs bei Anneliden.”Mittheil. a. d. zool. Station z. Neapel,Vol.II.1881.