Book - Manual of Human Embryology 17-2

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Grosser O. Lewis FT. and McMurrich JP. The Development of the Digestive Tract and of the Organs of Respiration. (1912) chapter 17, vol. 2, in Keibel F. and Mall FP. Manual of Human Embryology II. (1912) J. B. Lippincott Company, Philadelphia.

XVII. The Development of the Digestive Tract and of the Organs of Respiration: Introduction | Early Entodermal Tract | Mouth and Its Organs | Oesophagus | Stomach | Small Intestine | Large Intestine | Literature | Liver | Pancreas | Pharynx and its Derivatives | Respiratory Apparatus | Figures | Literature
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The Mouth and Its Organs

J. Playfair McMurrich

By J. Playfair McMurrich, Toronto.

The Mouth

The examination of a human embryo a little over 2 mm in length will reveal upon the ventral surface immediately in front of the yolk-sac a rounded elevation, the heart, and in front of this a somewhat pentagonal depression, the oral sinus, the anterior boundary of which is formed by the projecting frontal extremity of the brain, while the remaining sides are formed by the maxillary and mandibular processes of the first branchial arch (Fig. 248). The mandibular processes separate the sinus from the anterior surface of the pericardium, and their union in the middle line is marked by a groove, which forms the posterior angle of the sinus. The remaining angles are paired, the posterior ones being the angles between the maxillary and mandibular processes of either side, while the anterior ones are formed by the ventral ends of grooves which separate the maxillary process of either side from the frontal process. The floor of the sinus is formed by a thin pharyngeal membrane (Fig. 249, Pm), which separates it from the pharyngeal cavity and is lined upon its outer surface by ectoderm and upon its inner surface by endoderm, the two layers, indeed, being in contact throughout the entire extent of the membrane, no mesoderm intervening. Immediately anterior to the membrane a pocket-like evagination of the ectoderm toward the base of the brain occurs, forming what is known as Rathke's pouch (Fig. 249, Rp), destined to form the anterior lobe of the hypophysis cerebri.

Keibel Mall 2 248.jpg

Fig. 248. — Ventral view of the anterior portion of an embryo of 2.15 mm., from a reconstruction. (His.) Fr., frontal process; H ., heart; Mx., maxillary process; Man., mandibular process; Os., oral sinus.

Keibel Mall 2 249.jpg

Fig. 249. — Median longitudinal section of a rabbit embryo. (After Keibel.) Ch, chorda; Ekt, ectoderm; En, endoderm; H, heart; Rp, Rathke's pouch; Ph, pharynx; Pm, pharyngeal membrane; v.H, forebrain.

The oral sinus, however, does not correspond to the definitive month, which includes also a portion of the embryonic pharynx. Shortly after the stage just described (2.15 mm.), the pharyngeal membrane ruptures and disappears, with the exception of a part of its anterior border, which persists for a time as a transverse ridge upon the roof of the mouth, immediately posterior to Eathke 's pouch. Behind this ridge an evagination of the endoderm toward the base of the brain takes place, forming what is known, from its discoverer (Seessel, 1877), as Seessel's pouch (Fig. 250), a structure whose significance is uncertain.

Keibel Mall 2 250.jpg

Fig. 250. Median longitudinal section through the mouth region of an embryo chick of 5 days. (After Seessel.) Ch, chorda; H, Rathke's pouch; G, brain; M, mouth; N, Seessel's pouch; P, pharynx; In, infundibulum; Mn, mandibular process.

Seessel's pouch has been described (Nussbamn, 1896) as elongating in embryos of the dog until it came into contact with the hypophyseal downgrowth of the brain, whereupon its lumen disappeared and it eventually fragmented into a number of portions, the uppermost of which remained in connection with the hypophysis and became part of it.

By the disappearance of the pharyngeal membrane the oral sinus becomes continuous with the embryonic pharynx, and the anterior part of the digestive tract is placed in communication with the exterior ; the grooves, however, which separate the medial ends of the maxillary processes from the frontal process still maintain the open communication between the mouth cavity and the nasal pits. Later, the region of the frontal lobe which forms the anterior boundary of the oral sinus becomes a flat or slightly concave surface, whose lateral, thickened margins form the medial walls of the nasal fossae and terminate posteriorly in rounded elevations, the processus globulares (Fig. 251). In embryos of 8 mm. the nasal fossae are still open to the mouth, but later the posterior border of the lateral wall of each fossa approaches the corresponding processus globularis and eventually unites with it, the fossae thus becoming converted into pits completely shut off from the mouth (Hochstetter, 1892). Still later the medial ends of the maxillary processes also unite with the processus globulares, these gradually approach each other until they meet in the median line, and the anterior boundary of the mouth is completed, consisting of the processus globulares medially, and laterally of the right and left maxillary processes. At this stage the floor of the nasal pits is separated from the mouth cavity merely by a thin membrane, the bucconasal membrane, formed of the nasal epithelium in contact with that of the roof of the mouth cavity, and in embryos of 15. 5 mm. this membrane breaks through and the nasal and oral cavities are again in communication, but the communications, the primitive choance, are now behind the maxillary processes.

Keibel Mall 2 251.jpg

Fig. 251. — Face of human embryo of 8 mm. (After His.) mxp., maxillary process; np., nasal pit;

Keibel Mall 2 252.jpg

Fig. 252. — Roof of the mouth of human embryo showing the formation of the primary labial pg., processus globularis; os., oral sinus. grooves. (After His.) lg., primary labial groove; mp.. maxillary process; pg., processus globularis.

Several abnormalities may arise in connection with the development of the oral sinus, the most frequent of which is harelip, consisting 1 in a cleft extending through the upper lip slightly lateral to the middle line on either one or both sides and placing the vestibulum oris in communication with nasal pits. This finds its morphological explanation in a failure of the maxillary processes to unite with the processus globulares, whereby the original connection of the nasal pits with the oral sinus is retained. Other abnormalities of less frequent occurrence are dependent upon the imperfect or excessive development of the connection between the maxillary and mandibular processes from which the cheeks are developed, imperfection in this respect producing an abnormal broadening of the rima oris (macrostomia), and excess to its abnormal diminution (microstomia) or even to its suppression (astomia). Inhibition of the development of the mandibular processes may also occur, the two failing to meet in the median line and thus producing a more or less pronounced defect of the lower portion of the face (aprosopia).

The Formation of the Lips and Cheeks

Shortly after the fusion of the maxillary processes with the processus globulares is effected, a slight groove makes its appearance on the free border of the frontal process and eventually extends laterally on the maxillary processes. These are the primary labial grooves (Fig. 252), and are due to a downgrowth into the subjacent mesoderm of the epithelium covering the structures concerned. In embryos about 4 cm. in length a disintegration of the central cells of the downgrowths occurs, the result being a deepening of the grooves to form the secondary labial grooves (Bild, 1902), which separate the lips from the alveolar portions of the various processes (Fig. 254) and themselves form the vestibulum oris. The portion of the upper labial groove which forms on the processus globulares is at first partly interrupted in the median line by an anteroposterior furrow, which corresponds to the line of union of the two processus globulares. In some mammals, as for instance the rabbit, this furrow persists throughout life as a deep median slit in the upper lip, but in man it becomes almost obliterated and is represented in the adult only by the philtrum. The labial groove, however, does not extend as deeply into the tissue of the frontal lobe in the region of the furrow as it does more laterally, and there is consequently formed in the median line a slight fold lying in the sagittal plane and extending between the lip and the alveolar portion of the jaw, the frenulum labii superioris. Its development is associated with the occurrence of the intermaxillary suture, and a similar frenulum labii inferioris is formed opposite the intermandibular suture.

At the angle of the mouth the upper and lower primary labial grooves become continuous, and the epithelial downgrowths are here directed laterally and dorsally. By the disintegration of the central cells of the downgrowths in these regions the buccal cavities are formed, separating the alveolar portions of the maxillary and mandibular processes from the cheeks. The buccal cavities are thus merely lateral extensions of the labial grooves and the structure of the lips and cheeks is identical, both being formed in these early stages of an external epidermal layer and an internal mucous layer, these two meeting at the angles and margins of the mouth, an<J between the two a layer of mesenchyme. It is not until after the beginning of the second month of development that muscular tissue begins to make its appearance in the mesenchyme layer, wandering into it from the region of the second branchial arch and bringing with it branches of the facial nerve (see Vol. I, p. 513).

The condition in which the epidermal and mucous layers meet at the margins of the mouth does not, however, persist, but the mucous layer becomes everted to form the red portions of the lips, its meeting with the epidermal layer being some distance away from the actual rima oris. At birth, as was first pointed out by Luschka (1863), the red portion of the lip consists of two parts, an external pars glabra, whose surface is quite smooth, and a more proximal pars villosa, covered with numerous minute villosities which contain blood-vessels and may in some cases reach a length of 1 mm. (Fig. 253, A). These villosities, which also occur upon the mucosa of each cheek along a band extending from the angle of the mouth almost to the back of the buccal cavity (Fig. 253, B), make their appearance during the fourth month and are fully developed at the seventh month (Stieda, 1889) ; they disappear during the first weeks of extra-uterine life, but even in the adult the area occupied by the pars villosa can be distinguished by the papillae of its corium being more scattered and more irregular in height than those of the region representing the pars glabra. In addition to this differentiation of the red portions of the lips, there is in the upper lip, from the third month until shortly after birth, a well-marked tubercle, situated in the median line below the philtrum, from which it is separated by a portion of the pars glabra about 1 mm. in breadth (Fig. 253, B). At birth the tubercle is from 5 to 6 mm. broad and has a height of about 4 mm., and it bears along its median line a whitish raphe, continuous below with the frenulum. It is formed entirely from the pars villosa.

Keibel Mall 2 253.jpg

Fig. 253. — A, the lips of a new-born child, showing the villosities and the tubercle; B, the distribution of the villosities upon the vestibulum oris. (After Ramm.) a, opening of the parotid duct.

As stated, both the tubercle and the villosities of the pars villosa visually disappear during the first few weeks after birth, but indications of the tubercle are frequently to be seen even in the adult, and occasionally the pars villosa remains distinctly recognizable apart from its histological peculiarities, as a roughened projecting area of the red of the lip, separated from the pars glabra by a distinct groove. Such a condition forms what is termed a double lip.

The Formation of the Palate

The mouth cavity formed as described in the preceding pages does not, however, correspond with that of the adult, its roof being formed by the base of the skull, so that it includes portions of what will later be the nasal cavity as well as the mouth cavity proper. The separation of these two cavities is brought about by the formation of the palate, which takes place as follows : At the time of the formation of the labial grooves the maxillary processes have a triangular shape in transverse section, one of the angles being directed medially: As development proceeds, this angle enlarges to form a plate-like fold (Fig. 254, p), which projects downward toward the floor of the mouth, between the lateral surface of the tongue and the alveolar portion of the maxillary process. In these early stages the long axis of the tongue is directed almost vertically and its dorsal surface is still in contact with the base of the skull, but later, with the enlargement of the arch formed by the two mandibular processes, the tongue sinks down between these processes, its tip at the same time becoming bent downwards. By these changes the tongue is withdrawn from between the two palatal plates, and they gradually bend upward so that their free borders are directed medially instead of toward the floor of the mouth. Exactly how this change is effected has been a matter of discussion. His (1901) believed the withdrawal of the tongue from between the two palatal plates to be due to muscular action, and, with Dursy (1869), supposed the palatal processes were simply bent upward to their final horizontal position. Polzl (1905), however, opposed both these ideas, maintaining that the withdrawal of the tongue was due to changes in the proportions of the parts entering into the formation of the face and that the change of the palatal processes was due to a change in the direction of their growth and not a mere process of bending upward, basing this latter conclusion on the fact that the palatine nerve, which can be traced into the processes while they still have a vertical position, does not change its course in later stages. Schorr (1908) dissents from this conclusion, and finds that the change is really due to a bending up of the processes, a lively proliferation of the tissue on the oral surface of the processes taking place in the angle between them and the alveolar portion of the jaw (Fig. 255, A and B), so that this angle becomes gradually increased. He finds that the palatine nerve lies lateral to the region in which the proliferation takes place, a fact which explains its retention of its original vertical course ; its branches, however, which are directed medially, approach more nearly a horizontal direction as older embryos are examined.

Keibel Mall 2 254.jpg

Fig. 254. Roof of the mouth of a human embryo showing the formation of the palate. (After His.) dr., dental ridge; lg., secondary labial groove; mp., maxillary' process; p., palate; pg., processus globularis; ph., pharynx; Rp., Rathke's pouch.

The palatal processes are entirely confined to the maxillary processes, not extending upon the processus globulares (Fig. 254), and when they have assumed their horizontal position their free, borders are closer together anteriorly than posteriorly, owing to the curvature of the maxillary processes. As the palatal processes increase in breadth in the further course of development, their free borders gradually approach each other and eventually unite, at first anteriorly, the fusion later extending backwards. The mouth cavity proper thus becomes separated from the nasal cavities, these latter now opening posteriorly into the pharynx by the secondary or definitive choanae, which thus owe their existence to the development of the palate. Furthermore the development of the palate brings about the delimitation of the mouth cavity from the definitive pharynx by the formation of the arcus pharyngopalatini, these being the backward prolongation of the palatal processes upon the lateral walls of the pharynx. They pass downward and backward upon the pharyngeal wall almost in the line of the third branchial arches, the arcus glossopalatini and the sinus tonsillar es being formed respectively by the second branchial arch and the second branchial grooves.

Keibel Mall 2 255.jpg

Fig. 255. — Frontal section through the palatine process of a pig embryo of 24 mm. (A) and of 25 mm. (B). (After Schorr.) a., art. palatina; x., proliferating mesenchyme over AW., the angle between the palatine and alveolar processes; xx., proliferating mesenchyme over the dental ridge.

The palatal processes are thus derived entirely from the maxillary processes, and anteriorly, in the median line, there projects backward between them the lower border of the frontal process. With this the palatal processes eventually fnse, but at the meeting point in the median line there remains upon the oral surface a depression known as the incisive fossa. Furthermore, the fusion is not perfectly complete; the epithelia on both surfaces become perfectly continuous, but the intervening mesenchyme does not, a strip of epithelium extending through it from the fossa incisiva, upwards, backwards, and somewhat laterally, in the line of fusion of each palatal process with the corresponding processus globularis. The cells of the epithelial strip break down so that a lumen is formed in it, placing the mouth and nasal cavities again in communication anteriorly by what are known as the incisive canals (canals of Stenson). These usually become obliterated during further development, but occasionally they persist until adult life. Toward the close of the second month of development ossification begins to extend into the mesenchyme of the palatal processes from the alveolar processes, and the hard palate becomes readily distinguishable from the velum palati ; but for a considerable time, up to at least about the middle of the third month, the palate continues to show a distinct median raphe (Fig. 256, A), this indicating the line of fusion of the two palatal processes and terminating anteriorly in the incisive papilla. The uvula also remains distinctly notched for a considerable period (Fig. 256, A), an indication that it is really a bilateral structure and not, as it appears in the adult, a median unpaired organ. On either side of the median raphe on the hard palate from five to seven almost transverse ridges appear (Fig. 256, A), which represent the palatal ridges occurring in the lower mammals.* These ridges later develop minute fringe-like processes on their posterior borders, but at the same time they begin to undergo degeneration, the posterior ones breaking up into rows of papillae, while the regularity of the anterior ones is disturbed by the formation of cross branches. At birth (Fig. 256, B) the fringe-like processes have almost disappeared, as has also the posterior ridge, and the anterior ones have become very irregular. In this condition they persist throughout childhood, but in adult life they become still more reduced and may eventually disappear altogether.

Keibel Mall 2 256.jpg

Fig. 256. — Palate of (A) a human embryo of 5.5 cm. and of (B) a new-born child showing the palata ridges. (After Gegenbaur.) ±a, alveolar process; p., incisive papilla; pro, velum palati; -., median raphe: u, uvula.

Inhibition of the development of the palatal processes occasionally occurs, resulting in a failure of their fusion in the middle line, the defect constituting what is known as cleft palate. This may vary considerably in its extent, being limited in some cases to the velum palati, in others appearing as a perforation of the palate in the median line, and in others again involving the hard palate as well as the velum palati. In these last cases the cleft cannot continue forward in the median line beyond the anterior extremities of the maxillary processes, since it there meets with the unpaired frontal process, but it may be continued along the line of union of the maxillary and frontal processes on one or both sides, in which case it will usually be associated with harelip.

The Tongue

The tongue in the amniote vertebrates consists of two portions, quite distinct in their origin and represented in the adult by the body of the tongue anteriorly and the root posteriorly, the two being separated by a V-shaped groove, the sulcus terminalis. An examination of the floor of the mouth of an embryo of 5 mm. (Fig. 257) shows a rhomboidal depression in the median line between the ventral ends of the first and second branchial arches, and from this there projects dorsally a rounded tubercle, the tuberculum impar of His. Immediately behind this is a deep evagination of the epithelium, which is the median thyreoid evagination, and behind this again is a transverse elevation formed by the ventral ends of the second and third branchial arches, the copula. Since the apex of the V-shaped sulcus terminalis corresponds with the foramen caecum, and since this is the remains of the median thyreoid evagination, it would seem that the anterior portion of the tongue is formed from the region between the first and second branchial arches. It was held by His that it was formed by the enlargement of the tuberculum impar, although as early as 1869 Dursy had described the body of the tongue as having a paired origin, a condition more recently described as occurring in the pig by Born and in man by Kallius and by Hammar (1901). In embryos of 7.5 mm. a swelling appears in the anterior part of the mouth on each side of the median line (Fig. 258, t), and the two increase in size until they occupy the greater part of the interval between the lower ends of the first and second branchial arches, becoming separated from the former by an alveoloUngual groove. The two swellings eventually meet in the median line to form the main mass of the body of the tongue, the amount to which the tuberculum impar participates being probably small, Hammar, indeed, maintaining that it is merely a transitory structure and takes no part at all in the formation of the tongue.

Immediately behind the median thyreoid evagination the lower ends of the second and third branchial arches join to form a median elevation, the copula (Fig. 258, cop), on the floor of the mouth, and from the anterior portion of this, together with the neighboring portions of the second arch, the root of the tongue develops. His believed that the third arch also took part, but recent observers either limit extensively the participation of this arch or exclude it altogether.

Keibel Mall 2 257.jpg

Fig. 257. — Floor of the mouth and pharynx of an embryo of 5 mm. (After His.) F., furcula; Mn., mandibular arch; Ti., tuberculum impar; II-V, the branchial arches. The thyreoid evagination is indicated by a dotted line.

Keibel Mall 2 258.jpg

Fig. 258. — Floor of the mouth and pharynx of an embryo of 7.5 mm. (From a reconstruction.) cop., copula; F., furcula; t., anlage of the body of the tongue; Ti., tuberculum impar; I-III , branchial arches.

It must be remembered, however, that the tongue is a complex of mucous membrane and muscle tissue, and the statements given above indicate only the regions from which the mucosa is derived in the earlier stages of development. The origin of the musculature has not yet been thoroughly studied in the human embryo, but the fact that it is for the most part innervated by the hypoglossus is indication of its derivation from postbranchial myotomes. When first identifiable the various muscles have already reached the branchial region, so that His assigned the hyoglossus to the third arch, but it is probable that it had already undergone a considerable forward migration before it became recognizable. Indeed, even after it is distinctly differentiated its distribution in the tongue is materially extended, and there is reason for supposing that practically the whole of the tongue musculature undergoes an extensive migration from the postbranchial region, pushing forward beneath the mucons membrane of the floor of the pharynx and mouth until it occupies the elevations on the floor of the mouth mentioned above. During this migration it invades in succession the territories of the various branchial arches, and consequently the mucosa of the fully developed tongue is supplied by the nerves corresponding to these arches, that is to say, by the trigeminus and facialis anteriorly and by the glossopharyngeus and vagus posteriorly (Fig. 258).

If this view of the development of the tongue musculature be correct, it would seem that a more extensive area of the oropharyngeal mucosa is involved in the formation of the tongue than that associated with the elevations usually regarded as its origins. These undoubtedly represent the first indications of the organ, but with the later elaboration and increase in bulk of its musculature other portions of the mucosa become involved, the complicated innervation being thus produced. It is hardly accurate, therefore, to regard the mucosa of the tongue as being the product of the first and second branchial arches alone, even though its first indications are confined to them. Phylogenetically the copular portion of the tongue seems to be the older, being the only part present in the fishes. In the amphibia also it constitutes the main mass of the tongue, but anterior to it a glandular fold of the mucosa is formed on the floor of the mouth immediately behind the mandibular arch, and in later larval stages this unites with the copular portion representing the body of the amniote tongue. This in its origin is, therefore, essentially a glandular portion of the tongue, with which muscle-fibres, separated from the geniohyoideus, become associated ; but in the higher f orms its glandular character becomes subordinated and its muscle-fibres increase in number to form the genioglossus. The hyoglossus is probably a derivative of the geniohyoideus, and the intrinsic musculature is apparently derived from these two primary muscles, the transversus linguae and the vertical fibres from the genioglossus (Fig. 261, G-Grl) and the longitudinalis from the hyoglossus (Kallius).

Keibel Mall 2 259.jpg

Fig. 259. — Diagram of the distribution of the sensory nerves of the tongue. (After Zander.) The area supplied by the fifth (and seventh) nerve is indicated by the transverse lines, that supplied by the ninth nerve by the oblique lines, and that supplied by the tenth nerve by the small circles.

The fungiform papillae have become evident in embryos of 50 mm. as elevations of the submucous tissue which project upward into the epithelium, this frequently undergoing proliferation over the elevations so as to produce finger-like papillae, which are, however, merely temporary structures. The filiform papillae are at first indistinguishable from the fungiform, only becoming recognizable in embryos of 64 mm. In embryos of 100 mm. tastebuds begin to make their appearance upon the fungiform papillae, and somewhat later, about the beginning of the fifth month, both varieties begin to project above the general surface of the tongue, owing to the degeneration of the superficial layers of the epithelium in the intervals between the papillae. The development of the taste-buds on the fungiformes continues during the later fetal months, and at birth, as well as for some time after, the buds are greatly in excess of the number present in the adult, their subsquent reduction in number being associated with the, change in the nature of the food of the child occurring at the time of weaning (Stahr).

Keibel Mall 2 260.jpg

Fig. 260. — Diagrams illustrating the development of the vallate papillae. (After Graberg.)

The vallate papillae are represented in embryos of 90 mm. by two epithelial ridges, situated toward the posterior portion of the tongue and inclined toward one another in a V-shaped manner, the apex of the V practically corresponding with the mouth of the median thyreoid evagination. From these ridges downgrowths of the epithelium take place into the subjacent submucosa, each downgrowth having the form of a hollow truncated cone whose base is continuous with the mucosa and whose centre is occupied by a portion of the submucosa, which thus becomes surrounded by a solid wall of epithelial cells (Fig. 260, A). During the fourth month lateral outgrowths take place from the deeper edges of the wall, and at about the same time clefts begin to appear in its substance (Fig. 260, B) ; these increase in size and eventually open to the surface a trench, lined by epithelium, thus surrounding a papilla (Fig. 260, C). The lateral outgrowths from the deeper edges of the downgrowths also become hollow by the degeneration of their central cells and form the glands of Ebner, and during the development taste-buds differentiate from the basal layers of the epithelium. These make their appearance quite early in the development of the papillae, being recognizable even in a fetus of three months (Graberg), and increase in number as development proceeds, being formed not only on the sides of the papillae but also on their free horizontal surfaces, those in the latter situations, however, for the most part disappearing after birth. The development of the individual papillae is subject to considerable variation both in number and time, and, as a rule, is not completed until after birth. The foliate papillae appear much later than the other varieties, being indistinguishable in embryos of four and a half and five months although quite distinct in those of seven months (Tuckerman).

Anomalies of the tongue which may be assigned to interferences with the normal processes of development are not of frequent occurrence. A condition of diglossia has, however, been described, in which the anterior portion of the organ is divided throughout the greater or lesser portion of its extent, producing what might be spoken of as a forked tongue. This is especially interesting as indicating the development of the body of the tongue mainly from two primary anlagen, rather than from a single median structure such as the tuberculum impar.

The Salivary Glands

Of the glands of the mouth the most important are the salivary glands, — that is to say, the parotid, the submaxillary, the sublingual, and the alveololingual. The first three of these are individual glands, formed from a single epithelial outgrowth and having in the adult condition a single duct opening into the mouth cavity, that for the sublingual being known in anatomy as the ductus sublingualis major (duct of Bartholin). The alveololingual glands, on the contrary, consist of a group of glands each of which is provided with its own duct, and they are generally associated with the sublingual gland proper as the glandula sublingualis, a structure which, however, is not comparable morphologically to one of the other salivary glands, but rather to a group of them.

The first of the salivary glands to appear in the embryo is the parotid, which has been detected in an embryo of 8 mm. as a furrow in the floor of the alveolobuccal groove in the neighborhood of the angle of the mouth (Hammar). At first quite small, the furrow gradually elongates, and before the embryo has reached a length of 17 mm. it separates from the epithelium and forms a tubular structure lying beneath the epithelium of the alveolobuccal groove and opening into the mouth cavity at a point which corresponds with the anterior end of the original furrow. Mesenchymatous tissue gradually forces its way between the tube and the alveolobuccal epithelium, and the tube, increasing in length, pushes its way back over the masseter muscle to the neighborhood of the external ear. As it comes into this region the tube or duct, as it may be called, begins to branch at its posterior extremity, the branches being at first solid outgrowths from the wall of the duct, and, as these increase in number and size and become surrounded by a mesenchymatous capsule, the gland assumes the position and general form of the adult structure. The accessory parotid gland arises as an outgrowth from the duct as it passes over the masseter muscle, and its further development is similar to that of the main gland.

The account given above of the origin of the gland is based on the recent observations of Hammar, and these differ in some respects from those of earlier investigators (His, Chievitz), who first perceived the gland in embryos of seven and a half or eight weeks' development and describe it as formed from a solid outgrowth from the alveolobuccal epithelium. It is worthy of note that the gland is primarily lateral to the internal carotid artery, the posterior facial vein, and the facial nerve, and although these structures may eventually become more or less surrounded by its alveoli, yet their position is always medial to the principal ducts of the gland.

In embryos of the twelfth week Chievitz observed a branch arising from the parotid duct just where it crossed the anterior border of the masseter muscle and passing deeply to come into relation with the internal pterygoid, where it ended blindly in a small enlargement. The same structure was also observed in an embryo of ten weeks, but in this case it had lost its connection with the parotid duct, and the same condition may be observed in embryos of nine weeks or even earlier. What its significance may be is at present uncertain, but the possibility of its being the origin of cystic growths in the cheek is perhaps worthy of mention.

The submaxillary gland appears in embryos of the sixth week (13.2 mm.) as a ridge-like thickening of the epithelium of the alveololingual groove, the anterior end of the thickening lying some distance behind the frenulum linguae. The ridge later separates from the epithelium from behind forward, and the solid cord so formed grows downward and backward toward the submaxillary region, its enlarged terminal portion branching to form the gland proper, while the remainder of the ridge becomes the duct (Fig. 261, SMx) and gradually shifts its anterior connection with the epithelium forward until it reaches the adult position. During this development the duct acquires its lumen, although the buds which form the alveoli of the gland remain solid until a much later period.

The sublingual and alveololingual glands develop in a manner very similar to the submaxillary. They appear as solid downgrowths of the epithelium of the alveololingual groove (Fig. 261, SL), the sublingual beginning to form at about the eighth week immediately lateral to the anterior termination of the submaxillary duct, and the alveololingual s somewhat later and posterior to the larger sublinguals. Frequently, however, the sublinguals do not appear (Chievitz), the so-called sublingual gland of the adult then being formed entirely by the alveololinguals, and these also seem to be variable in number, Chievitz finding from 11 to 13 on the two sides in an embrvo of the twelfth week, while in one of 40 mm.

I have found 11 on the left side and 9 on the right, the left side also possessing a sublingual gland although it was absent on the right side.

The histogenetic development of the salivary glands is not completed until some time after birth, probably not until after the child is weaned. The canalization of the solid anlagen of the glands proceeds peripherally, and so long as the terminal branches remain solid they have the power of producing additional buds. When, however, the lumen is formed in a bud and it becomes an alveolus, its power of budding is lost, and the further increase in the size of the gland is due to the development of the investing connective tissue and to an increase in the size of the alveoli already present. The specific characters of the cells also become evident only after the canalization of the alveoli, mucin cells becoming distinguishable in the alveololingual glands of embryos of the 16th week and acinus cells in the parotids of those of five months. The demilune cells of Gianuzzi are developed from the cells lining the alveoli and are only secondarily overgrown by the mucous cells.

Keibel Mall 2 261.jpg

Fig. 261. — Transverse section of the lower jaw and tongue of an embryo of about 20 mm. D, digastricus; G.GL, genioglossus; GH., geniohyoideus; I.Al., inferior alveolar nerve; Man., mandibular ossification; Mk., Meckel's cartilage; My., mylohyoideus; SL., sublingual gland; S.Mx., submaxillary duct; T., tongue.

Anomalies of the salivary glands are of rather infrequent occurrence; a case of inhibition of the growth of the parotid has, however, been described, the gland being entirely confined to the buccal region, no trace of it occurring behind the masseter muscle.

The Teeth

At about the time when the primary labial grooves are formed — that is to say, in embryos of about 11 mm. — a ridge-like thickening of the epithelium appears upon what will be the alveolar portions of the maxillary and mandibular processes and also extends upon the portion of the upper jaw formed from the processus globular es (Fig. 253). These ridges are parallel with and immediately posterior (medial) to the labial grooves, and in later stages they penetrate more deeply into the mesenchyme in a somewhat oblique direction, so that they seem almost to be derivatives of the epithelium of the labial grooves (Fig. 262, A). From the deeper surface of each of these dental ridges a series of papillae project more deeply into the mesenchyme, and in embryos of 40 mm the deeper surface of each papilla has become concave and the concavity is occupied by a mass of condensed mesenchyme, the mesenchyme papilla, the epithelial and mesenchyme papillae together constituting a dental papilla (Fig. 262, B). The number of papillae so formed is normally ten in each jaw, one corresponding to each tooth of the milk dentition, and as they proceed in their development they gradually separate from the dental ridge, which, on its part, becomes prolonged backward in the mesenchyme beyond the point at which the papilla for the second molar of the milk dentition is formed. Three additional papillae appear on each side on these prolongations of the ridges, representing the permanent molars, that for the second molar forming, however, only in the sixth week after birth and that for the third molar not until the fifth year. As the papillae for the milk dentition separate from the dental ridges these begin to degenerate, becoming converted into a network of epithelial trabeculae (Fig. 263), except along their lingual border, where a continuous cord persists ; from this a second series of papillae arises, from which the permanent teeth, which replace the milk dentition, are formed. As the papillae for these teeth separate from the cord, it finally undergoes degeneration and, with the other remains of the dental ridges, eventually disappears, except for fragments of either the cord or the trabeculae which may persist imbedded the surrounding mesenchyme and are known as epithelial pearls.

Keibel Mall 2 262.jpg

Fig. 262. — Section through the dental ridge of the lower jaw of embryos of (A) 17 ram. and (B) 40 mm (After Rose.) LF. and LFL., labial groove; Pp., dental papilla; UK., lower jaw; uL., lower lip; ZL. dental ridge.

In each dental papilla two different structures are concerned, a mesenchyme papilla, from which the tooth pulp and dentine are formed, and an epithelial papilla, which invests the mesenchyme papilla like a cap and gives origin to the enamel, whence it is spoken of, in its later stages, as the enamel-organ. Nerves and blood-vessels make their way into the mesenchyme papilla, and certain of its cells arrange themselves in a single continuous layer over its surface and assume a columnar form, constituting the odontoblasts (Fig. 264, Od) by which the dentine is manufactured. This material appears to be formed by the transformation of a portion of the protoplasm of the odontoblasts into a gelatinous substance, which later becomes fibrillar and in which lime salts are eventually deposited. These deposits are at first in the form of spherical concretions, but later the interstices become filled up, numerous minute dentinal tubules, branching at their outer ends, traversing the matrix from within outwards and containing slender prolongations of the unaltered protoplasm of the odontoblasts, whose growth during the active period of dentine formation compensates for the loss of substance entailed in the formation of the matrix.

Keibel Mall 2 263.jpg

Fig. 263. — Reconstruction of the dental ridge and a papilla of an embryo of 30 cm. (After Rose.) D, dentine; S, enamel ; Zl, cord-like remnant of the dental ridge which gives rise to the papillae of the permanent teeth ; Ms, oral mucous membrane; ET, epithelial trabeculae representing the original dental ridge; Sp, enamel pulp.

This account of the formation of the dentine follows essentially the results of von Ebner. Recently von Korff has maintained that the dentine has a double origin, the first indication of it being bundles of connective-tissue fibrils, which are formed by the pulp cells of the mesenchyme papilla and extend outward between the odontoblasts. These latter structures produce the interfibrillar substance of the dentine and secrete the lime salts which are deposited in this. While results of von Korff bear the stamp of probability, it seems advisable to »u their further confirmation before adopting them in their entirety.

The dentine is formed from the outer ends of the odontoblasts and therefore lies immediately internal to the products of the enamel-organ. This differentiates (Fig. 264) into an outer epithelial layer consisting of more or less flattened cells, beneath which is a mass of tissue composed of stellate cells widely separated by the distention of the intercellular spaces, so that the tissue has a spongy appearance. This is the enamel-pulp, and internal to it is a single layer of large columnar cells, the ameloblasts, which are the active elements in the production of the enamel. The inner cells of the enamel-pulp are usually more closely aggregated than the rest, and form an epithelial-like layer external to the ameloblasts, which is termed the intermediate layer. As in the case of the odontoblasts the ameloblasts persist throughout the entire formation of the enamel, each cell producing one of the enamel-prisms. The first indication of the enamel is a delicate cuticular membrane covering the inner extremities of the ameloblasts, and to this succeeds the formation of a series of homogeneous columns, one corresponding to each ameloblast. Later the homogeneous material differentiates into bundles of fibrils, the enamel processes {processes of Tomes), imbedded in a homogeneous matrix, and, finally, the calcification of the columns ensues, this process being, according to some observers a calcification of the enamel processes, while others hold it to be a deposit of lime salts in the matrix surrounding the fibres. Even before the formation of the enamel is completed the degeneration of the enamel-organ begins, blood capillaries making their way through the outer epithelial layer into the enamel-pulp, which gradually becomes indistinguishable from the surrounding mesenchyme, and finally the layer of ameloblasts breaks up into fragments, some of which are usually to be found around the roots of the teeth even in the adult.

Keibel Mall 2 264.jpg

Fig. 264. — Section through a developing molar tooth of Didelphys. (After Rose.)
  • C - connective tissue
  • D - calcified dentine
  • D1 - uncalcified dentine
  • K - wall of dental alveolus
  • Od - odontoblasts
  • P - pulp cells
  • S - enamel
  • SEa - outer epithelial layer of the enamel organ
  • S.E.i - ameloblasts
  • S.P. - enamel-pulp
  • Str.i - intermediate layer of enamel-organ
  • T - Tomes's processes of the ameloblasts.

The cement which covers the dentine of the roots of the teeth is formed from the surrounding mesenchyme by a process identical with that by which membrane bone is formed.

As the teeth increase in size they gradually approach the surface of the alveolar processes and eventually break through the gum, not, however, at a point in the line of the original downgrowth of the dental ridge, but posterior to this, the first teeth to erupt being usually the median incisors, which make their appearance during the last half of the first year after birth. The remaining teeth of the milk dentition appear in succession up to about the middle of the third year. Shortly after their appearance, however, these teeth begin to undergo absorption, this being associated with the continued growth of the permanent teeth (Fig. 265). The milk-teeth lose their shiny appearance, their pulp dies, and an absorption of their roots occurs, beginning at the side in contact with the corresponding permanent tooth and being associated with the appearance of osteoclasts similar to those producing the absorption of ordinary bone. Their alveoli also undergo absorption, and finally their attachments become so feeble that the teeth are readily pulled or broken away.

Keibel Mall 2 265.jpg

Fig. 265. — Skull of a 5-year-old child showing the milk and permanent dentitions. (After Sobotta.) cp., permanent canine; ip., permanent incisor; mm., milk molars; mp.I, first permanent molar; mp.II, second permanent molar; pmp., permanent premolar.

The exact period of eruption of the various teeth varies considerably according to racial, climatic, and nutritive conditions, but the usual sequence is somewhat as follows :

Human Milk and Permanent Dentition Development Table
The milk dentition
Median incisors 6th to 8th month
Lateral incisors 8th to 12th month
First molars 12th to 16th month
Canines 1 7th to 20th month
Second molars 20th to 24th month
The permanent dentition
First molars 7th year
Median incisors 8th year
Lateral incisors 9th year
First premolars 10th year
Second premolars 11th year
Canines 13th to 14th year
Second molars 13th to 14th year
Third molars 17th to 40th year

Anomalies are not infrequent in connection with the development of the teeth, leading sometimes to a diminution and sometimes to an excess of the normal number. A case of total congenital absence of the teeth has been observed, and also cases in which there had apparently been a defect of the enamel-organ leading to the development of rudimentary teeth lacking enamel. The fusion of two neighboring tooth germs may also occur, as well as the reverse, that is to say, a splitting of a tooth germ so that an accessory tooth or indeed a number of small teeth may be present in the place of one of the normal teeth. More remarkable are the instances of heterotopy which occur, due apparently to the existence of aberrant processes of the dental ridges extending into regions beyond the alveolar processes. Thus, incisor teeth have been observed to form in the nasal cavity, in the maxillary sinus, and even in the orbit, and molars have developed upon the hard palate. Numerous cases of supernumerary dentitions have also been recorded, one or more teeth being replaced more than once. Many of these cases have been supposed to be really the belated development of the normal permanent tooth, but some do not seem referable to this condition, and must be regarded as due to the persistence in an active condition of portions of the dental ridge or to the awakening to functional activity of some of the epithelial pearls which are remnants of it.


Born, G. : Ueber die Derivate der embryonalen Schlundbogen und Schlundspalten bei Saugethieren. Arch, fur mikr. Anat. Bd. 22. 1883.

Chievitz, J. H. : Beitrage zur Entwicklungsgeschichte der Speicheldriisen. Arch. fur Anat. und Phys. Anat. Abth. 1885.

Dursy, E. : Zur Entwicklungsgeschichte des Kopfes des Menschen und der hoheren Wirbelthiere. Tubingen 1869.

Gegenbaur, C. : Die Gaumenf alten des Menschen. Morph. Jahrb. Bd. 4. 1878. Zur Phylogenese der Zunge. Morph. Jahrb. Bd. 21. 1894.

Graberg, J. : Beitrage zur Genese des Geschmacksknospen des Menschen. Morph. Arbeiten. Bd. 8. 1898.

Hammar, J. A.: Notiz iiber die Entwieklung der Zunge und der Mundspeicheldriisen beim Menschen. Anat. Anzeiger. Bd. 19. 1901.

His, W. : Anatomie menschlicher Embryonen. Heft 3. 1885. Die Entwieklung der menschlichen und tierischen Physiognomien. Arch, fur Anat. und Phys. Anat. Abth. 1892.

Hochstetter, F. : Ueber die Bildung der primitiven Choanen beim Menscben. Verbandl. Anat. Gesellscb. 1892.

Kallius, E.: Beitrage zur Entwicklung der Zunge. Verbandl. Anat. Gesellst. 1901.

Keibel, F. : Zur Entwicklungsgescbicbte der Chorda bei Saugern (Meerscbweincben und Kanincben). Arcb. fiir Anat. und Phys. Anat. Abtb. 1889. Zur Entwicklungsgescbicbte und vergleicbenden Anatomie der Nase und des oberen Mundrandes (Oberlippe) bei Vertebraten. Anat. Anzeiger. Bd. 8. 1893.

von Korpf, K. : Die Analogie in der Entwicklung der Knochen- und Zahnbeingrundsubstanz der Saugetiere, nebst kritiscben Bemerkungen iiber die Osteoblasten- und Odontoblastentbeorie. Arcb. fiir mikr. Anat. Bd. 69. 1907.

Neustatter, 0. : Ueber den Lippensaum beim Menscben, seinen Bau, seine Entwicklung und seine Bedeutung. Jenaische Zeitscbr. Bd. 29. 1895.

Nusbaum, J. : Einige neue Tbatsachen zur Entwicklungsgescbicbte der Hypophysis cerebri bei Saugethieren. Anat. Anzeiger. Bd. 12. 1896.

Polzl, A.: Zur Entwicklungsgeschichte des menschlichen Gaumens. Anat. Hefte. Bd. 27. 1905.

Ramm, M. : Ueber die Zotten der Mundlippen und der "Wangenschleimhaut beim Neugeborenen. Anat. Hefte. Bd. 29, 1905.

Rose, C. : Ueber die Entwicklung der Zahne des Menschen. Arch, fiir mikr. Anat. Bd. 38. 1891. Ueber die erste Anlage der Zahnleiste beim Menscben. Anat. Anzeiger. Bd. 8. 1893.

Schorr, G. : Zur Entwicklungsgeschichte des secundaren Gaumens bei einigen Saugethieren und beim Menschen. Anat. Hefte. Bd. 36 (Heft 108). 1908.

Seessel, A.: Zur Entwicklungsgeschichte des Vorderdarms. Arch, fiir Anat. und Phys. Anat. Abth. 1877.

Stahr, H. : Ueber die papillae f ungif ormes der Kinderzunge und ihre Bedeutung als Geschmacksorgan. Zeitschr. fiir Morph. und Anthropol. Bd. 4. 1901.

Stieda, A. : Ueber das Tuberculum labii superioris und die Zotten der Lippenscbleimhaut des Neugeborenen. Anat. Hefte. Bd. 13. 1899.

Tuckerman F. The Development of the Taste-Organs of Man. (1889) J Anat Physiol. 23(4): 559-82. PMID 17231815

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Grosser O. Lewis FT. and McMurrich JP. The Development of the Digestive Tract and of the Organs of Respiration. (1912) chapter 17, vol. 2, in Keibel F. and Mall FP. Manual of Human Embryology II. (1912) J. B. Lippincott Company, Philadelphia.

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