Book - Comparative Embryology of the Vertebrates 4-13

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Nelsen OE. Comparative embryology of the vertebrates (1953) Mcgraw-Hill Book Company, New York.

   Comparative Vertebrate Embryology 1953: 1. The Period of Preparation | 2. The Period of Fertilization | 3. The Development of Primitive Embryonic Form | 4. Histogenesis and Morphogenesis of the Organ Systems | 5. The Care of the Developing Embryo | Figures
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Part IV - Histogenesis and Morphogenesis of the Organ Systems

Part IV - Histogenesis and Morphogenesis of the Organ Systems: 12. Structure and Development of the Integumentary System | 13. Structure and Development of the Digestive System | 14. Development of the Respiratory-buoyancy System | 15. The Skeletal System | 16. The Muscular System | 17. The Circulatory System | 18. The Excretory and Reproductive System | 19. The Nervous System | 20. The Development of Coelomic Cavities | [[Book - Comparative Embryology of the Vertebrates 4-21|21. The Developing En

The Digestive System

A. Introduction

1. General Structure and Regions of the Early Digestive Tube or Primitive Metenteron

a. Definition

The word metenteron is applied to the gut tube which is developed from the archentcric conditions of the gastrula. The term primitive metenteron may be applied to the gut tube shortly after it is formed, that is, shortly after tabulation of the entoderm to form the primitive gut tube has occurred, while the word metenteron, unqualified, is applicable to the tubular gut, generally, throughout all stages of its development following the gastrular state.

b. Two Main Types of the Early Metenteron

Two types or morphological forms of early vertebrate metenterons are developed immediately after the gastrular stage. In one type, such as is found in the frog and other amphibia, ganoids, cyclostomes, and lungfishes, the walls of the gut tube are complete and the yolk material is enclosed principally within the substance of the midgut area of the tube (fig. 217). In the second type, on the other hand, most of the yolk material lies outside the confines of the primitive gut tube (fig. 217), and the midgut region of the primitive tube is open ventrally, the ventro-lateral walls of the tube being incomplete. The latter condition is found in elasmobranch fishes, reptiles, birds, and primitive mammals. In higher mammals, although yolk substance is greatly reduced, the arrangement is similar to that of the latter group. The teleost fishes represent a condition somewhat intermediate between these two major groups.

Fig. 278. Diagrams showing basic features of digestive-tube development in the vertebrates. (A) The regions of the primitive gut where outgrowths (diverticula) normally occur. (B) Basic cellular features of the gut tube. (C) Contributions of the basic cellular composition to the adult structure of the digestive tract. Consult Fig. 293 for actual structure of mucous layer in esophagus, stomach, and intestines.

2. Basic Structure of the Early Metenteron (Gut Tube)

(Consult figs. 278A; 279A; 280A; 281 A; and 282B.)

a. Basic Regions of the Primitive Metenteron

The primitive vertebrate metenteron possesses the following regions.

1) Stomodaeum. The stomodaeum lies at the anterior extremity of the gut tube, and represents an ectodermal contribution to the entodermal portion of the primitive gut. It results from an invagination of the epidermal tube directed toward the oral evagination of the foregut. The membrane, formed by the apposition of the oral evagination of the foregut and the stomodaeal invagination of the epidermal tube, constitutes the oral or pharyngeal membrane. Ectoderm and entoderm thus enter into the composition of the pharyngeal membrane. This membrane normally atrophies.

2) Head Gut or Seessel's Pocket. This structure represents the extreme anterior end of the foregut which projects forward toward the anterior end of the notochord and brain. It extends cephalad beyond the region of contact of the stomodaeum with the oral evagination of the foregut. During its earlier period, the head gut is intimately associated with the anterior end of the notochord and the pre-chordal plate mesoderm. The head gut ultimately degenerates. Its significance probably lies in its function as a part of the head organizer.

Fig. 279. Morphogenesis of the digestive structures in the dog fish, Sqiuilus acanthias. See also Figs. 29 1C and 296 A.

3) Foregut. The foregut comprises the anterior portion of the primitive metenteron from the region of the stomodaeum and Seessel's pocket, posteriorly to the intestinal area where arise the liver and pancreatic diverticula. It is divisible into four general regions:

  1. pharyngeal area,
  2. esophagus,
  3. stomach, and
  4. hepatopyloric segment.

4) Midgut. The midgut area of the gut tube is the general region lying between the foregut and hindgut regions. This segment of the primitive gut eventually differentiates into the greater part of the small intestine. In the early metenteron, the midgut area is concerned with the digestion of yolk material in such forms as the frog or with the elaboration of the yolk sac in the shark, chick, reptile, and mammalian embryos. In addition, it appears that the primitive blood cells also are elaborated in this area. (Sec Chap. 17.)

5) Hindgut. This portion of the early gut tube is located posteriorly, immediately anterior to the proctodacum.

6) Tail Gut (Post-anal Gut). The tail gut represents a dorsal, posterior continuation of the hindgut into the developing tail. As indicated in Chapter 10, it is extremely variable in the extent of its development. (Consult also fig. 217.)

7) Proctodaeum. The epidermal invagination, which meets the proctodaeal or ventral evagination of the hindgut, forms the proctodaeum. The anal membrane results when the proctodaeal inpushing meets the entodermal outpushing of the hindgut. The anal membrane is double, composed of entoderm and ectoderm. It is destined to disappear.

b. Basic Cellular Units of the Primitive Metenteron

Most of the lining tissue of the primitive metenteron is derived from the entoderm of the archenteric conditions of the late gastrula. Associated with the strictly entodermal portion of the primitive metenteron are two contributions of the epidermal tube as observed on pages 598 and 600, namely, the stomodaeum and the proctodaeum. Added to this lining tissue are mesenchymal contributions, derived from the medial or splanchnic layers of the hypomeric mesoderm (fig. 278B).

The glandular structures of the digestive tube are derived as modifications of the lining tissue of the stomodaeal, entodermal, and proctodaeal portions of the primitive gut tube, whereas muscular and connective tissues differentiate from mesenchyme (fig. 278C).

3. Areas of the Primitive Metenteron from which Evaginations (Diverticula) Normally Arise

Certain areas of the primitive metenteron tend to produce outgrowths (evaginations; diverticula). The following comprise these areas (fig. 278A).

a. Stomodaeum

In the middorsal area of the stomodaeum, a sac-like diverticulum or Rathke’s pouch, invaginates dorsally toward the infundibulum of the diencephalic portion of the brain. It remains open for a time and thus retains its connection with the oral epithelium. Later, however, it loses its connection with the oral cavity and becomes firmly attached to the infundibulum of the brain. It eventually forms the anterior lobe of the hypophysis or pituitary gland. (See chapters 1, 2, and 21.) Other diverticula of the oral (stomodaeal) cavity occur. These evaginations form the rudiment of the oral glands and will be discussed on page 617.

b. Pharynx

The pharyngeal area or pharynx represents the anterior portion of the foregut, interposed between the stomodaeum or oral cavity and the esophagus. This general region has four main functions:

  1. external respiration,
  2. food passage (alimentation),
  3. endocrine-gland formation, and
  4. development of buoyancy structures.

In most vertebrates, five or six pairs of lateral outgrowths, known as the visceral or branchial pouches are formed. A ventral outpocketing or outpocketings also occur in all vertebrates. The thyroid-gland diverticulum is the most constantly formed ventral outgrowth, but lung and air-bladder evaginations are conspicuous in most vertebrate species. Dorsal and dorso-lateral airbladder evaginations occur in many fishes.

c. Anterior Intestinal or Pyloric Area

The anterior intestinal area of the primitive gut, immediately caudal to the stomach region, is characterized by a tendency to form diverticula. Various types of outgrowths occur here, the most constant of which are the hepatic (liver) and the pancreatic evaginations. In lower vertebrates, such as teleost, ganoid, and some elasmobranch fishes, blind digestive pockets, the pyloric ceca, may be formed in this area.

d. Junction of Midgut and Hind gut

At the junction of the developing small and large intestin \s, outgrowths are common in many of the higher vertebrates. The diverticula which occur here may be large and pouch-like, as in certain mammals, or slender and elongated, as in birds.

e, Cloacal and Proctodaeal Area

The most prominent cloacal diverticula occur ventrally. Ventral urinary bladders arise in this area in many vertebrates. The allantoic diverticulum (Chap. 22) is a prominent outgrowth of the ventral wall of the cloaca. In the chick, the bursa of Fabricius projects dorsally from the area between the cloaca proper and the proctodaeum. Dorsal urinary bladders occur in fishes, arising as dorsal diverticula within this general area. The anal glands of certain mammals, such as the dog, represent proctodaeal evaginations.

Fig. 280. Morphogenesis of the digestive tract in the frog, Rana pipiens. (See Chap. 10.)

B. Development of the Digestive Tube or Metenteron

The following descriptions pertain mainly to the developing shark, frog, chick, and human embryos. Other forms are mentioned incidentally to emphasize certain aspects of digestive-tube development.

1. General Morphogenesis of the Digestive Tube

The general morphological changes of the developing digestive tubes of the shark, frog, chick, and human are shown in figures 279-282.

2. Histogenesis and Morphogenesis of Special Areas

a. Oral Cavity

1) General Characteristics of the Stomodaeal Invagination. The oral cavity arises as a simple stomodaeal invagination in most vertebrates. However, in the toadfish, Opsanus (Batrachus) tau, two stomodaeal invaginations occur which later fuse to give origin to a single oral cavity (Platt, 1891). In Amphioxus, the mouth originates on the left side of the head as shown in figure 249D and F; later, it migrates ventrally to a median position. In cyclostomes, the original invagination becomes partly everted secondarily, so that the pituitary invagination eventually lies on the upper portion of the head (fig. 283A, B).

2) Rudiments of the Jaws. In the shark embryo, the mandibular visceral arches bend to form U-shaped structures on either side of the forming oral cavity and thus give origin to the primitive framework of the upper and lower jaws (fig. 253). This condition holds true for other lower vertebrates, including the Amphibia. In the chick, the mandibular arch bends similarly to that in the shark embryo, but only the proximal portion of the upper jaw is present. The anterior or distal portion is displaced by mesenchyme from the head area (fig. 240). The latter condition is true also of the mammals (fig. 261). Regardless of whether or not all the jaw framework on either side of the forming oral cavity is derived from the original mandibular arch, the fact remains that in the formation of the jaws, a U-shaped, mesenchymal framework on either side is established in all the gnathostomous or jaw-possessing vertebrates.

3) Development of the Tongue. The "tongue" of the shark is essentially a fold of the oral membrane of the floor of the mouth, which overlies the basal (hypobranchial) portion of the hyoid visceral arch) A true, flexible tongue, however, is never developed in the shark or other fishes. Flexible, protrusile tongues are found almost entirely in forms which inhabit the land, where they are used for the acquisition and swallowing of food. The protrusile tongue, therefore, is a digestive-tract structure primarily, and its use in communication in the human and other species is a secondary adaptation.

( The tongue generally develops from folds or growths, associated with the floor of the oral cavity and anterior branchial region.') These lingual growths are associated with the ventral or lower jaw portions of the hyoid and mandibular visceral arches and the ventral area between these arches. However, in the frog, the tongue arises from a mass of tissue at the anterior portion of the floor of the mouth between the mandibular visceral arches. It is protruded from the oral cavity largely by the flow of lymph into the base of the tongue.

The tongue of the chick and other birds is developed as a fleshy, superficially cornified structure, overlying the anterior portion of the greatly modified hyoid apparatus. It arises from the tuberculum impar, a swelling located in the floor of the pharyngeal area between the first and second visceral arches and the copula protuberance which forms as a result of swellings on the lower ends of the second and third visceral arches and the intervening area. The copula forms the root of the tongue; the tuberculum impar contributes the middle portion; and the anterior part of the tongue arises from folds which grow forward from the anterior portion of the tuberculum impar (fig. 284).

Fig. 281. Morphogenesis of the gut structures in the chick, Callus (domesticus) gallus.

(in the human and pig embryos, the anterior portion or body of the tongue arises through the fusion of two ventro-medial swellings of the mandibular arches (fig. 285B). The root of the tongue takes its origin from areas of elevated tissue upon the ventral ends of the hyoid arches and in the adjacent area between the hyoid and first branchial visceral arches (fig. 285B). This elevated tissue is known as the copula. A small, insignificant area, the tuberculum impar, emerges from the medio-ventral area between the mandibular and hyoid visceral arches (fig. 285B). Stages in tongue development in the human embryo are shown in figure 285A-E.

4) Teeth: a) General Characteristics. Teeth are of two types:

  1. horny teeth and
  2. bony or true teeth.

Horny teeth are found in cyclostomatous fishes, the larval stages of frogs and toads, and in the prototherian mammal, Ornithorhynchus.

Most vertebrates possess true or bony teeth, although they are absent in some fishes (e.g., the sturgeon, pipefishes, and sea horses), turtles, and birds. Among the mammals, certain whales lack teeth, and, in Ornithorhynchus, vestigial bony teeth are formed before hatching, to be lost and supplanted by cornified epidermal teeth. Teeth are lacking also in the edentates, Myrmecophaga and ManLs

True or bone-like teeth have essentially the same general structure in all vertebrates. A tooth possesses three general areas (fig. 286E):

  1. crown,
  2. neck, and
  3. root.

The crown projects from the surface of epithelium overlying the jaw or oral cavity, while the root is attached to the jaw tissue. The neck is the restricted area lying between the root and the crown.

Teeth generally are composed of two substances, enamel and dentine. Some teeth, however, lack enamel. Examples of the latter are the teeth of sloths and armadillos. The tusks of elephants also represent greatly modified teeth without enamel. Some teeth have the enamel only on the anterior aspect, such as the incisors of rodents.

Teeth may be attached to the jaw area in various ways. In sharks, the teeth are embedded in the connective tissue overlying the jaws (fig. 287F), whereas in most teleosts, amphibia, reptiles, birds, and mammals, they are connected to the jaw itself (fig. 287A-D). In many vertebrates, such as crocodilians and mammals, the tooth is implanted in a socket or alveolus within the jaw tissue (fig. 287C, D). In other forms, the tooth is fused (i.e., ankylosed) to the upper surfaces of the jaw (fig. 287 A, B). A tooth inserted within a socket or alveolus of the jaw is spoken of as a thecodont tooth, while those teeth fused to the surface of the jaw are referred to either as acrodont or pleurodont teeth. If the tooth is ankylosed to the upper edge of the jaw, as in many teleosts and snakes, it falls within the acrodont group (fig. 287B), but if it is attached to the inner surface of the jaw’s edge, as in the frog and Necturus, it is of the pleurodont variety (fig. 287A).

Fig. 282. Morphogenesis of the digestive tract in the human. Observe differentiation of the cloaca in E-G, and the mesenteric supports including the omental bursa in G. (Based upon data from various sources.)

Fig. 283. Partial eversion of the oral cavity during development in the embryo of Petromyzon. (Left) Longitudinal section of the head region in 19-day embryo. (Redrawn and modified from Kingsley, 1912, Comparative Anatomy of Vertebrates, Blakiston, Phila.) (Right) Median longitudinal section of head region of adult Petromyzon. (Redrawn and modified from Neal and Rand, 1936, Comparative Anatomy, Blakiston, Phila.)

In most vertebrates, all the teeth of the dentition are similar and thus form a homodont dentition. In some teleosts, some reptiles, and in most mammals, the teeth composing the dentition are specialized in various areas. Such localized groups of specialized teeth within the dentition assume different shapes to suit specific functions. Consequently, the conical, canine teeth are for tearing; the incisor teeth are for biting or cutting; and the flat-surfaced, lophodont and bunodont teeth are for grinding and crushing. A dentition composed of teeth of heterogeneous morphology is a heterodont dentition.

b) Development of Teeth in the Shark Embryo. The development of teeth in the shark embryo is identical with that of the placoid scale previously described. However, the teeth of the shark are larger and more durably constructed than the placoid scale and they are developed from a dental lamina of epithelial cells which grows downward along the inner aspect of the jaw. From this epithelium, a continuous series of teeth is developed as indicated in figure 287E and F. Within the oral cavity and pharyngeal area, ordinary placoid scales are found. Teeth are continuously replaced throughout life in the shark from the dental lamina. The word poiyphyodont is applied to a condition where teeth are replaced continuously.

c) Development of Teeth in the Frog Tadpole. The mouth of the frog tadpole possesses prominent upper and lower lips (fig. 287H). Inside these lips are rows of horny epidermal teeth. Three or four rows are inside the upper lip, and four rows are found inside the lower lip. These horny teeth represent cornifications of epidermal cells. They are sloughed off and replaced continuously until the time of metamorphosis when they are dispensed with. The permanent teeth begin to form shortly before metamorphosis from an epithelial ridge (dental lamina) which grows inward into the deeper tissues around the medial portion of the upper jaw. The teeth develop from an enamel organ and dental papilla in a manner similar to that of the developing shark or mammalian tooth. After the young tooth is partially formed, it moves upward toward the jaw, where its development is completed and attachment to the jaw occurs. Teeth are replaced continuously during the life of the frog.

Fig. 284. Development of the tongue in the chick embryo.

d) Development of the Egg Tooth in the Chick. Modern birds do not develop teeth. However, an ingrowth of epithelium does occur which suggests a rudimentary condition of the dental lamina of the shark, amphibian, and mammalian embryo (fig. 2871). It is possible that this represents the rudiment of a basic condition for tooth development, one which is never realized, for the sharp edge of the horny beak takes the place of teeth. The egg tooth is a conical prominence, developed upon the upper anterior portion of the upper horny jaw (fig. 287J). It is lost shortly after hatching. It appears to function in breaking the shell at hatching time.

e) Development of Teeth in Mammals. As the oral cavity in the pig or the human embryo is formed, the external margins or primitive jaw area of he oral cavity soon become differentiated into three general areas (fig. 288A) :

  1. an external marginal elevation, the rudiment of the labium or lip,
  2. slightly mesial to the lip rudiment, a depressed area, the labial or abiogingival groove, and
  3. internal to this epithelial ingrowth, the gingiva or gum elevation.

The latter overlies the developing jaw. From the mesial aspect of the labial roove, an epithelial thickening forms which pushes inward into the tissue of he gum or gingiva. This thickened ridge of epithelium forms the dental amina (ledge). (See fig. 288B, C.)

Fig. 285. Development of the tongue in the human embryo. (A~D drawn and modified from Ziegler models. (A) Fourth week. (B) About fifth week. (C) 6th to 7th week; 1C mm. (D) 7th week; 14 mm. (E) Adult condition. Observe that the mandibular lingual swellings give origin to the body of the tongue, while the copula forms the root af the tongue.

Fig. 286. Development of thecodont teeth. (A) Early stage of developing premolar of human. (B) Cellular relationships of tooth-forming area greatly magnified. (C) Later stage in tooth development showing dental sac. (D) Vertical section of erupting milk tooth. (E) Vertical section of canine tooth, in situ. (Redrawn and modified from Morris, 1942, Human Anatomy, Blakiston, Phila. After Toldt.)

After the dental ledge is formed, epithelial buds arise at intervals along the sdge. These epithelial buds form the rudiments of the enamel organs. Each namel organ pushes downward into the mesenchyme of the gum and evenâ–¡ally forms a cup-shaped group of cells, enclosing a mass of mesenchyme, the dental papilla (fig. 288D, E). The enamel organ differentiates into three layers (fig. 28 8E):

  1. an inner enamel layer, surrounding the dental papilla,
  2. an outer enamel layer, and
  3. between these two layers, a mass of epithelial cells, giving origin to the enamel pulp.

The cells of the enamel pulp eventually form a stellate reticulum.

Development thus far serves to establish the basic mechanisms for tooth development. Further development of the tooth may be divided into two phases:

  1. formation of the dentine and enamel and
  2. development of the root of the tooth and its union with the alveolus or socket of the jaw.

The initial phase of tooth formation begins when the inner cells of the inner enamel layer of the enamel organ become differentiated into columnar epithelial cells. These cells form the amcloblasts (fig. 288E, F). Following this change in the cells of the inner enamel layer, the mesenchymal cells, facing the ameloblasts, become arranged into a layer of columnar odontoblasts (fig. 288F). The odontoblasts then begin to deposit the dentine of the tooth. The initial phase of formation of dentine consists first in the elaboration of an organic substance or matrix. The organic matrix then becomes impregnated with inorganic calcareous materials to form the dentine, a hard, borie-like substance. As the dentinal layer becomes thicker, the odontoblasts recede toward the dental pulp of the papilla. However, the odontoblasts do not withdraw entirely from the dentine already formed, as elongated, extremely fine extensions from the odontoblasts continue to remain within the dentine to form the dentinal fibers (fig. 286B).

Dentine is deposited by the odontoblasts; the ameloblasts deposit the enamel layer in the form of a cap, surrounding the dentine (fig. 286A, B). In doing so, a slight amount of organic substance is first deposited, and then the ameloblast constructs in some way a prismatic column of hard calcareous material at right angles to the dentinal surface (fig. 286B). The columnar prisms thus deposited around the dentine form an exceedingly hard cap for the dentine. As in the formation of the dentine, the elaboration of enamel begins at the crown or distal end of the tooth and proceeds rootward.

The development of the root of the tooth and its union with the jaw socket (alveolus) is a complicated procedure. This phase of tooth development is accomplished as follows: The mesenchyme, with its contained blood vessels and nerves of the dental papilla, lies within the developing dentinal layer of the forming tooth. At the base of the tooth (i.e., the end of the tooth opposite the crown), the mesenchyme of the dental papilla is continuous with the mesenchyme surrounding the developing tooth. Around the base, sides, and crown of the tooth, this mesenchyme condenses and forms the outer and inner layers of the dental sac (fig. 286C). The latter is a connective-tissue sac which surrounds the entire tooth, continuing around the outside of the outer enamel cells of the enamel organ. As the dentine and enamel are deposited, the process of deposition proceeds downward from the crown "oward the developing root of the tooth. However, in the root area, the cellular layers of the enamel organ are compressed against the dentine, where they form the epithelial sheath. The sheath eventually disintegrates and disappears. The formation of enamel thus becomes restricted to the upper or crown part of the tooth, the root portion consisting only of dentine. As the root area of the tooth lengthens downward, the tooth as a whole moves upward. Finally, the crown of the tooth erupts to the outside through the tissues of the gum (fig. 286D). The eruption, completion, and shedding of the milk or deciduous teeth in the human body occur apparently as shown in the following table.

Fig. 287. Tooth development and arrangement in various vertebrates. (A-D) Tooth relationships with the jaw. (Redrawn and modified from Rand, 1950, The Chordates, Blakiston, Phila. After Wilder.) (E) Dental ledge and developing teeth in the dog shark, Acanthias. (Redrawn and modified from Rand, 1950, The Chordates, Blakiston, Phila. After Kingsley.) (F) Section of the shark’s lower jaw indicating a continuous replacement of teeth, i.e., a polyphyodont condition. (Redrawn and modified from Rand, 1950, The Chordates, Blakiston, Phila.) (G) Incisor tooth of rodent. (Redrawn and modified from Rand, 1950, The Chordates, Blakiston, Phila. After Zittel.) (H) Horny teeth of 12 mm. frog tadpole. (I) Rudimentary dental lamina in upper jaw of chick. (Redrawn from Lillie, 1930, The Development of the Chick, Holt & Co., N. Y.) (J) Anterior portion of upper jaw of 18-day chick showing egg tooth.

The Milk Dentition

Median incisors Lateral incisors First molars Canines Second molars

6th to 8th month 8th to 12th month 12th to 16th month 17th to 20th month 20th to 24th month

The Permanent Dentition

First molars Median incisors Lateral incisors First premolars Second premolars Canines Second molars Third molars

7th year 8th year 9th year 10th year 11th year 13 th to 14th year 13th to 14th year 17th to 40th year

This table is taken from McMurrich, J. Playfair. 1922. Keibel and Mall, Manual of Human Embryology, page 354, Lippincott, Philadelphia.

At about the time of eruption, the tooth becomes cemented into the alveolus or socket of the jaw in the following manner:

  1. The inner layer of the dental sac (fig. 286D) forms a layer of cementoblasts which deposit a coating of cementum over the dentine of the root (fig. 286E). This occurs only after the epithelial sheath (enamellayer cells around the root) has been withdrawn or otherwise has disappeared.
  2. The cells of the outer layer of the dental sac become active in forming spongy bone.
  3. As the tooth reaches maturity, the two bony surfaces, i.e., the cementum of the root and the spongy bone of the jaw socket, gradually begin to approach each other. Then, as more cementum is deposited and more spongy bone is formed, the space between the cementum and the spongy bone of the alveolus becomes extremely narrow (fig. 286E).
  4. Finally, the dental-sac tissue between these two bony surfaces forms the peridental membrane, a thin, fibrous, connective-tissue layer whose fibers are attached to the cementum and to the spongy bone of the socket. In other words, the cemental bone of the root and the spongy bone of the socket become sutured together by means of the interlocking fibers of the peridental membrane. This type of suture, which is formed between the root of the tooth and the walls of the alveolar socket, is called a gomphosis (fig. 286E).

Fig. 288. Tooth development in the pig. (A) Upper and lower jaw region of 18 mm. pig embryo showing labial and gum areas with the labial groove insinuated between. (B) Section through snout and upper and lower jaws of 30-mm. pig embryo showing formation of nasal passageways, secondary palate, lip, gum, and jaw regions, and ingrowing dental ledge. (C) High-powered drawing of dental ledge shown in square C in figure B. (D) Section similar to B in 65-mm. pig embryo. (E) Enlargement of area marked E in D showing dental papilla and enamel organ. (F) Drawing showing juxtaposition of inner layer of enamel organ (the an^eloblast layer) and the odontoblast cells which differentiate from the mesenchyme of the dental papilla.

Fig. 289. Palatal conditions in frog, chick, and mammal. (A) Frog, adult. (B) Chick, 16-day embryo. (C) Human adult. (Redrawn and modified from Morris, 1942, Human Anatomy, Blakiston, Phila.) Only the anterior or hard palate is supported by bone, the soft palate being a fleshy continuation of the palate caudally toward the pharyngeal area. (D-F) Stages in development of the palate in the pig. (D) 20.5 mm. (E) 26.5 mm. (F) 29.5 mm.

The permanent teeth, which supplant the deciduous teeth, develop in much the same manner as the deciduous teeth. Man, like the majority of mammals, develops two sets of teeth and, consequently, is diphyodont. Some mammals, such as the mole, Scalopus, never cut the permanent teeth, while the guinea pig sheds its deciduous teeth in utero.

5) Formation of the Secondary Palate. In the fishes and the amphibia, a secondary palate, separating the oral cavity from an upper respiratory passageway, is not formed. The formation of a secondary palate begins in the turtle group and is well developed in the crocodilians and mammals. The bird also has a secondary palate, but it is built more tenuously than that of the crocodilian-mammalian group (fig. 289 A-C).

During secondary-palate formation in the mammal, the premaxillary, maxillary, and palatine bones develop secondary plate-like growths which proceed medially to fuse in the midline (fig. 289D-F). The secondary palate thus forms the roof of the oral cavity — the air passageway from the outside to the pharynx being restricted, when the mouth is closed, to the area above the secondary palate.

6 ) Formation of the Lips. Lips are ridge-like folds of tissue surrounding the external orifice of the oral cavity. They are exceptionally well developed in mammals, where they are present in the form of fleshy mobile structures'^ They are absent in the prototherian mammal, Ornithorhynchus, as well as in birds and turtles, where the horny edges of the beak displace the fleshy folds at the oral margin. Lips are much reduced in sharks, where the toothed jaws merge with the general epidermis of the skin, but arc present in most fishes, amphibia, and most reptiles. In general, lips are immobile or only slightly mobile structures in the lower vertebrates, although in some fishes they possess a mobility surpassed only in mammals.

In the formation of the lips, a labial groove or insinking of a narrow ledge of epidermal cells occurs along the edge of the forming mouth. The labial groove then divides the edge of the forming mouth into an outermost lip margin and the gum or jaw region (fig. 288A). In forms where the lip is mobile, the lip region becomes highly developed and the muscle tissue which invades this area comes to form the general mass of the lip.

Fig. 290. Oral glands. (A) Poison and labial glands of the rattlesnake, Crotalus horidus. (Redrawn from Kingsley, 1912, Comparative Anatomy of the Vertebrates, Blakiston, Phila.) (B) Loci of origin of salivary glands in human embryo. (Redrawn from Arey, 1946, Developmental Anatomy, Saunders, Phila.) (C) Position of mature salivary glands in human. (Redrawn and modified from Morris, 1942 Human Anatomy, Blakiston, Phila.)

Fig. 291. Diagrams of intestinal tracts in various fishes. (Redrawn from Dean, 1895, Fishes, Living and Fossil, Macmillan. N. Y.) (A) Petromyzon, the cyclostome. (B) Protopterus, the lungfish. (C) The shark.

7) Oral Glands. Mouth glands arc present throughout the vertebrate series. Mucus-secreting glands are the predominant type, but specialized glands, producing special secretions, appear in many instances. The cyclostomatous fish, for example, possesses a specialized gland which secretes an anticoagulating substance to prevent coagulation and stoppage of blood flow in the host hsh to which it may be temporarily attached by its sucker-like mouth. Meanwhile, it rasps the host’s flesh with its horny teeth and “sucks” the flowing blood. Salivary glands (i.e., glands forming the saliva) make their appearance in the amphibia. Such glands may be found on the amphibian tongue, where, as lingual glands, they secrete mucus and a watery fluid. Intermaxillary glands are present on the amphibian palate. The poison glands of the Gila monster and of snakes represent specialized oral glands (fig. 290A). Salivary glands are present also below the tongue and around the lips and palate in snakes. Birds, in general, possess salivary glands of various sorts. The mammals are characterized by the presence of highly developed, salivary glands, among which are the parotid, sublingual, and submaxillary glands. Unlike most of the salivary glands in other vertebrates, the mammalian salivary glands, in many species, secrete mucus and a watery fluid, together with a starch-splitting enzyme, ptyalin.

The submaxillary and sublingual glands in mammals arise as evaginations of the oral epithelii n in the groove between the forming lower jaw and the developing tongue. The place of origin is near the anterior limits of the tongue. Two of these epithelial outpushings occur on either side (fig. 290B). The submaxillary-gland and sublingual-gland ducts open at the side of the frenulum of the tongue (fig. 290C). The parotid glands arise as epithelial evaginations, at the angle of the mouth, from the groove which separates the forming jaw and the lip (fig. 290B, C).

Fig. 292. Developing stomach regions of the digestive tract. (A-C) Three stages in the development of the pig’s stomach. Arrows indicate formation of omental bursa which forms from the pocket-like enlargement of the dorsal mesogastrium and proceeds to the left forming the omental bursa as the pyloric end of the stomach rotates toward the right. The ventral aspect of the stomach is indicated by crosses. (D) Diagram of the ruminant stomach. The abomasum corresponds to the glandular stomach of the pig or human; the other areas represent esophageal modifications. (Redrawn from Kingsley, 1912, Comparative Anatomy of the Vertebrates, Blakiston, Phila.)

The various oral glands, such as the palatine, labial, tongue, and cheek glands of mammals and lower vertebrates, the poison glands of snakes, etc., arise as epithelial buds which grow out from the developing oral cavity in a manner similar to those of the parotid, submaxillary, and sublingual glands of mammals. The original epithelial outgrowths may branch and rebranch many times to produce large, compound, alveolar glands, as in the parotid, submaxillary, and sublingual glands of mammals and the poison glands of snakes.

b. Development of the Pharyngeal Area

1) Pharyngeal Pouches and Grooves. The pharynx is that region of the early digestive tube which lies between the oral cavity and the esophagus. In adult vertebrate species, the pharyngeal area is much modified and differentially developed. However, in the early embryo, it tends to assume a generalized sameness throughout the vertebrate series.

( The early formation of the pharynx results from a series of outpocketings of the entoderm of the foregut, associated with a corresponding series of epidermal inpushings; the latter tend to meet the entodermal outgrowthsr^s a result of these two sets of movements, the one outward and the other inward, the lateral plate mesoderm becomes isolated into dorso-ventral columns, the branchial or visceral arches, between the series of outpocketings and inpushings (figs. 252F; 260; 262). The entodermal pouches or outpocketir gs are called the branchial, pharyngeal, or visceral pouches, while the epidermal (ectodermal) inpushings form the visceral or branchial grooves (furrows). The mesodermal columns constitute the visceral arches?^

The number of branchial pouch-groove relationships, thus established, varies in different vertebrate species. In the cyclostomatous fish, Petrornyzon, there are seven; in Squalus acanthias, the shark, there are six. The latter number is present typically in a large number of fishes. In most frogs and salamanders, there are five, pouch-groove relationships with a vestigial sixth; in the chick, pig, and human, there are four. (In reptiles, birds, and mammals, the fourth pouch on either side may represent a fusion of two or three pouches.) The number of visceral arches, of course, varies with the number of pouch-groove relationships produced, the first pair of arches being formed just anterior to the first pair of pouches. The first pair of arches are called the mandibular visceral arches; the second pair constitute the hyoid visceral arches; and the remaining pairs form the branchial arches.

Within each visceral arch, three structures tend to differentiate:

  1. a skeletal arch,
  2. a muscle column, associated with the skeletal arch, and
  3. the aortal arch, a blood vessel.

In all water-living vertebrates, including those species which spend the larval period in the water, the entoderm of the branchial pouch and ectoderm of the branchial groove tend to fuse intimately and perforate to form the branchial or visceral clefts, with the exception of the first, pouch-groove relationship. The latter is variable. In the amphibia, the first pouch does not perforate but becomes associated with the developing ear. In land forms, on the other hand, the pouches, as a rule, remain imperforate or weakly so. As a rule, they continue unperforated in mammals. The ectoderm and entoderm of the branchialpouch-groove relationships is very thin in the chick, and openings (?) may appear in the more anterior pouches. (Note: The relation of these pouches to respiration is discussed in the following chapter.)

2) Pharyngeal Glands of Internal Secretion. An important developmental function of the pharynx is the formation of masses of epithelial cells from various parts of the entodermal wall which serve as endocrine glands. These glands are the thyroid, parathyroid, thymus, and ultimobranchial bodies. The places of origin of these cellular masses and their part in the formation of the endocrine system are discussed in Chapter 21.

3) Other Respiratory Diverticula. One of the primary functions of the pharyngeal area is respiration. In most water-living vertebrates, the pharyngeal pouches are adapted for respiratory purposes. However, in many water-dwelling species and in all land forms, a median ventral outpushing occurs which develops into the lungs or into structures which function as air bladders and lungs. (See Chap. 14.)

Fig. 293. Characteristics of the mucous membrane in different regions of the human digestive tract: (A and D) redrawn and modified from Maximow and Bloom, A Textbook of Histology, Saunders, Philadelphia; (B and C) redrawn from Bremer, A Textbook of Histology, Blakiston, Philadelphia. (A) Esophageal area. Stratified squamous epithelium together with esophageal and cardiac glands are characteristic. The esophageal glands are located in the submucous layer and are of the tubulo-alveolar variety. The cardiac glands are found in the upper and lower esophageal regions and are confined to the mucous layer. (B) Stomach region. The mucous layer of the stomach is featured by the presence of many glands composed of simple and branched tubules. These glands open into the bottom of the gastric pits which in turn form small, circular openings at the mucosal surface. (C) The mucosal walls of the small intestine present many finger-like processes, the villi, between the bases of which the intestinal glands or crypts of Lieberkuhn project downward toward the lamina muscularis mucosae. (D) The mucosa of the large intestine is devoid of villi, and the glands of Lieberkuhn are longer and straighter than in the small intestine.

c. Morphogenesis and Histogenesis of the Esoj)hagus^and the Stomach Region of the Metenteron

The esophageal and stomach areas of the gut develop from that segment of the foregut which extends from the pharyngeal area caudally to the area of the developing gut tube from which the liver and pancreatic diverticula arise. In Arnphioxus and certain of the lower vertebrates, a true stomach is not differentiated within this portion of the foregut. This condition is found in the cyclostome, Petromyzon, in the lungfish, Protopterus, and various other forms (fig. 291 A, B). In these species, this segment of the gut merely serves to transport food caudally to the intestine, and the histogenesis of its walls resembles that of the esophagus. On the other hand, a true stomach is developed in all other vertebrate species. The funetions of the stomach are to store food, to break it up into smaller pieces, and to digest it partially. As such, the stomach comprises that segment of the digestive tract which lies between the esophagus and intestine. It is well supplied with muscular tissue, is capable of great distentioUp and possesses glands for enzyme secretion.

In development, therefore, the foregut area between the primitive pharynx and the developing liver becomes divided into two general regions in most vertebrates:

  1. a more or less constricted, esophageal region, and
  2. a posteriorly expanded, stomach segment (figs. 279-282).

The latter tends to expand and to assume a general, V-shaped form, the portion nearest the esophagus comprising the cardiac region, and the part nearest the intestine forming the pyloric end.

Many variations in esophageal-stomach relationships are elaborated in different vertebrate species. In the formation of the stomach of the pig or human, for example, a generalized, typical, vertebrate condition may be assumed to exist. In these forms, the stomach area of the primitive gut gradually enlarges and assumes a broad, V-shaped form, with its distal or pyloric end rotated toward the right (fig. 292A-C). Eventually, the entodermal lining tissue shows four structural conditions:

(a) There is an esophageal area near the esophagus, where the character of the epithelial lining resembles that of the esophagus.

(b) A cardiac region occurs, where the epithelium is simple, columnar in form, and contains certain glands.

(c) There is a fundic region, capable of being greatly expanded. The internal lining of the fundic area produces numerous, simple, slightly branehed, tubular glands, wherein pepsin is secreted by the chief cells and hydrochloric acid by the parietal cells (fig. 293).

(d) The pyloric area is the last segment of the stomach and is joined to the intestine. It has numerous glands, producing a mucus-like secretion.

The pig’s stomach resembles closely that of the human.

If we compare the general morphogenesis of the stomach in the pig or human with that of the shark, frog, chick, or the cow, the following differences exist.

The shark stomach is composed mainly of fundic and pyloric segments (fig. 279C). The stomach of the frog closely resembles that of the pig (fig. 280F). Unlike the pig, however, the frog is able to evert the stomach by muscular action projecting it forward through the mouth to empty its contents. In the chick (fig. 28 IE), an area of the esophagus expands into a crop which functions mainly as a food-storage organ. A glandular stomach (proventriculus), comparable to the fundus of the pig, is formed posterior to the crop, while, still more caudally, a highly muscular gizzard or grinding organ is elaborated.

In the cow or sheep, an entirely different procedure of development produces a greatly enlarged, distorted, esophageal portion of the stomach. This esophageal area of the stomach comprises the rumen, the reticulum or honeycomb stomach, and the omasum (psalterium) or manyplies stomach. The distal end of the stomach of the cow or sheep is the abomasum or true stomach, comparable to that of the human or pig described above (fig. 292D).

d. Morphogenesis and Histogenesis of the Hepato-pancreatic Area

The hepato-pancreatic area of the digestive tract is a most important one. Its importance springs not only from the development of indispensable glands but also from the relationship of the liver to the developing circulatory system (Chap. 17) and the division and formation of the coelomic cavity. (See Chap. 20.)

1) Development of the Liver Rudiment. The liver begins in all vertebrates as a midventral outpushing of the primitive metenteron, immediately caudal to the Stomach. It originates thus between the foregut and midgut areas of the developing digestive tube.

Fig. 294. Development of the liver and pancreatic rudiments. (Diagrams CT, redrawn from Lillie, 1930, The development of the chick. Holt, N. Y. F redrawn from Thyng, 1908, Am. J. Anat.) (A) Developing liver rudiment in 10 mm. embryo of the dogshark, Squalus acanthias. (B) Developing liver in tadpole of Rana pipiens. (See also Figs. 221, 223, 225, 280.) (C) Developing liver rudiments in the 3rd-day chick. (D) Developing liver in early 4th-day chick. (E) Developing liver in late 4th-day chick. (F) Hepatic evagination in 7.5 mm. human embryo. (G) Relation of the fully developed liver to associated structures in various vertebrates. (Gl) Squalus acanthias. The liver is suspended from the posterior surface of the septum transversum by the coronary ligament. (G2 and G3) Frog, Rana pipiens. G2 transverse view; G3 sagittal view. (G4 and G5) 16-20 day chick. Callus domesticus. G4 transverse view. Observe that the liver lobes and peritoneal cavity have grown forward on either side of the heart and have separated the heart and pericardial cavity from the ventro-lateral body walls. G5 is a left ventral view of the heart, pericardial cavity, and liver. Left lobe of the liver is removed. Observe that the septum transversum is applied to the posterior wall of the parietal pericardium. G6 Mammal. The septum transversum has been completely displaced by developing diaphragmatic tissue. The liver is suspended from the caudal surface of the diaphragm by the coronary ligament.

Fig. 294. (See facing page for legend.)

a) Shark Embryo. In the 10- to 12-mm. shark embryo, Squalus acanthias, the liver rudiment arises as a midventral evagination of the gut which pushes downward and forward between the two parts of the ventral mesentery. It soon becomes divisible into three chambers, viz., a midventral chamber, the rudiment of the gallbladder, and two lateral chambers, the fundaments of the right and left lobes of the liver (figs. 279B; 294A).

b) Frog Embryo. In the frog, the liver rudiment appears as a ventrocaudal prolongation of the foregut area at the early, neural fold stage (figs. 220B; 223B). Later, the anterior end of the hepatic rudiment differentiates into the liver substance in close relation to the vitelline veins as the latter enter the heart, while the posterior extremity of the original hepatic rudiment differentiates into the gallbladder (figs. 280; 294B, G2, G3).

c) Chick Embryo. In the chick, two evaginations, one anterior and the other posterior, arise from the anterior wall of the anterior intestinal portal, beginning at about 50 to 55 hours of incubation (fig. 294C). These evaginations project anteriorly toward the sinus venosus of the heart, where they eventually come to surround the ductus venosus as it enters the sinus. (See Chap. 17.) At the end of the fourth day of incubation, secondary evaginations from the two primary outgrowths begin to produce a basket-like mass of tubules which surround the ductus venosus (fig. 294E). The gallbladder arises from the posterior hepatic outpushing toward the end of the third day of incubation (fig. 294D).

d) Pig Embryo. The liver diverticulum in the 4- to 5-mm. embryo of the pig begins as a bulbous outpushing of the foregut area, immediately caudal to the forming stomach (fig. 295E). This outpushing grows rapidly and sends out secondary evaginations, including the vesicular gallbladder. The latter is already a prominent structure in the 5. 5-mm. embryo (fig. 295A).

Fig. 295. Development of liver and pancreatic rudiments (Continued). (A) Diagram of early hepatic diverticulum in pig embryo of about 5.5 mm. (Redrawn and modified greatly from Thyng, 1908, Am. J. Anat.) For early growth of liver in pig, see Figs. 261A and 262. (B) Hepatic ducts, hepatic tubules, and hepatic canaliculi in relation to blood sinusoids. It is to be observed that the common bile duct ( 1 ) gives off branches, the hepatic ducts (2), from which arise the branches of the hepatic duct (3) which are continuous with the hepatic tubules or hepatic cord cells (4). Compare with Fig. 295C. (C) A portion of liver lobule of human. (Redrawn and modified from Maximow and Bloom, A Text-book of Histology, Saunders, Phila.) Blood sinusoids are shown in black; liver cells in stippled white; bile canaliculi shown in either white or black. (D) Section showing three pancreatic diverticula in 5-day chick embryo. (Redrawn from Lillie, 1930, The development of the chick, Holt, N. Y. After Choronschitsky.) (E) Pancreatic diverticula in 5.5 mm. pig embryo. (Redrawn from Thyng, 1908, Am. J. Anat. 7.) (F) Pancreatic diverticula in 20 mm. pig embryo. (Redrawn from Thyng, 1908, Am. J. Anat. 7.) (G) Pancreatic acini and islet of Langerhans.

Fig. 296. Development of coils in the digestive tracts in the dog shark, Squalus acarithias, and in the frog, Rana pipiens. (A) Squalus acanthias embryo of 110 mm. (B-F) Rana pipiens, digestive tube development, shown from ventral aspect. Arrows in B and C denote primary movements of the primitive gut tube resulting in condition shown in D.

e) Human Embryo. In the human embryo, the liver arises in a similar manner to that of the pig embryo from the ventral wall of the foregut, just posterior to the forming stomach (fig. 294F). The hepatic outpushing invades the area of the ventral mesentery and becomes intimately associated with the substance of the septum transversum (fig. 362H). Secondary evaginations or liver cords ramify extensively within the mesenchyme of the mesentery, and the vitelline or omphalomesenteric veins, as in other vertebrates, become broken up into sinusoids, surrounding the outgrowing hepatic cords. The gallbladder arises as a secondary outgrowth from the posterior wail of the original hepatic outgrowth (fig. 294F). The gallbladder rudiment enlarges distally and gives origin to the cystic duct which joins the common bile duct.

2) Histogenesis of the Liver. As the liver pushes out into the ventral mesentery, it tends to project forward below the forming stomach and the caudal limits of the heart (figs. 295A; 362H). Within the ventral mesentery, secondary evaginations or epithelial cords of entodermal cells sprout from the primary entodermal evagination of the entodermal lining of the gut (fig. 295A). These epithelial or liver cords grow in between the paired vitelline veins, and the veins become changed into a mass of capillary-like sinusoids. The liver cords come to lie in the interstices between the vitelline sinusoids (fig. 295B).

As the liver cords grow within the ventral mesentery, mesenchymal cells, given off from the medial surfaces of the mesentery, come to surround the liver cords and give origin to the connective-tissue substance of the liver. The outer surface of the ventral mesentery retains its integrity and functions as the peritoneal covering of the growing liver.

It is apparent that the growth of the epithelial (liver) cords progresses dichotomously, branching into a tree-like system of branches from the original hepatic diverticulum of the gut tube, thus forming the parenchyma of the liver (Bloom, ’26). The proximal portion of the original hepatic diverticulum forms the common bile duct, or ductus cholcdochus, whereas the larger branches of the hepatic cords develop lumina and form the duct system. The gallbladder represents an original diverticulum from the common-bile-duct rudiment. The liver cords appear to be hollow from the beginning. The bile capillaries thus apparently develop directly within the liver cords. The livercord cells probably assume their typical cuboidal shape under the influence of the surrounding young connective tissue and branches from the portal vein (Bloom, ’26). The ultimate relationship between hepatic cell cords, liver sinusoids, and bile ductules is shown in figure 295C.

In the majority of vertebrates, as the liver substance increases within the ventral mesentery below the stomach area, it expands the ventral mesentery enormously until the liver, with its coating of ventral mesentery, fills the coelomic space below the gut tube and posterior to the heart. The developing liver thus comes in contact with the ventral and lateral body walls and becomes fused to these walls. The anterior face of the liver, eventually, forms a partition across the coelomic cavity just caudal to the heart (figs. 261; 295A). The anterior face of the liver substance gradually separates and forms a primitive partition across the body cavity. This partition is the primary septum transversum (fig. 295A). (Sec also Chap. 20.)

As the liver rudiment develops in the pig embryo, the septum transversum forms essentially as described above, i.e., it develops as a modification of the ventral mesentery covering the anterior face of the liver. However, in the human embryo, the primary septum transversum develops precociously, forming a partition across the ventral area of the coelomic cavity between the developing heart and liver (fig. 362F— H). When the hepatic cords in the human embryo grow forward within the ventral mesentery, they secondarily become related to the previously formed, primitive septum transversum along the caudal aspect of the septum. The ends achieved in the human and pig embryos are much the same, therefore, and the anterior face of the developing liver and the septum transversum are intimately associated.

3) Development of the Rudiments of the Pancreas: a) Shark Embryo. In the embryo of Squalus acanthias, the shark, the pancreas arises as a dorsal diverticulum of the gut a short distance posterior to the gallbladder and hepatic outpushings (fig. 279B). It grows rapidly and, in the 18- to 20-mm. embryo, it is a much-branched gland with its pancreatic duct entering the duodenum slightly anterior to the beginning coils of the spiral valve.

b) Frog Embryo. In the frog, the pancreas arises from three diverticula, one dorsal and two ventral, near the liver rudiment (Kellicott, T3, p. 167). The dorsal diverticulum is solid and separates from the gut tissue. The two ventral diverticula arise together from the ventral portions of the gut but soon branch into two rudiments. As these rudiments enlarge and branch, they eventually unite with the dorsal diverticulum of the pancreas, and the three fuse to form one gland. The proximal portion of the original, ventral, pancreatic outpushing remains as the pancreatic duct and empties into the duodenum close to the bile duct.

c) Chick Embryo. As in the frog, three pancreatic diverticula arise in the chick. The dorsal one appears first as an outpushing into the dorsal mesentery at the end of the third and early fourth days of incubation (fig. 295D). The two ventral diverticula arise during the end of the fourth and early fifth days of incubation as two lateral diverticula of the posterior hepatic evaginations close to the latter’s origin from the duodenum. The three diverticula fuse into one pancreatic mass, but tend to retain the proximal portions of the original outpushings as pancreatic ducts. Two or even all three may persist in the adult.

Fig. 297. Developing coils in the digestive tube of the pig. (A) 12 mm. embryo. (B) 24 mm. embryo. (C) 35 mm. embryo. (D) Cecum and large intestine showing coils in 120 mm. embryo. (E) Coiling of large intestine of young adult pig. Observe haustra or lateral diverticula of colonic wall. (All figures redrawn and modified from Lineback, 1916, Am. J. Anat. 16.)

Fig. 298. Structural composition of walls of human digestive tract. (A) Diagrammatic representation of digestive tract structure. (B) Portion of wall of small intestine showing folds of mucosa. (A and B redrawn from Maximow and Bloom, 1942, A Textbook of Histology, Saunders, Phila. B after Braus.)

d) Pig Embryo. Two pancreatic diverticula make their appearance in the pig embryo. One, the ventral pancreatic diverticulum, arises from the proximal end of the hepatic evagination, while the other, the dorsal diverticulum, emerges as a separate dorsal outpushing from the duodenal area approximately opposite the hepatic diverticulum (fig. 295E). In the 20-mm. embryo of the pig, these two diverticula proceed in development as shown in figure 295F. At about the 24-mm. stage, the duct of the ventral pancreas is obliterated, the dorsal pancreatic duct (duct of Santorini) remaining ordinarily as the pancreatic duct of the adult (Thyng, ’08).

e) Human Embryo. Dorsal and ventral pancreatic evaginations occur in the human embryo in a manner similar to that in the pig. Both fuse into one mass, although the dorsal pancreas grows much faster and forms much of the bulk of the pancreatic tissue. The ventral pancreas swings dorsally as the stomach and duodenal area of the intestine are rotated toward the right side of the peritoneal cavity. In doing so, the dorsal pancreas appropriates the duct of the ventral pancreas proximaliy toward the intestine, while distally it retains its own duct. This combined duct, or duct of Wirsung, first observed by Wirsung in 1642 (see Lewis, T2), is the pancreatic duct of the adult. Occasionally, two ducts opening into the intestine are retained, the original dorsal duct, the accessory duct or duct of Santorini, described by Santorini (see Lewis, ’12), and the duct of Wirsung or ventral pancreatic duct. The latter condition appears to be normal in the dog.

4) Histogenesis of the Pancreas. The original pancreatic diverticula branch, rebranch, and form an elaborate duct system. The secretory portions of the pancreas or the acini arise as terminal outgrowths of the distal portions of the duct system. The pancreas thus is a compound alveolar (acinous) gland. The loose connective tissue of the pancreas forms the surrounding mesenchyme, derived from the mesenteric tissue.

Two types of secretory cells bud off from the developing duct system. The majority form the acini of the pancreatic gland and pour their secretions into the duct system. This constitutes the exocrine aspect of the pancreas. Other cell masses bud off from the duct system and give origin to the islets of Langerhans. The latter form the endocrine portion of the pancreas (fig. 295G) .

e. Morphogenesis and Histogenesis of the Intestine

1) Morphogenesis of the Intestine in the Fish Group. In the fishes, the intestinal rudiment of the digestive tube does not undergo extensive elongation during development. A relatively short tube is formed as shown in figure 279C, although some coiling of the intestine does occur in teleost fishes. A distinct, small and large division of the intestine is not formed; intestinal and rectal areas only are developed. Specialized rectal outgrowths develop in sharks (fig. 279C), while, in teleost fishes, pyloric evaginations or cecae are formed.

2) Morphogenesis of the Intestine in Amphibia, Reptiles, Birds, and Mammals. The development of the intestine in this group of vertebrates involves considerable elongation and coiling (figs. 280, 281, 282). Two general divisions of the intestine are formed, a small intestine, developed from the midgut portion of the primitive metenteron, and a hindgut or colon, derived from the hindgut portion of the gut tube. A rectal area is formed at the caudal end of the hindgut. There is a tendency also for enlargements or extensions to occur in the area of junction between the small intestine and colon in the birds and mammals.

3) Torsion and Rotation of the Intestine During Development. Twisting and rotation of the stomach and intestine is a general feature of alimentarytract development. In the shark embryo, the stomach is rotated in such a way that its pyloric end is pulled upward toward the liver, forming a J-shaped structure (fig. 296A). Also, the duodenal and valvular areas of the intestine are rotated vertically, and the place of attachment of the dorsal mesentery moves into a ventro-lateral position.

The developing stomach and intestine of the frog embryo presents a remarkable and precise rotative procedure. In the early stages, the primitive metenteron is a simple tube, continuing from the forming stomodaeum caudad to the proctodaeum (fig. 280B). At the 6- to 7-mm. stage, the stomach-liver area begins to rotate toward the right as indicated in figure 296B. At about 7 to 9 mm., the stomach-liver area is projected to the right and anteriad, while the midgut and hindgut regions move toward the left (see arrows, fig. 296C). At the stage of development when the larvae approximate 10 mm. in length, the stomach and intestinal areas are arranged as in figure 296D. Through the larval stages to the time of metamorphosis, the midgut or small intestinal area becomes greatly extended and coiled as shown in figure 296E. At the time of metamorphosis, the small intestine becomes greatly reduced in relative length (fig. 296F),

The chick embryo manifests similar gastrointestinal torsion. The duodenal area of the intestine and the gizzard are pulled forward toward the liver, while the small intestine becomes coiled and lies to a great extent in the umbilical stalk, to be retracted later into the abdominal area.

At the 10-mm. stage in the pig, the digestive tract consists of a simple tubular structure as shown in figure 297A (Lineback, T6). In this figure, the pyloric-duodenal area is projected forward toward the liver, where the pyloric-duodenal area eventually is tied to the liver on the right side of the peritoneal cavity, with the result that the forming stomach lies transversely across the upper part of the abdominal cavity. The cecal and large intestinal areas are rotated around the small intestine (see arrow, fig. 297A), when the latter lies herniated within the umbilical cord. In figure 297B is shown the condition in the 24-mm. pig. It is to be observed that there is now a half rotation of the large intestine around the small intestine, the latter being considerably coiled, while in figure 297C a complete rotation of 360 degrees is shown.

Aside from these rotational movements, extensive coiling of the gut tube occurs, especially in the higher vertebrates. For example, the small intestine of the frog becomes coiled extensively during the larval period (fig. 296E). Reference to figure 297D and E shows a similar coiling of the large intestine of the pig.

Rotational movements of the intestine in the human embryo also occur. For example, in the human embryo of about 23 mm., a condition is present, comparable to that of the pig embryo of 24 mm., and the future large intestine has been rotated 180 degrees around the small intestine as shown in figure 282F. Unlike the pig, however, a complete rotation of the gut is not effected. Also, the large intestine does not later form into a double coil as in the pig. In the human embryo soon after the intestine is retracted from its herniated position in the umbilical cord (fig. 282G), the cecal area of the large intestine becomes fixed to the right side of the peritoneal cavity near the crest of the ilium (Hunter, ’28). The ascending, transverse, and descending portions of the large intestine are then developed (fig. 364G, H).

4) Histogenesis of the Intestine. During histogenesis of the intestine, two prominent modifications of the internal lining or mucous membrane tend to occur:

(a) Small finger-like projections or villi are formed which project inwardly into the lumen (fig. 298A); and

(b) the internal lining may project inwardly in the form of extensive elongated folds.

In many fishes, such as the sharks, lungfishes, ganoids, and cyclostomes, elaborate folds of the mucosa, known as the spiral folds or valves, are formed (fig. 29 1C). Similarly, in higher vertebrates, elongated folds may occur, such as the valves of Kerkring in the human and pig small intestine (fig. 298B).

Another conspicuous feature of the early histogenesis of the entodermal layer is the formation of epithelial membranes and plugs. The pharyngeal membrane is formed by the stomodaeal ectoderm and pharyngeal epithelial layers. The proctodaeal membrane is similarly constructed. This structure serves as a temporary blocking device between external and internal media. Under normal conditions these membranes degenerate and disappear, although occasionally they may persist. Epithelial plugs, temporarily obliterating the lumen of the digestive tract, appear with regularity in many vertebrates. Such temporary obstruction, for example, may appear in the developing digestive tract of the chick or in the human esophagus, duodenum, and other areas of the digestive tract.

/. Differentiation of the Cloaca

As previously observed, the caudal end of the intestine expands into the cloaca, an enlarged area which eventually receives the urinary products as well as the intestinal substances. The differentiation of this area is considered in Chapter 18.

C. Physiological Aspects of the Developing Gut Tube

Within the developing digestive tubes of the shark, reptiles, birds, and mammals, a brownish-green, pigmented material appears during the latter phases of embryonic development. This material is composed of cells, bile pigments, mucus, etc. It is discharged during the period just before or after parturition. Fetal swallowing of ammionic fluid, gastrointestinal motility, the presence of enzymes, fetal digestion and absorption, and defecation are wellestablished facts in the physiology of the developing digestive tract of the mammalian fetus (Windle, ’40, Chap. VII).


Bloom, W. 1926. The embryogenesis of human bile capillaries and ducts. Am. J. Anat. 36:451.

Hunter, R. H. 1928. A note on the development of the ascending colon. J. Anat. 62:297.

Kellicott, W. E. 1913. Outlines of Chordate Development. Henry Holt & Co., New York.

Lewis FT. The form of the stomach in human embryos with notes upon the nomenclature of the stomach. (1912) Amer. J Anat. 13(4): 477-503.

Lineback, P. E. 1916. The development of the spiral coil in the large intestine of the pig. Am. J. Anat. 20:483.

Platt, J. B. 1891. Further contribution to the morphology of the vertebrate head. Anat, Anz. 6:251.

Thyng FW. Models of the pancreas in embryos of the pig, rabbit, cat, and man. (1908) Amer. J Anat. 7(4): 489–503.

Windle, W. F. 1940. Physiology of the Fetus. W. B. Saunders Co., Philadelphia.

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