Book - Developmental Anatomy 1924-5

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Arey LB. Developmental Anatomy. (1924) W.B. Saunders Company, Philadelphia.

   Developmental Anatomy 1924: 1 The Germ Cells and Fertilization | 2 Cleavage and the Origin of the Germ Layers | 3 Implantation and Fetal Membranes | 4 Age, Body Form and Growth Changes | 5 The Digestive System | 6 The Respiratory System | 7 The Mesenteries and Coelom | 8 The Urogenital System | 9 The Vascular System | 10 The Skeletal System | 11 The Muscular System | 12 The Integumentary System | 13 The Central Nervous System | 14 The Peripheral Nervous System | 15 The Sense Organs | C16 The Study of Chick Embryos | 17 The Study of Pig Embryos | Figures Leslie Arey.jpg
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Part II. Organogenesis Entodermal Derivatives

Chapter V The Digestive System

The entoderm of the embryonic disc is at first directly continuous with the entodermal lining of the yolk sac, and merely forms a roof to that organ (Fig. 40 C). As the embryo grows and expands, while its connection with the yolk sac lags in development, the entoderm necessarily takes the form of a blind tube within the cylindrical body. This extends first into the head region as the fore-gut (Figs. 40 C and 43), then tailward as the hind-gut (Fig. 44). The intermediate region, open ventrally through the narrower yolk stalk into the yolk sac, is sometimes termed the mid-gut (Fig. 71), but its existence is brief for the yolk stalk loses its connection with the gut during the sixth week.

At each end, the gut comes into direct contact ventrally with the ectoderm. The plates thus formed are the pharyngeal and cloacal membranes (Fig. 71). The pharyngeal membrane forms the floor of a depression known as the oral fossa, or stomodeum ; this fossa is bounded by the fronto-nasal, maxillary, and mandibular processes (Figs. 61 and 62). At the beginning of the fifth week (2.5 to 3 mm. embryos), the pharyngeal membrane ruptures and the oral fossa and fore-gut become continuous. The oral fossa develops into the front part of the mouth cavity, which is therefore ectodermal. The remainder of the mouth cavity, the respiratory tract, and the alimentary canal to a point well along the small intestine are all derived from the entodermal fore-gut.

The caudal end of the entodermal tube comprises the cloaca, which soon receives the allantoic, urinary, and genital ducts (Figs. 87, 91 and 94). The cloaca promptly begins to subdivide into a dorsal rectum and a ventral urogenital sinus (Figs. 139 to 142). At the same time, the cloacal membrane is separatedinto anal and urogenitalmembranes (Figs. 71, 95 and 96). The anal membrane ruptures at about the ninth week, and an external depression, the proctodeum, therefore becomes continuous with the hindgut (Figs. 96 and 142). It constitutes the anal canal, which, like the front part of the mouth cavity, is lined with ectoderm. The hind-gut itself forms some of the small intestine, the colon, and the rest of the rectum (Figs. 93 to 96). It will be noticed that the primitive entodermal tube extends a little beyond the cloacal membrane (Figs. 71, 91 and 139); this tail-gut, or postanal gut, soon dwindles and disappears.

The entoderm forms only the epithelial lining of these organs. AU other coats develop from the investing splanchnic mesoderm. The original low epithelium of the gut differentiates into the several types of simple epithelium formed in the digestive and respiratory systems, as well as into the pseudostratified and stratified forms. The various glands are primarily epithelial outgrowths.

Fig. 71. - Diagrams showing the human alimentary canal in median sagittal section. X 35. A, 2 mm. (modified after His): B, 2.5 mm. (after Thompson).

It is impossible to determine the exact junction of ectoderm and entoderm in the mouth, but in general the roof and peripheral portions are ectodermal. The salivary glands are considered to be from ectoderm, as are the enamel of the teeth, a portion of the tongue epithelium, and much of the lining of the nose and palate. Although these structures do not strictly belong with entodermal derivatives, it is simplest to consider them with the systems of which they are integral parts.

The Mouth

Lips and Cheeks

During the fifth week, these separate from the jaws proper by the ingrowth of epithelial plates which promptly begin to thin and form the vestibule (Figs. 74, 76 and 79).

The Palate

The roof of the original mouth cavity is the base of the skull. When the membranes which separate the olfactory pits from the mouth rupture, their orifices, the primitive choance, also open into the common oral cavity. The nasal passages next become separate by partitioning off a portion of the mouth cavity and adding it to their original extent. They then communicate with the pharynx by the secondary, definitive choance. The horizontal septum which thus divides mouth from nasal ^passage is the palate. The details of its formation follow ; At first the jaws are closed and the tongue extends up between shelf like folds of the maxilla, the lateral palatine processes (Figs. 73 A and 74), which project downward (Fig. 72 A). Soon the mandible drops, owing to growth changes, and the tongue is withdrawn. This allows the palatine folds to bend upward to the horizontal plane (Fig. 75), approach, and fuse (Fig. 72 B). The shift of the lateral palatine processes is an active bending, due to cellular proliferation on their under sides (Fig. 75). The union of the halves of the palate begins about the end of the second month and progresses backward toward the pharynx (Fig. 73 B). Coincidently, bone appears in the front part and forms the hard palate; more caudad, ossification fails, and this region constitutes the soft palate and its free apex the uvida. The unfused, backward prolongations of the palatine folds give rise to the pharyngo-palatine arches, which delimit oral cavity from pharynx. The palate shows a median seam, and, for a time, the uvula is notched; both are indicative of the mode of origin.

Fig. 72. - Sections through the jaws of pig embryos, to show the development of the palate (Prentiss). X 8. A, 22 mm.; B, 34 mm.

Fig. 73 - Dissections to show the development of the palate in pig embryos (Prentiss). X 5. A , The upper jaw and palatine processes of a 22 mm. embryo in ventral view; B, fusion of the palatine processes in a 35 mm. embryo.

74. - The roof of the mouth of a two-months - human embryo, groove, primitive choanse and developing palate (after His). showing the labial ,X 9 .

Fig. 75. - Section through the jaws of a 25 mm. pig embryo, to show the change in position of one palatine process due to unequal growth (Prentiss).

The median nasal lobes of the original fronto-nasal process also develop median palatine processes, so-called, which do not contribute to the palate but form the premaxillary portion of the upper jaw (Figs. 73 and 74). Fusion with the palate is incomplete and in the midplane there is a gap, the incisive foramen, flanked by the incisive canals (of Stenson). These become covered with mucous membrane, although they sometimes are patent at birth.

Fig. 76. - Early stages in the development of the teeth (Rose). . 4 , Seven weeks (X 90); B, nine weeks (X 45).


The lateral palatine processes occasionally fail to unite in the middle line, producing a defect known as deft palate. The extent of the defect varies considerably, in some cases involving only the soft palate, while in other cases both soft and hard palate are cleft. It may be associated also with hare lip.

The Teeth

The teeth have a double origin. The enamel is from ectoderm ; the dentine, pulp, and cement are mesodermal.

The Enamel Organ. - When the labial groove is forming in embryos of six weeks, a horizontal shelf develops from it and extends backward into the substance of the jaw (Fig. 76). This curved dental ridge, or lamina, is parallel with the adjacent labial groove and lies mesial to it (Fig. 82). At intervals a series of thickenings develop, the anlages of the enamel organs; these will form enamel and serve as the moulds of the future teeth (Figs. 76 B and 77). Early in the third month the ventral side of each enamel organ becomes concave, like an inverted cup, and the concavity is occupied by dense mesenchymal tissue, the dental papilla, which will differentiate into dentine and pulp (Figs. 77 and 78). An enamel organ and its associated dental papilla is the basis of each tooth (Fig. 79). Ten such anlages of the decidual, or milk teeth, are present in each jaw (Fig. 82). Their connection with the dental ridge is eventually lost.

Fig. 77. - Models of the early development of three teeth, one in section (Lewis and StohrL .

Fig. 78. - Section through an upper incisor from a fetus of three months (Prentiss). X 70.

The compact internal cells of the enamel organ transform into a reticulum resembling mesenchyme, termed the enamel pulp (Fig. 78). The outer enamel cells, at first cuboidal, flatten out as a fibrous layer. Neither of these components contributes to tooth formation. The inner enamel cells line the cup-shaped concavity of the enamel organ. Over the crown of the tooth these cells are designated amelohlasis, for they become columnar and produce the enamel layer along their basal ends (Fig. 80). The enamel is laid down first as an uncalcified, fibrillar layer which then calcifies in the form of enamel prisms, one for each ameloblast. The enamel is deposited first at the apex of the crown and then downward .

Fig. 79. - Parasagittal section through the mandible and tongue of a three-months - fetus, showing the relations of the first incisor anlage (Prentiss). X 14.

Fig. 80. - Section through a portion of the crown of a developing tooth, showing the various layers (Tourneux in Heisler).

Toward the root. The enamel cells about the future root of the tooth remain cuboidal or low columnar in form, come into contact with the outer enamel cells, and the two layers constitute the epithelial sheath of the root (Fig. 81); it does not produce enamel prisms.

The Dental Papilla - At the end of the fourth month, the outermost cells of the dental i)apilla arrange themselves as a definite layer of columnar epithelium. Since they produce the dentine, or dental bone, these cells are known as odontoblasts (Fig. 8i). When the dentine layer is deposited, the odontoblast cells remain internal to it, but branched processes from them (the dentinal fibers of Tomes) extend into the dentine and occupy the dental canalicnli (Fig. 80). Internal to the odontoblasts, the remaining mesenchymal cells differentiate into the dental pulp, popularly known as the - nerve - of the tooth. This is composed of a framework of reticular tissue in which are found blood vessels, lym.phatics, and nerve fibers. The odontoblast layer persists throughout life and intermittently lays down dentine, so that eventually the root canal may be obliterated,

Fig. 81. - Longitudinal section of a decidual tooth of a newborn dog. X 42. Above the enamel, on either side, are artificial shrinkage spaces (Lewis and Stohr).

The Dental Sac. - The mesenchymal tissue surrounding the anlage of the tooth gives rise to a dense outer layer and a more open inner layer of fibrous connective tissue. These form the dental sac (Fig. 8i). Over the root of the tooth a layer of osteoblasts, or bone forming cells, develops, and when the epithelial sheath of the enamel organ disintegrates, they deposit about the dentine an investment of specialized bone, known as the cement. Cementum contains typical bone cells but no Haversian systems. As the tooth grows and fills its alveolar socket, the dental sac.

Fig. 82. - Dental lamina and anlages of the upper milk teeth in a three-months - fetus (Rose).

Becomes a thin, vascular layer, the peridental membrane. This has fibrous attachments to both the alveolar bone and the cement and holds the tooth in place.

Eruption. - ^When the crown of the tooth is fully developed the enamel organ disintegrates, and, as the root continues to grow, the crown approaches the surface and breaks through the gum. The periods of eruption of the various milk, or decidual teeth vary with race, climate, and nutritive conditions Usually they are cut in the following sequence: .

Median Incisors sixth to eighth month.

Lateral Incisors eighth to twelfth month.

First Molars twelfth to sixteenth month.

Canines seventeenth to twentieth month.

Second Molars twentieth to thirty-sixth month.

The permanent teeth develop precisely like the temporary set. The anlages of those permanent teeth which correspond to the milk dentition arise in another series along the free edge of the dental lamina (Fig. 77 H) and come to lie mesad of the decidual teeth (Fig. 83). In addition, three permanent molars are developed on each side, both above and below, from a backward or alioral extension of the dental lamina, entirely free from the oral epithelium (Fig. 82). The anlages of the first permanent molars appear at the end of the fourth month, those of the second molars at six weeks after birth, while the anlages of the third permanent molars, or wisdom teeth, are not found until the fifth year. The permanent dentition of thirty-two teeth is then complete.

Before the permanent teeth begin to erupt, the roots of the milk teeth undergo partial resorption, their dental pulp dies, and they are eventually shed. Toward the sixth year, before the loss of the decidual teeth begins, each jaw may contain twenty-six teeth (Fig. 83). The permanent teeth are cut as follows : .

First Molars

Median Incisors

Lateral Incisors

First Premolars

Second Premolars

Canines I

Second Molars )

Third Molars (Wisdom Teeth)

seventh year, eighth year, ninth year, tenth year, eleventh year.

Thirteenth to fourteenth year, seventeenth to fortieth year.

Fig 83. - Skull of a live-year-old child, showing the positions of the decidual and permanent teeth (Sobotta-McMurrich).

The teeth of vertebrates are homologues of the placoid scales of elasmobranch fishes (sharks and skates). The teeth of the shark resemble enlarged scales, and many generations are produced in the adult fish. In some mammalian embryos, three, or even four, dentitions are present. The primitive teeth of mammals were of the canine type, and from this conical tooth the incisors and molars have arisen. Just how the cusped tooth differentiated - whether by the fusion of originally separate units, or by the development of cusps on a single primitive tooth - is debated.

Fig. 84. - Dissections showing the development of the tongue in pig embryos (Prentiss). X 12. A, 7 mm.; B, 9 mm.; C, 13 mm.


Dental anomalies are frequent. They may consist in the congenital absence of some or all of the teeth, or in the production of more than the normal number. Defective teeth are frequently associated with hare lip. Cases have been noted in which, owing to a defect of the enamel organ, the enamel was wanting. Third dentitions have been recorded, and occasionally fourth molars are developed behind the wisdom teeth.

The Tongue

The tongue develo])s as two distinct portions, the body and the root, separated from each other by a V-sha]ied groove, the sulcus tcnninalis (Fig. 85 B). In both human and pig embryos, the body of the tongue is represented by three anlages that appear in front of the second branchial arches. These are the median, somewhat triangular iuhcrculitm uupar, and the paired lateral swellings of the first, or mandibular arches - all of which are present in human embryos of 5 mm. (Figs. 84 A and 85 . 4 ). At this stage, a median ventral elevation, formed by the union of the second branchial arches, constitutes the copula. This, with the ])ortions of the second arches lateral to it, forms later the root of the tongue. Between it and the tuberculum impar is the point of evagination of the thyroid gland, represented in the adult by the foramen cecum (Fig. 85). I'he copula also connects the tuberculum impar with a rounded prominence that is developed in the midventral line from the bases of the third and fourth branchial arches. This is the anlage of the epiglottis (Figs. 84 and 85).

Fig. 85. - Stages in the development of the human tongue (adapted). A, 6 mm.; B, 15 mm. Contributions from the first three branchial arches are indicated respectively by parallel lines, dots and crosses; the tuberculum impar is marked by circles.

In later stages (Figs. 84 B, C and 85 B), the lateral mandibular anlages, bounded laterally by the alveolo-lingual grooves, increase rapidly in size and fuse with the tuberculum impar, which lags behind in development, and, according to recent investigators, atrophies completely. The epiglottis enlarges and becomes concave on its ventral surface. Caudad, and in early stages continuous with it, are two thick, rounded folds, the arytenoid ridges. Between these is the slit-like glottis, leading into the laryn.

In fetuses of 11 weeks, the fungiform and filiform papilla; may be distinguished as elevations. Taste buds appear in the fungiform papillae at 14 weeks and are much more numerous in the fetus than in the adult.

The vallate papilla: develop on a V-shaped epithelial ridge whose apex corresponds to the site of the thyroid evagination (Fig. 85 B). After the thirteenth week, circular epithelial down-growths occur at intervals along the ridges and take the form of inverted and hollow truncated cones (Fig. 86 A). During the fourth month circular clefts appears in the epithelial down-growths, thus separating the walls of the vallate papillae from the surrounding epithelium and forming the trench from which this type of papilla derives its name (Fig. 86 B). At the same time, lateral outgrowths arise from the bases of the epithelial cones, hollow out and form the ducts and glands of Ebner (Fig. 86 C). The taste buds of the vallate papillae also are formed early, appearing in embryos of three months. Foliate papilla: probably develop at about six months.

The foregoing account applies to the early origin of the mucous membrane alone. The musculature of the tongue is supplied chiefly by the hypoglossal nerve, and both nerve and muscles belong historically to the post-branchial region. If not in the development of each present-day embryo, at least in the past the musculature has migrated cephalad and invaded the branchial region beneath the mucous membrane (cf. p. 229). At the same time, the tongue may be said to extend caudad until its root is covered by the epithelium of the third and fourth branchial arches. This is shown by the fact that the sensory portions of the nn. trigeminus a.nA facialis, the nerves of the first and second arches, supply the body of the tongue, while the nn. glossopharyngeus and vagus, the nerves of the third and fourth arches, supply chiefly the root.

Fig. 86. - Diagrams showing the development of the vallate papillae of the tongue (Graberg in McMurrich). a, Valley; b, von Ebner's gland.

Anomalies. - Faulty development or incomplete fusion of the several anlages causes variable degrees of absence or bifurcation of the tongue.

The Salivary Glands. - The glands of the mouth are all regarded as derivatives of the ectodermal epithelium. They complete their differentiation only after birth.

The parotid is the first to develop. Its anlage has been observed in 8 mm. embryos, near the angle of the mouth, as a keel-like flange in the floor of the groove which divides cheek from jaw. The flange elongates, and, in embryos of seven weeks, separates from the parent epithelium, forming a tubular structure that opens into the mouth cavit\" near the front end of the original furrow. The tube grows back into the region of the external ear, branches, and forms the main body of the gland in this region, while the stem portion of the tube becomes the parotid duct. Acinus cells are present at five months.

The submaxillary gland arises at n mm. as an epithelial ridge in the groove between the jaw and the tongue, its cephalic end located near the frenulum. The caudal end of the ridge soon begins to separate from the epithelium and extend backward and ventrad into the submaxillary region, where it enlarges and branches to form the gland proper ; its cephalic, unbranched portion, persisting as the duct, soon hollows out (Fig. 79).

Fig. 87. - Diagrammatic ventral view of pharynx, digestive tube and mesonephroi of a 4 to 5 mm. embryo (adapted by Prentiss). X about 30. The liver and yolk sac are cut away. The tubules of the right mesonephros are shown diagrammatically.

The sublingual gland appears by the eighth week as several solid evaginations of epithelium from the jaw-tongue groove (Fig. 79). This group, usually regarded as a sublingual gland, really consists of the sublingual proper, with its ductus major, and of about ten equivalent alveololingual glands. Mucin cells have appeared by the sixteenth week.

Pharyngeal Pouches. - There are developed early from the lateral wall of the entodermal pharynx paired outpocketings which are formed in succession cephalo-caudad. In 4 to 5 mm. embryms, five pairs of such pharyngeal {branchial) pouches are present, the fifth pair being rudimentary (Figs. 87 and 91). Meanwhile, the pharynx has flattened and broadened, so that it is triangular in ventral view (Figs. 87 and 88).

From each pharyngeal pouch develop small dorsal and large ventral diverticula. All five pouches come into contact with the ectoderm of corresponding branchial grooves, fuse with it, and form the closing plates. Although the closing plates become perforate in human embryos only occasionally, these pouches and grooves, nevertheless, are homologous to the functional branchial clefts of fishes and tailed amphibia. The first and second pharytigeal pouches soon connect with the pharyngeal cavity through wide common openings. The third and fourth pouches grow laterad and their diverticula communicate with the pharynx through narrow ducts in lo to 12 mm. embryos (Fig. 88). When the cervical sinus (p. 77) is formed, the ectoderm of the second, third, and fourth branchial clefts is drawn out to produce the transient branchial and cervical ducts and the cervical vesicle. These are fused at the closing plates with the entoderm of the corresponding pharyngeal pouches, the fate of the entodermal pouches is varied and spectacular. Although they do not continue as parts of the digestive apparatus, their embryonic relations justify their inclusion in the present section. The first differentiates into the tympanic cavity of the middle ear and into the auditory (Eustachian) tube. The second becomes the palatine tonsil in part. The third, fourth, and fifth pouches give rise to a series of ductless glands: the thymus, parathyroids, and the ultimobranchial bodies.

Fig. 88. - Reconstruction of the pharynx and fore-gut of a 12 mm. human embryo, seen in dorsal view (Hammar-Prentiss). The ectodermal structures are stippled.

The Tonsils. - By the growth and lateral expansion of the pharynx, the second pouch is absorbed into the pharyngeal wall, its dorsal angle alone persisting, to be transformed into the tonsillar and su proton sillar fossae. Crypts arise at the end of the third month by the hollowing of solid epithelial ingrowths, whereas a mound of mesodermal lymphoid tissue hrst presses against the epithelium at the middle of the fourth month. This association constitutes the palatine tonsil.

A Subepithelial infiltration of lymphocytes during the sixth month gives rise to the median pharyngeal tonsil, which like the lingual tonsil is not of pharyngeal pouch origin. Immediately caudad is a recess, the pharyngeal bursa, formed by a protracted connection of the epithelium with the notochord (Huber). It bears no relation to the original blind termination of the fore-gut known as Seesel - s pouch. According to Hammar, the lateral pharyngeal recess (of Rosenmfiller) is not a persistent portion of the second pouch, as His asserted.

The Thymus. - The thymus anlages appear in 10 mm. embryos as ventral and medial prolongations of the third pair of pouches (Figs. 88 and 89). The ducts connecting the diverticula with the pharynx soon disappear so that the anlages are set free. At first, they are hollow tubes which soon lose their cavities and migrate caudally into the thorax, usually passing ventral to the left innominate vein. Their upper ends become attentuate and atrophy, but may persist as accessory thymus lobes. The enlarged lower ends of the anlages form the body of the gland, which is thus a paired structure (Fig. 90). At 1 1 weeks the thymus still contains solid cords and small closed vesicles of entodermal cells. From this stage on, the gland becomes more and more lymphoid in character. Its final position is in the thorax, dorsal to the upper end of the sternum. It grows under normal conditions until puberty, after which involution begins. This process proceeds slowly in healthy individuals, rapidly in case of disease. True atrophy of the parenchyma enters at about the fiftieth year.

The ventral diverticulum of the fourth pouch is a rudimentary thymic anlage which usually atrophies.

It is now generally believed that the entodermal epithelium of the thymus is converted into reticular tissue and thymic corpuscles. The latter are the atrophic and hyalinized remains of embryonic tubules and cords (Marine, 1915). The lymphoid cells were regarded by Stohr as entodermal in origin, but most observers derive them from the mesoderm.

Fig. 89. - Diagram of the pharynx and its derivatives (adapted by Prentiss). I-V, first to fifth pharjmgeal pouches.

The Parathyroid Glands. - Each dorsal diverticulum of the third and fourth pharyngeal pouches gives rise to a small mass of epithelial cells termed a parathyroid gland (Fig. 89). Two pairs of these bodies are thus formed, and, with the atrophy of the ducts of the pharyngeal pouches, they are set free and migrate caudalw^ard. The}* eventually lodge in the dorsal surface of the thyroid gland; the pair from the third pouches lies, one on each side, at its caudal border, the ]iair from the fourth pouches at the cranial border (Fig. go). Their solid bodies are broken up into masses and cords of jiolyhedral entodermal cells intermingled with blood vessels. In postfetal life, lumina may appear in the cell masses and fill with a colloid-like secretion.

Fig. 90. - Reconstruction of the thymus, thyroid and parathj-roid glands in a human embryo of two months (Tourneaux and Verdun). X 15.

The Ultimobranchial Bodies. - These bodies, also called postbranchial, are usually rated as derivatives of the fifth pharyngeal pouches (Fig. 89).

Fig. 91. - Reconstruction of a 4.2 human embryo (His- Prentiss). X 25.

By the atrophy of the ducts of the fourth pouches they are set free and migrate caudad with the parathyroids. Each forms a hollow vesicle which has been erroneously termed the lateral thyroid. It takes no part in forming thyroid tissue, but atrophies. Kingsbury (1915) denies the origin of the ultimobranchial body from any specific pouch, and asserts it is “merely formed by a continued growth activity in the branchial entoderm .

The Thyroid Gland. - In embryos with five to six primitive segments (1.4 mm.) there appears in the midventral wall of the pharynx, between the first and second branchial arches, a small outpocketing, the thyroid milage. In 2.5 mm. embryos it has become a stalked vesicle (Figs. 71 j 5 and 87). Its stalk, the thyroglossal duct, opens at the aboral border of the tuberculum impar of the tongue (Figs. 85 B); this spot is represented permanently by the foramen cecum (Fig. g6). The duct soon atrophies and the bilobled gland anlage (Fig. 89) loses its lumen and breaks up into irregular, solid, anastomosing plates of tissue as it migrates caudad. The thyroid assumes a transverse position with a lobe on each side of the trachea and larynx (Fig. 90). In embryos of eight weeks, discontinuous lumina begin to appear in swollen portions of the plates; these represent the primitive thyroid follicles. Colloid soon forms.

Anomalies. - Persistent portion of the thyroglossal duct may form cysts or even fistulce.


The several accessory coats of the digestive tube are all derived from splanchnic mesoderm wTich invests the entoderm of the primitive gut. In each division of the tube the circular muscle layer develops before the longitudinal layer.

The Esophagus. - The esophagus in 4 to 5 mm. embryos is a very short tube, extending from pharynx to stomach (Fig. 91). As the heart and diaphragm recede into the thorax, it grows rapidly in length (Figs. 95 and 96). In embryos of 8 mm. the esophageal epithelium is composed of two layers of columnar cells, but at birth they number nine or ten. The esophagus remains so broadly attached to the dorsal body wall that there is never a distinct mesentery (Fig. 124).

During the eighth week vacuoles appear in the epithelium and increase the size of the lumen, which, however, is at no time occluded. Glands begin to develop at four months. The circular muscle layer is indicated at six weeks but the longitudinal fibers do not form a definite layer until u weeks.

Anomalies. - There may be atresia. This usually involves a fistulous relation with the trachea; the esophagus is divided transversely, the trachea opening into the lower segment, while the upper portion ends as a blind sac.

The Stomach

The stomach appears in embryos of 4 to 5 mm. as a laterally flattened, fusiform enlargement of the fore-gut, caudal to the lung anlages (Figs. 93 and 94). Its wall is composed of three layers: the entodermal epithelium, a thick mesenchymal layer, and the peritoneal mesothelium (Fig. 114). The stomach is attached dorsally to the body wall by its mesentery, the greater omentum, and ventrally to the liver by the lesser omentum (Fig. 112 B). The dorsal border of the stomach soon bulges locally to form the fundus, and also grows more rapidly than the ventral wall throughout its extent, thus producing the convex greater curvature (Fig. 92). The whole stomach becomes curved, and its cranial end is displaced to the left by the enlarging liver (Fig. 88). This forms a ventral concavity, the lesser curvature, and produces the first flexure of the duodenum.

Fig. 93. - Median sagittal section of a 5 mm. human embryo, to show the digestive canal (Ingalls-Prentiss). X 14.

The rapid growth of the gastric wall along the greater curvature also causes the stomach to rotate about a long axis until its greater curvature, or primitive dorsal wall, lies to the left, its lesser curvature, or ventral wall, to the right (Fig. 114). The original right side is now dorsal, the left side ventral in position, and the caudal, or pyloric end of the stomach is ventral and to the right of its cardiac, or cephalic end. The whole organ extends obliquely across the peritoneal cavity from left to right.

These changes in position progress rapidly and are already completed early in the second month.

The rotation of the stomach explains the asymmetrical position of the vagus nerves of the adult organ, the left nerve supplying the ventral wall of the stomach, originally the left wall, while the right vagus supplies the dorsal wall, originally the right. At the end of the seventh week the stomach has reached its permanent position, the cardia having descended through about ten segments, the pylorus through six or seven.

Fig. 94. - Reconstruction of a 5 mm. human embryo, showing the entodermal canal and its derivatives (His in Kollmann). X 25.

Gastric pits are indicated in embryos of seven weeks, and at 14 weeks the glands begin to differentiate. The gastric pits number 270,000 at birth but increase by fission to nearly seven million in the adult. At seven w'eeks, the circular muscle layer is indicated by condensed mesenchyme; a heavier ring forms the pyloric sphincter. During the fourth month the cardiac region shows a few longitudinal muscles fibers, which become distinct in the pyloric region at seven months.

The Intestine

In 5 mm embryos (Fig. 93), the intestine, beginning at the stomach, consists of the duodenum (from which are given off the hepatic diverticulum and dorsal pancreas), and the cephalic and caudal limbs of the intestinal loop, which bends ventrad and connects with the yolk stalk. Caudally, the intestinal tube expands into the cloaca. It is sup]:)orted from the dorsal body wall by the mesentery (Fig. 94).

From 5 to 9 mm., the ventral flexing of the intestinal loop becomes more marked and the attachment of the yolk stalk to it normally disappears (Fig. 95). At this stage there is formed in the caudal limb of the intestinal loop an enlargement, due to a ventral bulging of the gut wall, that marks the anlage of the cecum and the boundary line between the large and small intestine. Succeeding changes in the intestine consist; (i) in its torsion and coiling, due to rapid elongation, and (2) in the differentiation of its several regions. As the gut elongates in 9 to 10 mm. embryos, the intestinal loop rotates. As a result, the originally caudal limb lies at the left and cranial to its cephalic limb (Fig. 95) of the small intestine soon lengthens so rapidly that the coelom can no longer accommodate it, and, at seven weeks, it protrudes into the umbilical cord and forms loops there (Fig. 96). This constitutes a normal umbilical hernia. Six primary loops occur and these may be recognized in the arrangement of the adult intestine. In embryos of ten weeks, spatial readjustments have allowed the intestine to return from the umbilical cord into the abdominal cavity; the coelom of the cord is obliterated soon after.

Fig. 95. - Median sagittal section of a 9 mm. human embryo, showing the digestive canal (Mall-Prentiss). X 9.

Vacuoles appear in the duodenal wall of embryos six to nine weeks old and epithelial septa completely block its lumen. The remainder of the small intestine becomes vacuolated but not occluded. Villi develop as rounded elevations of the epithelium at eight weeks. They begin to form at the cephalic end of the jejunum, and, at four months are found throughout the small intestine. Intestinal glands appear as ingrowths of the epithelium about the bases of the villi. They develop first in the duodenum at 14 weeks. The duodenal glands (of Brunner) are said to appear during the fourth month. In embryos of six weeks the circular muscle layer of the intestine first forms, but the longitudinal layer is not distinct until the end of the third month.

Fig. 96. - Median sagittal section of a 17 mm. human embryo, showing the digestive canal (Mall-Prentiss). X 5.

The large intestine, as seen in 9 mm. embryos (Fig 95), forms a tube extending from the cecum to the cloaca. It does not lengthen so rapidly as the small intestine, and, when the intestine is withdrawn from the umbilical cord, its cranial, or cecal end lies on the right side and dorsal to the small intestine (Fig. 97). It extends across to the left side as the transverse colon, then, bending abruptly caudad as the descending colon, returns by its sigmoid segment to the median plane and continues into the rectum. In fetuses from three to six months old, the lengthening of the colon causes the cecum and cephalic end of the colon to descend toward the pelvis (Fig. 97). The ascending colon is thus established in the position which it occupies in the adult. The distal end of the cecal anlage continues to elongate, but early lags in transverse development; as a result, the vermiform process is distinct from the cecum at the end of the third month. These structures make a sharp U-shaped bend with the colon at ten weeks, and this flexure gives rise to the colic valve.

Fig. 97. - Later changes in the intestine and dorsal mesentery of the human fetus (Tourneux in Heisler). i. Stomach; 2, duodenum; 3, small intestine; 4, colon, 5, yolk stalk; 6, cecum; 7, greater omentum; 8, mesoduodenum; 9, mesentery, 10, mesocolon. The arrow points to the orifice of the omental bursa. The ventral mesentery is not shown.

Fig. 98. - Development of the cecum and vermiform process (adapted from Kollman and Paterson). A, Two months; B, three months; C, early infancy; D, five years.

The Rectum

The terminal portion of the intestine is derived by the horizontal division of the cloaca; Figs. 95 and 96, and 140 to 142 illustrate the process which is described in full on p. 145. When the anal membrane ruptures at the ninth week, the ectodermal proctodeum is added to the entodermal rectum.

The circular muscle layer of the large intestine appears first at two months, the longitudinal layer at three months. Between the third and seventh months villi are present.

Glandular secretions and desquamated entodermal cells, together with swallowed amniotic fluid, containing lanugo hairs and vernix caseosa, collect in the fetal intestine. This mass, yellow to brown in color, is known as meconium. At birth the intestine and its contents are perfectly sterile, but a bacterial flora is promptly acquired.

Anomalies. - The intestine may show atresia. This occurs most often in the duodenum as a retention of the embryonic occlusion. When the anal membrane fails to rupture, an imperforate anus results. If the rectum does not separate completely from the cloaca, a common urogenital and rectal cavity remains. Rarely there is nonrotation of the intestine and the colon lies on the left side. Two per cent of all adults show a persistence of the proximal end of the yolk stalk to form a pouch, Meckel's diverticulum of the ileum (p. 52). Congenital umbilical hernia is due either to the continuance of the normally transitory embryonic condition or to a secondary protrusion of the viscera. Other hernias are explained on pp. 134 and 163.

The Liver

Fig. 99. - Model of the liver anlage of a 4 mm. human embryo (Bremer). X 160. In., Intestine; Pa., pancreas; V., veins in contact with liver trabeculse.

In embryos of 2.5 mm, the liver anlage is present as a median ventral outgrowth from the entoderm of the fore-gut, just cranial to the yolk stalk (Fig. 71). Its thick walls enclose a cavity which is continuous with that of the gut. This hepatic diverticitluni becomes embedded at once in a mass of splanchnic mesoderm, the septum transversum (Fig. 91). Cranially, the septum will contribute later to the formation of the diaphragm; caudally, in the region of the liver anlage, it becomes Glisson - s capsule and the ventral mesentery (Figs, no and in). Thus, from the first, the liver is in close relation to the septum transversum, and later when the septum becomes the diaphragm, the liver remains attached to it (Fig. 113).

In embryos 4 to 5 mm. long, solid cords of cells proliferate from the ventral and cranial portion of the hepatic diverticulum (Fig. 91). These cords anastomose and form a crescentic mass with wings extending upward on either side of the gut (Fig. 93). This mass, a network of solid trabecula?, is the glandular portion of the liver, whereas the primitive, hollow diverticulum differentiates later into the gall bladder and the large biliary ducts. Referring to Fig. 183, it will be seen that the early liver anlage lies between the vitelline veins and is in close proximity to them laterally. The veins send anastomosing branches into the ventral mesentery. The trabeculce of the expanding liver grow between and about these venous plexuses, and the plexuses in turn make their way .

Fig. 100. - The trabeculae and sinusoids of the liver in section (after Minot). X 300. Tr., Trabeculae of liver cells; Si., sinusoids.

Between and around the liver cords (Fig. 99). The vitelline veins, on their way to the heart, are thus surrounded by the liver and largely subdivide into a network of vessels, termed sinusoids. The endothelium of the sinusoids is closely applied to the cords of liver cells, which, in the early stages, contain no bile capillaries (Fig. 100).

The glandular portion of the liver grows rapidly, and, in embryos of 7 to 8 mm., is connected with the primitive hepatic diverticulum by a single cord of cells only, the hepatic duct (Fig. loi A). That portion of the hepatic diverticulum distal to the hepatic duct is now differentiated into the terminal, solid gall bladder and its cystic duct; the proximal portion forms the ductus choledochus. In embryos of 10 mm. (Fig. loi B), the gall bladder and ducts have become longer and more slender and the hepatic duct receives a right and left branch from the corresponding lobes of the liver. The gall bladder is without a lumen up to the 15 mm stage, but later its cavity appears, surrounded by a wall of high, columnar epithelium.

The glandular portion of the liver develops fast and is largest relative to the size of the body at nine weeks. In certain regions the liver tissue undergoes degeneration, and especially is this true in the peripheral portion of the left lobe. In general, the external lobes of the liver are moulded under the influence of the fetal vitelline and umbilical trunks. The development of the ligaments of the liver is described on p. 126.

Fig. 101. - Reconstructions of the hepatic diverticulum and pancreatic anlages in human embryos. A, 7.5 mm. (Thyng). X 50; B, 10 mm. (Prentiss). X 33.

Fig. 102. - Diagrams illustrating the formation of liver lobules (Mall), a, Hepatic side; d, portal side; b and c, successive stages of the hepatic vein; e and f, successive stages of the portal vein.

During the development of the liver the endothelial cells of the sinusoids become stellate in outline, and thus form an incomplete layer. From the second month of fetal life to some time after birth, blood cells are actively differentiating between the hepatic cells and the endothelium of the sinusoids. At eight weeks hollow interlobular ducts appear spreading inward from the hepatic duct along the larger branches of the portal vein. In fetuses of ten weeks bile capillaries with cuticular borders are present, most numerous near the interlobular ducts with which some of them connect. At birth, or shortly after, the number of liver cells surrounding a bile capillary is reduced to two, three, or four. Secretion of the bile commences at about the end of the third fetal month.

The lobules, or vascular units of the liver, are formed, according to Mall, by the peculiar and regular manner in which the veins of the liver branch. The primary branches of the portal vein extend along the periphery of each primitive lobule, parallel to similar branches of the hepatic veins that drain the blood from the center of the lobule (Fig. 102). As development proceeds, each primary branch becomes a stem, giving off on either side secondary branches which bear the same relation to each other and to new lobules as did the primary branches to the first lobule. This process is repeated during fetal and early postnatal life until thousands of liver lobules are developed until the 20 mm. stage, the portal vein alone supplies the liver. The hepatic artery, from the cadiac axis, comes into relation first with the hepatic duct and gall bladder. Later, it grows into the connective tissue about the larger bile ducts and the branches of the portal vein, and also supplies the capsule of the liver.

Anomalies. - A common anomaly of the liver consists in its subdivision into multiple lobes. Absence or duplication of the gall bladder and of the ducts may occur. In some animals (horse; elephant) the gall bladder is absent normally.

The Pancreas

Two pancreatic anlages are developed almost simultaneously in embryos of 3 to 4 mm. The dorsal pancreas arises as a hollow outpocketing of the dorsal duodenal wall, just cranial to the hepatic diverticulum (Figs. 93 and 94). At 7.5 mm., it is separated from the duodenum by a slight constriction and extends into the dorsal mesentery (Fig. 10 1 A). The ventral pancreas develops in the inferior angle between the hepatic diverticulum and the gut, and its wall is at first continuous with both. With the elongation of the ductus choledochus, its origin is transferred to this portion of the diverticulum.

Fig. 103. - Stages in the development of the human pancreas (Kollman). 4 , 8 mm. ; B, 20 mm.

Of the two pancreatic anlages, the dorsal grows more rapidly, and, in 10 mm. embryos, forms an elongated structure with a central duct and irregular nodules upon its surface (Fig. loi B). The ventral pancreas is smaller and develops a short, slender duct that opens into the ductus choledochus. When the stomach and duodenum rotate, the pancreatic ducts shift their positions as well. At the same time, growth and bending of the bile duct to the right bring the ventral pancreas into close proximity with the dorsal pancreas (Figs. 101 and 103).

In embryos of 20 mm., the tubules of the dorsal and ventral pancreatic anlages interlock (Fig. 103 B). Eventually, anastomosis takes place between the two ducts, and the duct of the ventral pancreas, plus the distal segment of the dorsal duct, persists as the functional pancreatic duct (of Wirsung) of the adult. The proximal portion of the dorsal pancreatic duct forms the accessory duct (of Santorini), which remains pervious, but becomes a tributary of the chief pancreatic duct. The ventral pancreas forms part of the head and uncinate process of the adult gland. The dorsal pancreas participates in forming the head and uncinate process, and comprises the whole of the body and tail.

In 10 mm. embryos the portal vein separates the two pancreatic anlages, and later they partially surround the vein. The alveoli of the gland are derived from the ducts as darkly staining cellular buds in fetuses of ten weeks. The islands, characteristic of the pancreas, also bud from the ducts (and alevoli, Mironescu, 1910) and appear first in the tail a week later. Owing to the shift in the position of the stomach and duodenum during development, the pancreas takes up a transverse position.


The ventral pancreas may arise directly from the intestinal wall, and paired ventral anlages also occur. Accessory pancreases are not uncommon. Both the dorsal and ventral ducts persist in the horse and dog; in the sheep and man, the ventral duct normally becomes of chief importance; in the pig and ox, the dorsal duct.

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   Developmental Anatomy 1924: 1 The Germ Cells and Fertilization | 2 Cleavage and the Origin of the Germ Layers | 3 Implantation and Fetal Membranes | 4 Age, Body Form and Growth Changes | 5 The Digestive System | 6 The Respiratory System | 7 The Mesenteries and Coelom | 8 The Urogenital System | 9 The Vascular System | 10 The Skeletal System | 11 The Muscular System | 12 The Integumentary System | 13 The Central Nervous System | 14 The Peripheral Nervous System | 15 The Sense Organs | C16 The Study of Chick Embryos | 17 The Study of Pig Embryos | Figures Leslie Arey.jpg


Arey LB. Developmental Anatomy. (1924) W.B. Saunders Company, Philadelphia.

Cite this page: Hill, M.A. (2024, April 16) Embryology Book - Developmental Anatomy 1924-5. Retrieved from

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