McMurrich1914 Chapter 10
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McMurrich JP. The Development Of The Human Body. (1914) P. Blakiston's Son & Co., Philadelphia, Pennsylvania.
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- 1 Chapter X. The Development of the Digestive Tract and Glands
Chapter X. The Development of the Digestive Tract and Glands
The greatest portion of the digestive tract is formed by the constriction off of the dorsal portion of the yolk-sac, as shown in Fig. 52, the result being the formation of a cylinder, closed at either end, and composed of a layer of splanchnic mesoderm lined on its inner surface by endoderm. This cylinder is termed archenteron and has connected with it the yolk-stalk and the allantois, the latter communicating with its somewhat dilated terminal portion, which also receives the ducts of the primitive kidneys and is known as the cloaca (Fig. 170).
At a very early stage of development the anterior end of the embryo begins to project slightly in front of the yolk-sac, so that a shallow depression is formed between the two structures. As the constriction of the embryo from the sac proceeds, the anterior portion of the brain becomes bent ventrally and the heart makes its appearance immediately in front of the anterior surface of the yolk-sac, and so the depression mentioned above becomes deepened (Fig. 169) to form the oral sinus. The floor of this, lined by ectoderm, is immediately opposite the anterior end of the archenteron, and, since mesoderm does not develop in this region, the ectoderm of the sinus and the endoderm of the archenteron are directly in contact, forming a thin pharyngeal membrane separating the two cavities (Fig. 169, pm) In embryos of 2.15 mm. this membrane is still existent, but soon after it becomes perforated and finally disappears, so that the archenteron and oral sinus become continuous.
Toward its posterior end trr; archenteron comes into somewhat similar relations with the ectoderm, though a marked difference is noticeable in that the area over which the cloacal endoderm is in contact with the ectoderm to form the cloacal membrane (Fig. 170, cm) lies a little in front of the actual end of the archenteric cylinder, the portion of the latter which lies posterior to the membrane forming what has been termed the postanal gut (p. an). This diminishes in size during development and early disappears altogether, and the pouch-like fold seen in Fig. 170 between the intestinal portion of the archenteron and the allantoic stalk (al) deepening until its floor comes into contact with the cloacal membrane, the cloaca becomes divided into a ventral portion, with which the allantois and the primitive excretory ducts (w) are connected, and a dorsal portion which becomes the lower end of the rectum. This latter abuts upon the dorsal portion of the cloacal membrane, and this eventually ruptures, so that the posterior communication of the archenteron with the exterior becomes established. This rupture, however, does not occur until a comparatively late period of development, until after the embryo has reached the fetal stage; nor does the position of the membrane correspond with the adult anus, since later there is a considerable development of mesoderm around the mouth of the cloaca, bulging out, as it were, the surrounding ectoderm, more especially anteriorly where it forms the large genital tubercle (see Chapter XIII), and posteriorly where it produces the anal tubercle. This appears as a rounded elevation on each side of the median line, immediately behind the cloacal membrane and separated from the root of the caudal projection by a depression, the precaudal recess. Later the two elevations unite across the median line to form a transverse ridge, the ends of which curve
Fig. 169. - Reconstruction of the Anterior Portion of an Embryo of 2.15 MM.
ab. Aortic bulb; h, heart; 0, auditory capsule; op, optic evagination;/>?w, pharyngeal membrane. - (His.)
forward and eventually meet in front of the original anal orifice. From the mesoderm of the circular elevation thus produced the external sphincter ani muscle is formed, and it would seem that so much of the lower end of the rectum as corresponds to this muscle is formed by the inner surface of the elevation and is therefore ectodermal. The definitive anus being at the end of this terminal portion of the gut is therefore some distance away from the position of the original cloacal membrane.
Fig. 170. - Reconstruction of the Hind End of an Embryo 6.5 mm. Long al, Allantois; b, belly-stalk; cl, cloaca; cm, cloacal membrane; i, intestine; n, spinal cord; nc, notochord; p.an, postanal gut; ur, outgrowth to form ureter and metanephros; w, Wolffian duct. - (Keibel.)
It will be noticed that the digestive tract thus formed consists of three distinct portions, an anterior, short, ectodermal portion, an endodermal portion representing the original archenteron, and a posterior short portion which is also ectodermal. The differentiation of the tract into its various regions and the formation of the various organs found in relation with these may now be considered.
The Development of the Mouth Region
The deepening of the oral sinus by the development of the first branchial arch and its separation into the oral and nasal cavities by the development of the palate have already been described (p. 99), but, for the sake of continuity in description, the latter process may be briefly recalled. At first the nasal pits communicate with the oral sinus by grooves lying one on each side of the fronto-nasal process, but by the union of the latter, through its processus globularis, with the maxillary processes these communications are interrupted and the floors of the nasal pits are separated from the oral cavity by thin bucco-nasal membranes, formed of the nasal epithelium in contact with that of the oral cavity. In embryos of about 15 mm. these membranes break through and disappear, and the nasal and oral cavities are again in communication, but the communications are now behind the maxillary processes and constitute what are termed the primitive choance. The oral cavity at this stage does not, however, correspond with the adult mouth cavity, since there is as yet no palate, the roof of the oral cavity being the base of the skull. From the maxillopalatine portions of the upper jaw, shelf-like ridges begin to grow, being at first directed downward so that their surfaces are parallel with the sides of the tongue, which projects up between them. Later, however, they become bent upward to a horizontal position (Fig. 171) and eventually meet in the median line to form the palate, separating the nasal cavities from the mouth cavity. All that portion of the original oral cavity which lies behind the posterior edge of the palatal shelf is now known as the pharynx, the boundary between this and the mouth cavity being emphasized by the prolongation backward and downward of the posterior angles of the palatal shelf as ridges, which form the pharyn go -palatine arches (posterior pillars of the fauces) . The nasal cavities now communicate with the upper part of the pharynx (naso-pharynx) by the posterior choanae. The palatal processes are entirely derived from the maxillary processes, the premaxillary portion of the upper jaw, which is a derivative of the fronto-nasal processes, not taking part in their formation/ Consequently a gap exists between the palatal
shelves and the premaxillae for a time, by which the nasal and mouth cavities communicate; it places the organ of Jacobson (see p. 429) in communication with the mouth cavity and may persist until after birth. Later it becomes closed over by mucous membrane, but may be recognized in the dried skull as the foramen incisivum (anterior palatine canal).
Occasionally there is a failure of the union of the palatal plates, the condition known as cleft palate resulting. The inhibition of development which brings about this condition may take place at different stages, but frequently it occurs while the plates still have an almost vertical direction. Typically cleft palate is a deficiency in the median line of the roof of the
Fig. 171. - View of the Roof of the Oral Fossa of Embryo showing the Lipgroove and the Formation of the Palate. - (His.) mouth, not affecting the upper jaw, but very frequently it is combined with the defect which produces hare-lip (see p. 100), in which case the cleft may be continued through the upper jaw between its maxillary and premaxillary portions on either or both sides, according to the extent of the defect.
At about the fifth week of development a downgrowth of epithelium into the substance of both the maxillary and fronto-nasal processes above and the mandibular process below takes place, and the surface of the downgrowth becomes marked by a deepening groove (Fig. 171), which separates an anterior fold, the Up, from the jaw proper (Fig. 172). Mention should also be made of the fact that at an early stage of development a pouch is formed in the median line of the roof of the oral sinus, just in front of the pharyngeal membrane, by an outgrowth of the epithelium. This pouch, known as Rathke's pouch, comes in contact above with a downgrowth from the floor of the brain and forms with it the pituitary body (seep. 399).
The Development of the Teeth. - When the epithelial downgrowth which gives rise to the lip groove is formed, a horizontal outgrowth develops from it which extends backward into the substance of the jaw, forming what is termed the dental shelf (Fig. 172, A). This at first is situated on the anterior surface of the jaw, but with the continued development of the lip fold it is gradually shifted until it comes to lie upon the free surface (Fig. 172, B), where its superficial edge is marked by a distinct groove, the dental groove (Fig. 171). At first the dental shelf of each jaw is a continuous plate of cells, uniform in thickness throughout its entire width, but later ten thickenings develop upon its deep edge, and beneath each of these the mesoderm condenses to form a dental papilla, over the surface of which the thickening moulds itself to form a cap, termed the enamel organ (Fig. 172, B). These ten papillae in each jaw, with their enamel caps, represent the teeth of the first dentition.
The papillae do not, however, project into the very edge of the dental shelf, but obliquely into what, in the lower jaw, was originally its under surface (Fig. 172, B), so that the edge of the shelf is free to grow still deeper into the surface of the jaw. This it does, and upon the extension so formed there is developed in each jaw a second set of thickenings, beneath each of which a dental papilla again appears. These tooth-germs represent the incisors, canines, and premolars of the permanent dentition. The lateral edges of the dental shelf being continued outward toward the articulations of the jaws as prolongations which are not connected with the surface epithelium, opportunity is afforded for the development of three additional thickenings on each side in each jaw, and, papillae developing beneath these, twelve additional tooth-germs are formed. These represent the permanent molars; their formation is much later than that of the other teeth, the germ of the second molar not appearing until about the sixth week after birth, while that of the third is delayed until about the fifth year.
As the tooth-germs increase in size, they approach nearer and nearer to the surface of the jaw, and at the same time the enamel organs separate from the dental shelf until their connection with it is a mere neck of epithelial cells. In the meantime the dental shelf itself has been undergoing degeneration and is reduced to a reticulum
Fig. 172. - Transverse Sections through the Lower Jaw showing the Formation of the Dental Shelf in Embryos of (A) 17 mm. and (B) 40 mm. - (Rose.) which eventually completely disappears, though fragments of it may occasionally persist and give rise to various malformations. With the disappearance of the last remains of the shelf, the various toothgerms naturally lose all connection with one another.
It will be seen, from what has been said, that each tooth-germ consists of two portions, one of which, the enamel organ, is derived from the ectoderm, while the other, the dental papilla, is mesenchymatous. Each of these gives rise to a definite portion of the fully formed tooth, the enamel organ, as its name indicates, producing the enamel, while from the dental papilla the dentine and pulp are formed.
The cells of the enamel organ which are in contact with the surface of the papilla, at an early stage assume a cylindrical form and become arranged in a definite layer, the enamel membrane (Fig. 173, SEi), while the remaining cells (SEa) apparently degenerate eventually, though they persist for a time to form what has been termed the enamel pulp. The formation of the enamel seems to be due to the direct transformation of the enamel cells, the process beginning at the basal portion of each cell, and as a result, the enamel consists of a series of prisms, each of which represents one of the cells of the enamel membrane. The transformation proceeds until the cells have become completely converted into enamel prisms, except at their very tips, which form a thin membrane, the enamel cuticle, which is shed soon after the eruption of the teeth.
The dental papillae are at first composed of a closely packed mass of mesenchyme cells, which later become differentiated into connective tissue into which blood-vessels and nerves penetrate. The superficial cells form a more or less definite layer (Fig. 173, od), and are termed odontoblasts, having the function of manufacturing the dentine. This they accomplish in the same manner as that in which the periosteal osteoblasts produce bone, depositing the dentine between their surfaces and the adjacent surface of the enamel. The outer surface of each odontoblast is drawn out into a number of exceedingly fine processes which extend into the dentine to occupy the minute dentinal tubules, just as processes of the osteoblasts occupy the canaliculi of bone.
At an early stage the enamel membrane forms an almost complete investment for the dental papilla (Fig. 173), but as the ossification of the tooth proceeds, it recedes from the lower part, until finally it is confined entirely to the crown. The dentine forming the roots of the tooth then becomes enclosed in a layer of cement, which is true bone and serves to unite the tooth firmly to the walls of its socket. As the tooth increases in size, its extremity is brought nearer to the surface of the gum and eventually breaks through, the eruption of the first teeth usually taking place during the last half of the first year after birth. The growth of the permanent teeth
Fig. 173. - Section through the First Molar Tooth of a Rat, Twelve Days Old. Ap, Periosteum; E, dentine; Ep, epidermis; Od, odontoblasts; S, enamel; SEa and SEi, outer and inner layers of the enamel organ; SE, portion of the enamel organ which does not produce enamel. - (von Brunn.)
proceeds slowly at first, but later it becomes more rapid and produces pressure upon the roots of the primary teeth. These roots then undergo partial absorption, and the teeth are thus loosened
DEVELOPMENT OF THE TONGUE 289 in their sockets and are readily- pushed out by the further growth of the permanent teeth.
The dates and order of the eruption of the teeth are subject to considerable variation, but the usual sequence is somewhat as follows: Primary Dentition.
Median incisors 6th to 8th month.
Lateral incisors 8th to 12 month.
First molars Beginning of 2d year.
Canines iÂ£ years.
Second molars 3 to 3^ years.
The teeth of the lower jaw generally precede those of the upper.
First molars 7th year.
Middle incisors 8th year.
Lateral incisors 9th year.
First premolars 10th year.
Second premolars nth year.
13th to 14th years.
Second molars J Third molars 17th to 40th years.
In a considerable percentage of individuals the third molars (wisdom teeth) never break through the gums, and frequently when they do so they fail to reach the level of the other teeth, and so are only partly functional. These and other peculiarities of a structural nature shown by these teeth indicate that they are undergoing a retrogressive evolution.
The Development of the Tongue
Strictly speaking, the tongue is largely a development of the pharyngeal region of the digestive tract and only secondarily grows forward into the floor of the mouth. In embryos of about 3 mm. there may be seen in the median line of the floor of the mouth, between the ventral ends of the first and second branchial arches, a small rounded elevation which has been termed the tuberculum impar (Fig. 174, Ti). It was at one time believed that this gave rise to the anterior portion of the tongue, but recent observations seem to show that it reaches its greatest development in embryos of about 8 mm., after which it becomes less prominent and finally unrecognizable. But before 19
this occurs a swelling appears in the anterior part of the mouth on each side of the median line (Fig. 174, t), and these gradually increase
Fig. 174. - Floor of the Mouth and Pharynx of an Embryo of 7.5 mm., from a Reconstruction.
Cop, Copula; /, furcula; t, swelling that gives rise to the body of the tongue; Ti, tuberculum impar; I-III, branchial arches.
in size and eventually unite in the median line to form the main mass of the body of the tongue. They are separated from the neighboring portions of the first branchial arch by a deep groove, the alveolo-lingual groove, and posteriorly are separated from the second arch by a groove which later becomes distinctly V-shaped (Fig. 175), a deep depression, which gives rise to the thyreoid body lying at the apex of the V. Behind the thyreoid pouch the ventral ends of the second and third branchial arches unite to form an elevation, the copula (Fig. 174, cop), and from this and the adjacent portions of the second and third arches the posterior portion of the tongue develops. The tongue then consists of two distinct portions, which even
Fig. 175. - The Floor of the Pharynx of an Embryo of about 20 MM.
ep, Epiglottis; fc, foramen caecum; t 1 and t 2 median and lateral portions of the tongue. - (His.)
tually fuse together, but the groove which originally separated them remains more or less clearly distinguishable (Fig. 175), the vallate papillae (see p. 430) developing immediately anterior to it.
The tongue is essentially a muscular organ, being formed of a central mass of muscular tissue, enclosed at the sides and dorsally by mucous membrane derived from the floor of the mouth and pharynx. The muscular tissue consists partly of fibers limited to the substance of the tongue and forming the m. lingualis, and also of a number of extrinsic muscles, the hyoglossi, genioglossi, styloglossi, glos so palatini, and chondroglossi. The last two muscles are innervated by the vagus nerve, and the remaining extrinsic muscles receive fibers from the hypoglossal, while the lingualis is supplied partly by the hypoglossal and partly, apparently, by the facial through the chorda tympani. That the facial should take part in the supply is what might be expected from the mode of development of the tongue, but the hypoglossal has been seen to correspond to certain primarily postcranial metameres (p. 169), and its relation to structures taking part in the formation of an organ belonging to the anterior part of the pharynx seems somewhat anomalous. It may be supposed that in the evolution of the tongue the extrinsic muscles, together with a certain amount of the lingualis, have grown into the tongue thickenings from regions situated much further back, for the most part from behind the last branchial arch.
Such an invasion of the tongue by muscles from posterior segments would explain the distribution of its sensory nerves (Fig. 176). The anterior portion, from its position, would naturally be supplied by branches from the fifth and seventh nerves, while the posterior portion might be expected to be supplied by the seventh. There seems, however, to have been a dislocation forward, if it may be so expressed, of the mucous membrane, the sensory distribution of the ninth nerve extending forward upon the posterior part of the anterior portion of the tongue, while a considerable amount of the posterior portion is supplied by the tenth nerve. The distribution of the sensory fibers of the facial is probably confined entirely to the anterior portion, though further information is needed to determine the exact distribution of both the motor and sensory fibers of this nerve in the tongue.
The Development of the Salivary Glands
In embryos of about 8 mm. a slight furrow may be observed in the floor of the groove which connects the lip grooves of the upper and lower jaws at the angle of the mouth and may be known as the cheek groove. In later stages this furrow deepens and eventually becomes closed in to form a hollow tubular structure, which in embryos of 17 mm.
has separated from the epithelium of the floor of the cheek groove except at its anterior end and has become embedded in the connective tissue of the cheek. This tube is readily recognizable as the parotid gland and duct, and from the latter as it passes across the masseter muscle a pouch-like outgrowth is early formed which probably represents the soda parotidis.
Fig. 176. - Diagram of the Distribution of the Sensory Nerves of the Tongue. The area supplied by the fifth (and seventh) nerve is indicated by the transverse lines; that of the ninth by the oblique lines; and that of the tenth by the small circles. - (Zander.) The submaxillary gland and duct appear in embryos of about 13 mm. as a longitudinal ridge-like thickening of the epithelium of the floor of the alveolo-lingual groove (see p. 290). This ridge gradually separates from behind forward from the floor of the groove and sinks into the subjacent connective tissue, retaining, however, its connection with the epithelium at its anterior end, which indicates the position of the opening of the duct. In the vicinity of this there appear in embryos of 24.4 mm. five small bud-like downgrowths of the epithelium (Fig. 177, SL), which later increase considerably in number as well as in size, and constitute a group of glands which are generally spoken of as the sublingual gland.
As these representatives of the various glands increase in length,
Fig. 177. - Transverse Section of the Lower Jaw and Tongue of an Embryo of about 20 mm. D, Digastric muscle; GGl., genioglossus, GH.\ geniohyoid; T.Al, inferior alveolar nerve; Man, mandible; MK, Meckel's cartilage; My, mylohyoid; SL, sublingual gland; S.Mx, submaxillary duct; T, tongue.
they become lobed at their deeper ends, and the lobes later give rise to secondary outgrowths which branch repeatedly, the terminal branches becoming the alveoli of the glands. A lumen early appears in the duct portions of the structures, the alveoli remaining solid for a longer time, although they eventually also become hollow.
It is to be noted that each parotid and submaxillary consists of a single primary outgrowth, and is therefore a single structure and not a union of a number of originally separate parts. The sublingual glands of adult anatomy are usually described as opening upon the floor of the mouth by a number of separate ducts. This arises from the fact that the majority of the glands which form in the vicinity of the opening of Wharton's duct remain quite small, only one of them on each side giving rise to the sublingual gland proper. The small glands have been termed the alveolo-lingual glands, and each one of them is equivalent to a parotid or submaxillary gland. In other words, there are in reality not three pairs of salivary glands, but from fourteen to sixteen pairs, there being usually from eleven to thirteen alveolo-lingual glands on each side.
The Development of the Pharynx
The pharynx represents the most anterior part of the archenteron, that portion in which the branchial arches develop, and in the embryo it is relatively much longer than in the adult, the diminution being brought about by the folding in of the posterior arches and the formation of the sinus prsecervicalis already described (p. 97). Between the various branchial arches, grooves occur, representing the endodermal portions of the grooves which separate the arches. During development the first of these becomes converted into the tympanic cavity of the ear and the Eustachian tube (see Chapter XV) ; the second disappears in its upper part, the lower persisting as the fossa in which the tonsil is situated; while the lower parts of the remaining two are represented by the sinus piriformis of the larynx (His), and also leave traces of their existence in detached portions of their epithelium which form what are termed the branchial epithelial bodies , and take part in the formation of the thyreoid and thymus glands.
In the floor of the pharynx behind the thickenings which produce the tongue there is to be found in early stages a pair of thickenings passing horizontally backward and uniting in front so that they resemble an inverted U (Fig. 178, /). These ridges, which form what is termed the furcula (His), are concerned in the formation of parts of the larynx (see p. 334). In the part of the roof of the pharynx which comes to lie between the openings of the Eustachian tubes, a collection cf lymphatic tissue takes place beneath the mucous membrane, forming the pharyngeal tonsil, and immediately behind this there is formed in the median line an upwardly projecting pouch, the pharyngeal bursa, first certainly noticeable in embryos 6.5 mm. in length.
This bursa has very generally been regarded as the persistent remains of Rathke's pouch (p. 285), especially since it is much more pronounced in fetal than in adult life. It has been shown, however, that it is formed quite independently of and posterior to the true Rathke's pouch (Killian), though what its significance may be is still uncertain.
The tonsils are formed from the epithelium of the second branchial groove. At about the fourth month solid buds begin to grow from the epithelium into the subjacent mesenchyme, and depressions appear on the surface of this region. Later the buds become hollow by a cornification of their central cells, and open upon the floor of the depressions which represent the crypts of the tonsil. In the meantime lymphocytes, concerning whose origin there is a difference of opinion, collect in the subjacent mesenchyme and eventually aggregate to form lymphatic follicles in close relation with the buds. Whether the lymphocytes wander out from the blood into the mesenchyme or are derived directly from the epithelium or the mesenchyme cells is the question at issue.
The tonsil may grow to a size sufficient to fill up completely the groove in which it forms, but not infrequently a marked depression, the fossa supratonsillaris, exists above it and represents a portion of the original second branchial furrow.
The groove of Rosenmuller, which was at one time thought to be also a remnant of the second furrow, is a secondary depression which appears in embryos of 11.5 cm. behind the opening of the Eustachian tube, in about the region of the third branchial furrow.
The Development of the Branchial Epithelial Bodies. - These are structures which arise either as thickenings or as outpouchings of the epithelium lining the lower portions of the inner branchial furrows. Five pairs of these structures are developed and, in addition, there is a single unpaired median body. This last makes its appearance in embryos of about 3 mm., and gives rise to the major portion of the thyreoid body. It is situated immediately behind the anterior portion of the tongue, at the apex of the groove between this and the posterior portion, and is first a slight pouch -like depression. As it deepens, its extremity becomes bilobed, and after the embryo has reached a length of 6 mm. it becomes completely separated from the floor of the pharynx. The point of its original origin is, however, permanently marked by a circular depression, the foramen cacum (Fig. 175, fc). Later the bilobed body migrates down the neck and becomes a solid transversely elongated mass (Fig. 179, th), into the substance of which trabecule of connective tissue extend, dividing it into a network of anastomosing cords which
Fig. 178. - The Floor of the Pharynx of an Embryo of 2.15 MM.
/. Furcula; t, tuberculum impair. - (His.)
Fig. 179. - Reconstructions of the Branchial Epithelial Bodies of Embryos.
of (a) 14 mm. and (b) 26 mm.
ao, Aorta; Ith, lateral thyreoid; ph, pharynx; pth 1 and pth 2 , parathyreoids; th, thyreoid; thy, thymus; vc, vena cava superior. - (Tourneux and Verdun.) later divide transversely to form follicles. When the embryo has reached a length of 2.6 cm., a cylindrical outgrowth arises from the anterior surface of the mass, usually a little to the left of the median line, and extends up the neck a varying distance, forming, when it persists until adult life, the so-called pyramid of the thyreoid body.
This account of the pyramid follows the statements made by recent workers on the question (Tourneux and Verdun) ; His has claimed that it is the remains of the stalk connecting the thyreoid with the floor of the pharynx, and which he terms the thyreo- glossal duct.
Two other pairs of bodies enter into intimate relations with the thyreoid, forming what have been termed the parathyreoid bodies (Fig. 179, pth 1 and pth 2 ). One of these pairs arises as a thickening of the dorsal portion of the fourth branchial groove and the other comes from the corresponding portion of the third groove. The members of the former pair, after separating from their pthm IV points of origin, come to lie on the dorsal surface of the lateral sd ^v^B - pthm ill portions of the thyreoid body (Fig. 180, pthm IV) in close proximity to the lateral thyreoids, while those of the other pair, passing further backward, come to rest behind the lower border of the thyreoid (Fig. 180, pthm III). The cells of these bodies do not become divided into cords by the ingrowth of connective tissue to the same extent as those of the thyreoids, nor do they become separated into follicles, so that the bodies are readily distinguishable by their structure from the thyreoid.
From the ventral portion of the third branchial groove a pair of evaginations develop, similar to those which produce the lateral thyreoids. These elongate greatly, and growing downward ventrally to the thyreoid and separating from their points of origin, come to lie below the thyreoids, forming the thymus gland (Fig. 179, thy). As development proceeds they pass further backward and come eventually to rest upon the anterior surface of the peri
Fig. 180. - Thyreoid, Tyhmtjs and Epithelial Bodies of a New-born Child.
pthm 111 and pthm IV, Para thyreoids; sd, thyreoid; thm III, thymus; thm 7T", lateral thyreoid. - (Groschuff.)
cardium. The cavity which they at first contain is early obliterated and the glands assume a lobed appearance and become traversed by trabecular of connective tissue. Lymphocytes, derived, according to some recent observations, directly from the epithelium of the glands, make their appearance and gradually increase in number until the original epithelial cells are represented only by a number of peculiar spherical structures, consisting of cells arranged in concentric layers and known as Hassall's corpuscles.
The glands increase in size until about the fifteenth year, after
Fig. 181. - Diagram showing the Origin of the Various Branchial Epithelial Bodies.
Ith, Lateral thyreoids; pp, ultimobranchial bodies; pht 1 and phi 2 , parathyreoids; th, median thyreoid; thy, thymus; I to IV, branchial grooves. - (Kohn.) which they gradually undergo degeneration into a mass of fibrous and adipose tissue.
A pair of evaginations very similar to those that give rise to the thymus are also formed from the ventral portion of the fourth branchial groove (Figs. 179, A and 181, lih). As a rule they completely disappear in later stages of development, but occasionally they undergo differentiation into small masses of thymus-like tissue, which remain associated with the parathyreoids from the same arch (Fig. 180, thm IV). They have been termed lateral thyreoids, but the term is a misnomer, since they take no essential part in the formation of the thyreoid body.
Finally, a pair of outgrowths arise from the floor of the pharynx just behind the fifth branchial arch, in the region where the fifth groove, if developed, would occur. These ultimo-branchial bodies, as they have been called, usually undergo degeneration at an early stage and disappear completely, though occasionally they persist as cystic structures embedded in the substance of the thyreoid.
The relation of these various structures to the branchial grooves is shown by the annexed diagram (Fig. 181), and from it, it will be seen that the bodies derived from the third and fourth grooves are serially equivalent. Comparative embryology makes this fact still more evident, since, in the lower vertebrates, each branchial groove contributes to the formation of the thymus gland. The terminology used above for the various bodies is that generally applied to the mammalian organs, but it would be better, for the sake of comparison with other vertebrates, to adopt the nomenclature proposed by Groschuff, who terms each lateral thyreoid a thymus IV, while each thymus lobe is a thymus III. Similarly the parathyreoids are termed parathymus III and IV, the term thyreoid being limited to the median thyreoid.
The Musculature of the Pharynx. - The pharynx differs from other portions of the archenteron in the fact that its walls are furnished with voluntary muscles, the principal of which are the constrictors and the stylo-pharyngeus. This peculiarity arises from the relations of the pharynx to the branchial arches. It has been seen that in the higher mammalia the dorsal ends of the third, fourth, and fifth branchial cartilages disappear; the muscles originally associated with these structures persist, however, and give rise to the muscles of the pharynx, which consequently are innervated by the ninth and tenth nerves.
The Development of the (Esophagus. - From the ventral side of the lower portion of the pharynx an evagination develops at an early stage which is destined to give rise to the organs of
respiration; the development of this may, however, be convenientlypostponed to a later chapter (Chapter XII) .
The oesophagus is at first a very short portion of the archenteron (Fig. 182, A), but as the heart and diaphragm recede into the thorax, it elongates (Fig. 182, B) until it eventually forms a considerable portion of the digestive tract. Its endodermal lining, like that of the rest of the digestive tract except the pharynx, is surrounded
Fig. 182. - Reconstructions of the Digestive Tract of Embryos of (^4) 4.2 mm.
and (2?) 5 MM.
all, Allantois; cl, cloaca; I, lung; li, liver; Rp, Rathke's pouch; 5, stomach; t, tongue; th> thyreoid body; Wd, Wolffian duct; y, yolk-stalk. - (His.) by splanchnic mesoderm whose cells become converted into nonstriated muscular tissue, which, by the fourth month, has separated into an inner circular and an outer longitudinal layer.
The Development of the Stomach and Intestines
By the time the embryo has reached a length of about 5 mm. its constriction from the yolk-sac has proceeded so far that a portion of the digestive tract anterior to the yolk-sac can be recognized as the stomach and a portion posterior as the intestine. As first the stomach is a simple, spindle-shaped enlargement (Fig. 182) and the intestine a tube without any coils or bends, but since in later stages the intestine grows much more rapidly in length than the abdominal cavity, a coiling of the intestine becomes necessary.
The elongation of the stomach early produces changes in its position, its lower end bending over toward the right, while its upper end, owing to the development of the liver, is forced somewhat toward the left. At the same time the entire organ undergoes a rotation about its longitudinal axis through nearly ninety degrees, so that, as the result of the combination of these two changes, what was originally its ventral border becomes its lesser curvature and what was originally its left surface becomes its ventral surface.
Hence it is that the left vagus nerve passes over the ventral and the right over the dorsal surface of the stomach in the adult.
In the meantime the elongation of the oesophagus has carried the stomach further away from the lower end of the pharynx, and from being spindle-shaped it has become more pyriform, as in the adult. The fundus, it may be noted, is not due to a general enlargement of the organ but to a local outpouching of the upper dorsal portion of its wall.
The growth of the intestine results in its being thrown into a loop opposite the point where the yolk-stalk is still connected with it, the loop projecting ventrally into the portion of the ccelomic cavity which is contained within the umbilical cord, and being placed so that its upper limb lies to the right of the lower one. Upon the latter a slight pouch-like lateral outgrowth appears which is the beginning of the cacum and marks the line of union of the future small and large intestine. The small intestine, continuing to lengthen more rapidly than the large, assumes a sinuous course (Fig. 183), in which it is possible to recognize six primary coils which continue to be recognizable until advanced stages of development and even in the adult (Mall). The first of these is at first indistinguishable from the pyloric portion of the stomach and can be recognized as the duodenum only by the fact that it has connected with it the ducts of the liver and pancreas; as development proceeds, however, its caliber diminishes and it assumes the appearance of a portion of the intestine.
The remaining coils elongate rapidly and are thrown into numerous secondary coils, all of which are still contained within the
Fig. 183. - Reconstruction of Embryo of 20 mm. C, Caecum; K, kidney; L, liver; S, stomach; SC, suprarenal bodies; W, mesonephros. - (Mall.)
coelom of theumbilical cord (Fig. 184). When the embryo has reached a length of about 40 mm. the coils rather suddenly return to the abdominal cavity, and now the caecum is thrown over toward the right, so that it comes to lie immediately beneath the liver on the right side of the abdominal cavity, a position which it retains until about the fourth month after birth (Treves). The portion of the large intestine which formerly projected into the umbilical ccelom now lies transversely across the upper part of the abdomen, crossing in front of the duodenum and having the remaining portion of the small intestine below it. The elongation continuing, the secondary coils of the small intestine become more numerous and the lower portion of the large intestine is thrown into a loop which extends transversely across the lower part of the abdominal cavity and represents the sigmoid flexure of the colon. At the time of birth this portion of the large intestine is relatively much longer than in the adult, amounting to nearly half the entire length of the colon (Treves), but after the fourth month after birth a readjustment of the relative
Fig. 184. - Reconstruction of the Intestine of an Embryo of 19 mm. The Figures on the Intestine Indicate the Primary Coils. - (Mall.) lengths of the parts of the colon occurs, the sigmoid flexure becoming shorter and the rest of the colon proportionally longer, whereby the caecum is pushed downward until it lies in the right iliac fossa, the ascending colon being thus established.
When this condition has been reached, the duodenum, after passing downward for a short distance so as to pass dorsally to the transverse colon, bends toward the left and the secondary coils derived from the second and third primary coils come to occupy the left upper portion of the abdominal cavity. Those from the fourth primary coil pass across the middle line and occupy the right upper part of the abdomen, those from the fifth cross back again to the left lumbar and iliac regions, and those of the sixth take possession of the false pelvis and the right iliac region (Fig. 185).
Slight variations from this arrangement are not infrequent, but it occurs with sufficient frequency to be regarded as the normal. A failure
Fig. 185. - Representation of the Coilings of the Intestine in the Adult Condition. The Numbers indicate the Primary Coils. - (Mall.) in the readjustment of the relative lengths of the different parts of the colon may also occasionally occur, in which case the caecum will retain its embryonic position beneath the liver.
The yolk-stalk is continuous with the intestine at the extremity of the loop which extends out into the umbilical coelom, and when the primary coils become apparent its point of attachment lies in the region of the sixth coil. As a rule, the caliber of the stalk does not increase proportionally with that of the intestine, and eventually its embryomic portion disappears completely. Occasionally, however, this portion of it does partake of the increase in size which occurs in the intestine, and it forms a blind pouch of varying length, known as Meckel's diverticulum (see p. 113).
The ccecum has been seen to arise as a lateral outgrowth at a time when the intestine is first drawn out into the umbilicus. During subsequent development it continues to increase in size until it forms a conical pouch arising from the colon just where it is joined by the small intestine (Fig. 186). The enlargement of its terminal portion does not keep pace, however, with that of the portion nearest the intestine, but it becomes gradually more and more marked off from it by its lesser caliber and gives rise to the vermiform appendix. At birth the original conical form Fig. 186. - caecum of of the entire outgrowth is still quite evident, B â€ž . *Â°' 3 ^ c, Colon; 1, ileum.
though it is more properly described as funnelshaped, but later the proximal part, continuing to increase in diameter at the same rate as the colon, becomes sharply separated from the appendix, forming the caecum of adult anatomy.
Up to the time when the embryo has reached a length of 14 mm., the inner surface of the intestine is quite smooth, but when a length of 19 mm. has been reached, the mucous membrane of the upper portion becomes thrown into longitudinal folds, and later these make their appearance throughout its entire length (Fig. 187). Later, in embryos of 60 mm., these folds break up into numbers of conical processes, the villi, which increase in number with the development of the intestine, the new villi appearing in the intervals between those already present. Villi are formed as well in the large as in the small intestine, but in the former they decrease in size as development proceeds and practically disappear toward the end of fetal life.
In the early stages the endodermal lining of the digestive tract assumes a considerable thickness, the lumen of the oesophagus and upper part of the small intestine being reduced to a very small caliber. In later stages a rapid increase in the size of the lumen occurs, apparently associated with the formation of cavities or vacuoles in the endodermal epithelium. These increase in size, the neighboring cells arrange themselves in an epithelial layer around their walls and they eventually break through into the general lumen. They are sometimes sufficiently large to give the appearance of diverticula of the gut, but later they flatten out, their cavities becoming portions of the general lumen.
In the case of the duodenum the thickening of the endodermal lining proceeds to such an, extent that in embryos of from 12.5 mm. to 14.5 mm. the lumen is completely obliterated immediately below the opening of the hepatic and pancreatic ducts. This condition is interesting in connection with the occasional occurrence in new-born children of an atresia of the duodenum. Under normal conditions, however, the lumen is restored by the process of vacuolization described above.
Fig. 187. - Reconstruction of a Portion of the Intestine of an Embryo of 28 mm. showing the longitudinal folds from which the villi are formed.
The Development of the Liver
The liver makes its appearance in embryos of about 3 mm. as a longitudinal groove upon the ventral surface of the archenteron just below the stomach and between it and the umbilicus. The endodermal cells lining the anterior portion of the groove early undergo a rapid proliferation, and form a solid mass which projects ventrally into the substance of a horizontal shelf, the septum transversum (see p. 318), attached to the ventral wall of the body. This solid mass (Fig. 188, L) forms the beginning of the liver proper, while the lower portion of the groove, which remains hollow, represents the future gall-bladder (Fig. 188, B). Constrictions appearing between the intestine and both the hepatic and cystic portions of the organ gradually separate these from the intestine, until they are united to it only by a stalk which represents the ductus choledochus (Fig. 188).
The further development of the liver, so far as its external
. SS 2
Fig. 188. - Reconstruction of the Liver Outgrowths of Rabbit Embryos of (a) 5 mm. and (b) of 8 mm.
B, Gall-bladder; d, duodenum; DV, ductus venosus;L, liver; p, dorsal pancreas; pm, ventral pancreas; rL, right lobe of the liver; S, stomach. - (Hammar.)
form is concerned, consists in the rapid enlargement of the hepatic portion until it occupies the greater part of the upper half of the abdominal cavity, its ventral edge extending as far down as the umbilicus. In the rabbit its substance becomes divided into four lobes corresponding to the four veins, umbilical and vitelline, which traverse it, and the same condition occurs in the human embryo, although the lobes are not so clearly indicated upon' the surface as in the rabbit. The two vitelline lobes are in close apposition and may almost be regarded as one, a median ventral lobe which embraces the ductus venosus (Fig. 188, B, DV), while the umbilical lobes are more lateral and dorsal and represent the right (rL) and left lobes of the adult liver. The remaining definite lobes, the caudate (Spigelian) and quadrate, are of later formation, standing in relation to the vessels which cross the lower surface of the liver.
The ductus choledochus is at first wide and short, and near its proximal end gives rise to a small outgrowth on each side, one of which becomes the ventral pancreas (Fig. 188, B, pm). Later the duct elongates and becomes more slender, and the gall-bladder is
Fig. 189. - Transverse Section through the Liver oe an Embryo of Four Months. in, Intestine; I, liver; W, Wolffian body. - (Toldt and Zuckerkandl.) constricted off from it, the connecting stalk becoming the cystic duct. The hepatic ducts are apparently developed from the liver substance and are relatively late in appearing.
Shortly after the hepatic portion has been differentiated its substance becomes permeated by numerous blood-vessels (sinusoids) and so divided into anastomosing trabeculae (Fig. 189). These are at first irregular in size and shape, but later they become more slender and more regularly cylindrical, forming what have been termed the hepatic cylinders. In the center of each cylinder, where the cells which form it meet together, a fine canal appears, the beginning of a bile capillary, the cylinders thus becoming converted into tubes with fine lumina. This occurs at about the fourth week of development and at this time a cross-section of a cylinder shows it to be composed of about three or four hepatic cells (Fig. 190, A), among which are to be seen groups of smaller cells (e) which are erythrocytes, the liver having assumed by this time its haematopoietic function (see p. 225). This condition of affairs persists until birth, but
Fig. 190.- - Transverse Sections of Portions of the Liver of (.4) a Fetus of Six Months and (B) a Child of Four Years.
be, Bile capillary; e, erythrocyte; he, hepatic cylinder. - (Toldt and Zuckerkandl.)
later the cylinders undergo an elongation, the cells of which they are composed slipping over one another apparently, so that the cylinders become thinner as well as longer and show for the most part only two cells in a transverse section (Fig. 190, B); and in still later periods the two cells, instead of lying opposite one another, may alternate, so that the cylinders become even more slender.
The bile capillaries seem to make their appearance first in cylinders which lie in close relation to branches of the portal vein (Fig. 191) ,
and thence extend throughout the neighboring cylinders, anastomosing with capillaries developing in relation to neighboring portal branches. As the extension so proceeds the older capillaries continue to enlarge and later become transformed into bile-ducts (Fig. 191, C), the cells of the cylinders in which these capillaries were situated becoming converted into the epithelial lining of the ducts.
The lobules, which form so characteristic a feature of the adult liver, are late in appearing, not being fully developed until some time after birth. They depend upon the relative arrangement of the branches of the portal and hepatic veins; these at first occupy distinct territories of the liver substance, being separated from one another by practically the entire thickness of the liver, although of
Fig. 191. - Injected Bile Capillaries of Pig Embryos of (A) 8 cm., (B) 16 cm., and (C) of Adult Pig. - (Hendrickson.) course connected by the sinusoidal capillaries which lie between the hepatic cylinders. During development the two sets of branches extend more deeply into the liver substance, each invading the territory of the other, but they can readily be distinguished from one another by the fact that the portal branches are enclosed within a sheath of connective tissue (Glisson's capsule) which is lacking to the hepatic vessels. At about the time of birth the branches of the hepatic veins give off at intervals bunches of terminal vessels, around which branches of the portal vein arrange themselves, the liver tissue becoming divided up into a number of areas which may be termed hepatic islands, each of which is surrounded by a number of portal branches and contains numerous dichotomously branching hepatic terminals. Later the portal branches sink into the substance of the islands, which thus become lobed, and finally the sinking in extends so far that the original island becomes separated into a number of smaller areas or lobules, each containing, as a rule, a single hepatic terminal (the intralobular vein) and being surrounded by a number of portal terminals (interlobular veins) , the two systems being united by the capillaries which separate the cylinders contained within the area. The lobules are at first very small, but later they increase in size by the extension of the hepatic cylinders.
Frequently in the human liver lobules are to be found containing two intralobular veins, a condition with results from an imperfect subdivision of a lobe of the original hepatic island.
The liver early assumes a relatively large size, its weight at one time being equal to that of the rest of the body, and though in later embryonic stages its relative size diminishes, yet at birth it is still a voluminous organ, occupying the greater portion of the upper half of the abdominal cavity and extending far over into the left hypochondrium. Just after birth there is, however, a cessation of growth, and the subsequent increase proceeds at a much slower rate than that of the rest of the body, so that its relative size bcomes still more diminished (see Chap. XVII). The cessation of growth affects principally the left lobe and is accompanied by an actual degeneration of portions of the liver tissue, the cells disappearing completely, while the ducts and blood-vessels originally present persist, the former constituting the vasa aberrantia of adult anatomy. These are usually especially noticeable at the left edge of the liver, between the folds of the left lateral ligament, but they may also be found along the line of the vena cava, around the gall-bladder, and in the region of the left longitudinal fissure.
The Development of the Pancreas
The pancreas arises a little later than the liver, as two or three separate outgrowths, one from the dorsal surface of the duodenum (Fig. 192, DP) usually a little above the liver outgrowth, and one or two from the lower part
of the common bile-duct. Of the latter outgrowths, that upon the left side (Vps) may be wanting and, if formed, early disappears, while that of the right side (Vpd) continues its development to form what has been termed the ventral pancreas. Both this and the dorsal pancreas continue to elongate, the latter lying to the left of ^^ the portal vein, while the former, at first situated to the right of the vein, later grows across its ventral surface so as to come into contact with the dorsal gland, with which it fuses so intimately that no separation line can be distinguished. The body and tail of the adult pancreas represent the original dorsal outgrowth, while the right ventral pancreas becomes the head.
Both the dorsal and ventral outgrowths early become lobed, and the lobes becoming secondarily lobed and this lobation repeating itself several times, the compound tubular structure of the adult gland is acquired, the very numerous terminal lobules becoming the secreting acini, while the remaining portions become the ducts. Of the principal ducts, there are at first two; that of the dorsal pancreas, the duct of Santorini, opens into the duodenum on its dorsal surface, while that of the ventral outgrowth, the duct of Wirsung, opens into the ductus choledochus. When the fusion of the two portions of the gland occurs, an anastomosis of branches of the two ducts develops and the proximal portion of the duct of Santorini may
Fig. iq2. - Reconstruction of the Pancreatic Outgrowths of an Embryo of 7.5 MM.
D, Duodenum; Dc, ductus communis choledochus; DP, dorsal pancreas; Vpd, and Vps, right and left ventral pancreas. - (Helly.)
degenerate, so that the secretion of the entire gland empties into the common bile-duct through the duct of Wirsung.
In the connective tissue which separates the lobules of the gland, groups of cells occur, which have no connection with the ducts of the gland, and form what are termed the areas ofLangerhans. They arise by a differentiation of the cells which form the original pancreatic outgrowths, and have been distinguished in the dorsal pancreas of the guinea-pig while it is still a solid outgrowth. They gradually separate from the remaining cells of the outgrowth and come to lie in the mesenchyme of the gland in groups into which, finally, bloodvessels penetrate.
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