Book - A Laboratory Manual and Text-book of Embryology 7

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Prentiss CW. and Arey LB. A laboratory manual and text-book of embryology. (1918) W.B. Saunders Company, Philadelphia and London.

Human Embryology 1917: The Germ Cells | Germ Layers | Chick Embryos | Fetal Membranes | Pig Embryos | Dissecting Pig Embryos | Entodermal Canal | Urogenital System | Vascular System | Histogenesis | Skeleton and Muscles | Central Nervous System | Peripheral Nervous System | Embryology History
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Pages where the terms "Historic Textbook" and "Historic Embryology" appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms and interpretations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)


Chapter VII. Entodermal Canal and its Derivatives

When the head- and tail-folds of the embryo develop, there are formed both cranial and caudal to the spherical vitelline sac bUnd entodermal tubes, the foregut and kind-gut respectively (Figs. 79 and 167 A). The region between these intestinal tubes, open ventrally into the yolk sac, is sometimes termed the mid-gut.


Fig. 167. — Diagiams showing in median sagittal section the human alimentary canal, phaiyngeal and cloftcal membranes. X 35. A, 2 mm. embiyo (modified after His); B, 2.5 mm. embiyo (alter Thompson).

As the embryo and the yolk sac at first grow more rapidly than the connecting region between them, this region is apparently constricted and becomes the yolk stalk or vitelline duct. At either end the entoderm comes into contact ventrally with the ectoderm. Thus there are formed the pharyngeal membrane of the fore-gut, the cloacal membrane of the hind-gut. In 2 mm. embryos the pharyngeal membrane separates the ventral ectodermal cavity, or slomodanim, from the pharyngeal cavity of the fore-gut. Cranial to the membrane is the ectodermal diverticulum, Rathke's pocket. In 2.5 to 3 mm. embryos (Fig. 167 B) the pharyngeal membrane ruptures and the stomodajum and pharynx become continuous. The blind termination of the fore-gut apparently forms Seessel^s pocket.


The fore-gut later forms part of the oral cavity and is further differentiated into the pharynx and its derivatives, and into the esophagus, respiratory organs, stomach, duodenum, jejunum, and a portion of the ileum. From the duodenum arise the liver and pancreas. The hind-gut, beginning at the attachment of the yolk stalk extends caudally to the cloaca, into which the allantois opens in 2 mm. embryos. The hind-gut is differentiated into the ileum, caecum, colon, and rectum. The cloaca is subdivided into the rectum and urogenital sinus (for its development see Chapter VIII). At the same time the cloacal membrane is separated into a urogenital membrane and into an anal membrane. The latter eventually ruptures, forming the anus. The yolk stalk usually loses its connection with the entodermal tube in embryos of about 7 mm. (Fig. 179).

We have seen how the palatine processes divide the primitive oral cavity into the nasal passages and mouth cavity of the adult, and have described the development of the tongue, teeth, and salivary glands — organs derived wholly or in part from the ectoderm. It remains to trace the development of the pharynx and the intestinal tract and their derivatives.

Pharyngeal Pouches

There are developed early from the lateral wall of the pharynx paired outgrowths which are formed in succession cephalo-caudad. In 4 to 5 mm. embryos, five pairs of such pharyngeal pouclies are present, the fifth pair being rudimentary (Figs. 86 and 87). Meantime, the pharynx has been flattened dorso-ventrally and broadened laterally and cephalad, so that it is triangular in ventral view (Figs. 87 and 168).

From each pharyngeal pouch develop small dorsal and large ventral diverticula. All five pouches come into contact with the ectoderm of the branchial clefts, fuse with it, and form the closing plates. Only occasionally do the closing plates become perforate in human embryos. The first and second pharyngeal pouches soon connect with the pharyngeal cavity through wide openings. The third and fourth pouches grow laterad and their diverticula communicate with the pharynx through narrow ducts in 10 to 12 mm. embryos (Fig. 168). When the cervical sinus (p. 97) is formed, the ectoderm of the second, third, and fourth branchial clefts is drawn out to produce the transient branchial and cervical duels and the cervical vesicle. These are fused at the closing plates with the entoderm of the second, third, and fourth pharyngeal pouches. the pharynx the first two pouches acquire a common opening into it. The first pouch later differentiates into the tympanic cavity of the middle ear and into the auditory {Eustachian) tube. By the growth and lateral expansion of the pharynx, the second pouch is absorbed into the pharyngeal wall, its dorsal angle alone periling, to be later transformed into the tonsillar and supratonsUlar fossa. The third, fourth, and fifth pouches give rise to a series of ductless glands, the thymus, paratkyreoids, and the tdtimobranchtal bodies.


Fig. 168. — A reconstruction of the pharynx and [ore-gut o[ view (after Hammar). The ectodermal i 1.7 mm. human embo'o seen in dormi stippled.


A mound of lymphoid tissue presses against the epithelium of the tonsillar fossa in 140 mm. (C R ?) fetuses and forms the palatine tonsil. The lymphocytes are probably of mesodermal origin (Hammar, Maximow).

Imperfect closure of the branchial clefts (usually the second) leads to the formation of cysts, diverticula, or even of fistulse. According to Hammar (Arch. f. mikr. Anat., Bd. 61, 1903), the lateral pharyngeal recess (of Rosenmiiller) is not a persistent portion of the second pouch as His asserted.

A subepithelial infiltration of lymphocytes during the sixth month give$ rise to the median pharyngeal topisilj which like the lingual tonsil is not of pharyngeal pouch origin. Immediately caudad is a recess, the pharyngeal bursas formed by a persistent connection of the epithelium with the notochord (Huber).

The Thymus

The thymus anlage appears in 10 mm. embryos as a ventral and medial prolongation of the third pair of pouches (Figs. 168 and 169). The ducts connecting the diverticula with the pharynx soon disappear so that the thsmaus anlages are set free. At first hollow tubes, they soon lose their cavities and their lower ends enlarge and migrate caudally into the thorax, passing usually ventral to the left vena anonyma. Their upper ends become attentuate and atrophy, but may persist as an accessory thymus lobe (Kohn). The enlarged lower ends of the anlagcs form the body of the gland, which is thus a paired structure (Fig. 170). At 50 mm. (C R) the thymus still contains solid cords and small closed vesicles of entodermal cells. From this stage on, in development, the gland becomes more and more lymphoid in character. Its final position is in the thorax, dorsal to the cranial end of the sternum. It grows under normal conditions until puberty, after which its degeneration begins. This process proceeds slowly in healthy individuals, rapidly in case of disease. The thymus may function normally until after the fortieth year.


Fig. 169. — Diagram in ventral view of the pharynx and pharyngeal pouches, showing the origin of the thymus and thyreoid glands and of the epithelial bodies (modified after Groschuff and Kohn). I-W first to fifth pharj-ngeal pouches.


The ventral divertiadum of the fourth pouch is a rudimentary thymic aniage. It usually atrophies.


It is now generally believed that the e nt ode rmal epithelium of the thymus is converted into relicular tissue and thymic corpuscles. The "lymphoid" cells are regarded by Hammar, Maximow, and recently by Badertscher (Amer, Jour, Anat., vol. 17, 1915) as immigrant lymphocytes derived from the mesoderm. According to Stohr, they are not true lymphocytes, but are derived from the thymic epithelium. Weill (Arch. f. mikr. Anat., Bd. 83, 1913) has observed the development of granular leucocytes in the human thymus gland.


Fig. 170. — Reconstruction of the thymus and thyreoid glands (Touroeaux and Verdun). X 15.


The Epithelial Bodies or Parathyreoids

The dorsal diverticula of the third and fourth pharyngeal pouches each give rise to a small mass of epithelial celb termed an epithelial body (Fig. 169). 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 caudalward. They eventually lodge in the dorsal surface of the thyreoid gland, the pair from the third pouch l>ing one on each side at the caudal border of the thyreoid in line with the thymus anlages (Fig. 170). The pair of epithelial bodies derived from the fourth pouches are located on each side near the cranial border of the thyreoid. From their ultimate relation to the thyreoid tissue the epithelial bodies are often termed Parathyreoid glands. The solid body is broken up into masses and cords of polygonal entodermal cells intermingled with blood vessels. In postfetal life, lumina may appear in the cell masses and fill with a colloid-like secretion.

The Ultimobranchial or Postbrachial

The ultimobranchial body is the derivative of the fifth phar3aigeal pouch (Fig. 169). With the atrophy of the duct of the fourth pouch it is set free and migrates caudad with the paraihyreoids. It forms a hollow vesicle which has l)een erroneously termed the lateral thyreoid. According to Grosser (Keibel and Mall, vol. 2) and Verdun, it takes no part in forming thyreoid tissue, but atrophies. Kingsbury (Anat. Anz., Bd. 47, 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 Thyreoid Gland

In embryos with five to six primitive segments (1.4 mm.) there appears in the mid-ventral wall of the pharynx, between the first and second branchial arches, a small out-pocketing, the thyreoid anlage. In 2.5 mm. embryos it has become a stalked vesicle (Figs. 167 B and 87) Its stalk, the thyreoglossal duct, opens at the aboral border of the tuberculum impar of the tongue (Fig. 157 -4); this spot is represented permanently by the forameti ccecum (Fig. 180). The duct soon atrophies and the bilobed gland anlage (Fig. 169) loses its lumen and breaks up into irregular solid anastomosing plates of tissue as it migrates caudad. It takes up a transverse position with a lobe on each side of the trachea and larynx (Fig. 170). In embryos of 24 mm. discontinuous lumina begin to appear in swollen portions of the plates; these represent the primitive thyreoid follicles (Norris, Amcr. Jour. Anat., vol. 20, 1916).

Larynx, Trachea and Lungs

In embryos of 23 segments, the anlage of the respiratory organs appears as a groove in the floor of the entodermal tube just caudal to the pharyngeal pouches. This groove produces an external ridge on the ventral wall of the tube, a ridge which becomes larger and rounded at its caudal end (Fig. 171). The laryngotracheal groove and the ridge are the anlages of the larynx and trachea. The rounded end of the ridge is the unpaired anlage of the lungs.


Externally two lateral longitudinal grooves mark off the dorsal esophagus from the ventral respiratory anlages. The lung anlage rapidly increases in size and becomes bilobed in embn'os of 4 to 5 mm. A fusion of the lateral furrows progressing cephalad, constricts first the lung anlages and then the trachea from the esophagus. At the same time the laryngeal portion of the groove and ridge advances cranially until it lies bt^twcen the fourth branchial arches (Fig. 87). At 5 mm. the respiratory apparatus consists of the laryngeal groove and ridge, the tubular trachea, and the two limg buds (Fig. 165 D).


The Larynx

In embryos of 5 to 6 mm. the oral end of the laryngeal griHJve is bounded on either side by two rounded prominences, the arytawid swellings, which, continuous orally with a transverse ridge, form the furcula of His (Fig. 157 B). The transverse ridge becomes the epiglottis^ and, as we saw in connection with the development of the tongue, it is derived from the third and fourth branchial arches. In embryos of 15 mm. the arytenoid swellings are bent near the middle. Their caudal portions become parallel, while their cephalic jX)rtions


Fig. 171. — Diagrams of stages in the early de\'eIopment of the trai hea ami lun^* '/f human tmStf^m (based on reconstructions by Bremer, Broman, Grosser, and Narath;, / aU/ul Hi A, IS mm \ /I, 4 mm.; C, stage B in side view; D, 5 mm.; E, 7 mm.

diverge nearly ^t right angles CFig. 172), ITic r>f)ening Int/i the larynx thuH Incomes T-shaped and ends blindly, as the bryngeal epithch'um han fuM'd. In 40 mm. fetuses (C R) this fusion is dissolved, the ar>'Ufnoid swcrllingii are witlulrawii from contact with the epi^ttis, and the entrance t/i the larynx \Hrnmu*% oval in form (Fig. 173). At 27 mm- the ttntricUs iA the larynx sufptsir and ui M lum, (C R) their margins indicate the ptmiUm of the vrxal r/;fd«, 'IV tifiiMlum of the vocal cords is without cifia. llie ehL%tu: an/1 muM le filiers iff the <//rdfe are developed by the fifth mfmth.


The thyreoid cartilage is formed as two lateral plates, each of which has two centers oE

These plates grow vcntrad and fuse in the median line. Thf aniagcs uf ihe cricoid and arytenoid cartilages are al first lie cariilagc centers develop for the arytenoids. The cricoid is at first ji iveniually forms a complete ring. he criioid may iherefore be regarded . a yiKhJiricd tracheal ring. The comic .;:<■ ,\:ri:!jgfs represent separated pori^r* oi ihe arytenoids. The cunci/orm 'i-^jfis are derived from the cartilage the epiglottis.


Flg. 172.— The Ui>Tii of IfiO to Z» mm. hunun(otu$e$vSouli£3iidBudier^ X6. From a vlion. hJ.. Bise ot tongue: r., cfM^ttis; f.i.j.. inlcRin-tenoid itsiurr: oJ- orifice of luyiu; fl.jj.. pU\-3 ar>^HfH>.>tt>c»: fl.pk.r.. plica pharieKJe >-iip>-ct>idotn'.Ti; cun., cundfonn tuberde; com.,


The Trachea

This gradually olongalt's during dcwlopment and its coliimnir ;p;OitJi:i:i bfwmcs dliatwl. Mus*-lc lilvr? and the anlages of the cartilage


The Lungs

Soon after the lung anlages or stem buds are formed (in 5 nun. embryos), the right bronchial bud becomes larger and is directed more caudally {Fig. 171). At 7 mm. the stem bronchi give rise to two bronchial buds on the right side, to one on the left. The smaller bronchial bud on the right side is the apical bud. The right and left chief buds, known as ventral bronchi, soon bifurcate. There are thus formed three bronchial rami on the right side, two on the left, and these correspond to the primitive lobes of the lungs (Fig. 174).

On the left side, an apical bud is interpreted as being derived from the first ventral bronchus (Fig. 174). It develops later and remains small so thai a lobe corresponding to ihe upper lobe of the right lung is not developed (NaralH). The upper lobe of the left lung thus would correspond to the upper and middle lobes of ihc right lung.


Fig. 175.— Transverse section through the lungs and pleural cavities of a 10 mm. human embryo. X 2.1.


The bronchial anlages continue to branch in such a way that the stem bud is retained as the main bronchial stem (Fig. 174). That is, the branching is monopodial, not dichotomous, lateral buds being given oS from the stem bud proximal to its growing tip. Only in the later stages of development has dichotomous branching of the bronchi and the formation of two equal buds been described. Such buds, formed dichotomously, do not remain of equal ^ze (Flint, Amer. Jour. Anat., vol. 6, 1906).


The entodermal anlages of the lungs and trachea are developed in a median mass of mesenchyme dorsal and cranial to the peritoneal cavity. This tissue forms a broad mesentery termed the mediastinum (Fig. 175). The right and left stem buds of the lungs grow out laterad, carrying with them folds of the mesoderm. The branching of the bronchial buds takes place within this tissue which is covered by the mesothelial lining of the body cavity. The terminal branches of the bronchi are lined with entodermal cells which flatten out and form the resf>iral0ry epithdiitm of the adult lungs. The surrounding mesenchyme differentiates into the muscle, connective tissue, and cartilage plates of the lung, tracheal, and bronchial walls. Into it grow blood vessels and nerve fibers. When the pleural cavities are separated from the pericardial and peritoneal cavities, the mesothelium covering the lungs, with the connective tissue underlying it, becomes the visceral pleura. The corresponding layers lining the thoracic wall form the parietal pleura. These layers are derived respectively from the visceral (splanchnic) and parietal (somatic) mesoderm of the embryo.


Fig. 17(i,~- Ventral view of the lungs of a lO.S mm. embryo showing the pulmonary arteries and i-eins

(His in MeMurrich|. X 27. Ep., Apical bronchus; /, //, primary bronchi.

In 11 mm. embryos the two pulmonary arteries, from the sixth pair of aortic arches, course lateral then dorsal to the stem bronchi (Fig. 176). The right pulmonary artery passes ventral to the apical bronchus of the right lung. The single pulmonary vein receives two branches from each lung, two larger veins from each lower lobe, two smaller veins from each upper lobe and the middle lobe of the right side. These four pulmonary branches course ventrad and drain into the pulmonary trunk. When this common stem is taken up into the wall of the left atrium, the four pulmonary veins open directly into the latter.


According to KolUker, the air cells or alveoli of the lungs begin to form in the sixth monih and their development is completed during pregnancy. Elastic tissue appears during the fourth month in ihe largest bronchi. The abundant connective tissue found between the btiinchial branches in early fetal life becomes reduced in its relative amount as the alveoli of the lungs are developed.


Before birth the lungs arc relatively small, compact, and possess sharp margins. They lie in the dorsal portion o( the pleural cavities. After birth they normally fill with air, expanding and completely filling the pleural cavities. Their margins become rounded and the compact fetal lung tissue, which resembles that of a gland in structure, becomes light and spongy, owing to the enormous increase in the size of the alveoli and blood vessels. Because of .the greater amount of blood admitted to the lungs after birth, their weight is suddenly increased.


In the most common anomaly involving the esophagus and trachea the former is divided transversely, the trachea opening into the lower portion of the esophagus, while the upper portion of the esophagus ends blindly.

Esophagus, Stomach and Intestine

Esophagus

The esophagus in 4 to 5 mm. embryos is a short tube, flattened laterally, extending from the pharynx to the stomach. It grows rapidly in length and in 7.5 mm. embryos its diameter decreases both relatively and absolutely (Forssner). At this stage the esophageal epithelium is composed of two layers of columnar cells.

In 20 mm. embryos, vacuoles appear in the epithelium and increase the size of the lumen which remains open throughout. In later stages the wall of the esophagus is folded, and ciliated epithelial cells appear at 44 mm. (C R). The number of cell layers in the epithelium increases, until, at birth, they number nine or ten. Glands are developed as epithelial ingrowths. The circular muscle layer is indicated at 10 mm. but the longitudinal muscle fibers do not form a definite layer until 55 mm. (C R). (F. T. Lewis in Keibel and Mall, vol. 2.) These layers appear in similar time-sequence throughout the entire digestive tract.

Stomach

The stomach appears in embryos of 4 to 5 mm. as a laterallyflattened, fusiform enlargement of the fore-gut caudal to the lung anlages (Figs. 177 and 1 78) . Its epithelium is early thicker than that of the esophagus and is surrounded by a thick layer of splanchnic mesoderm. It is attached dorsally to the body wall by its mesentery, the greater omentum, and ventrally to the liver by the lesser omentum (Fig. 190 J5). The dorsal border of the stomach both enlarges locally to form the fundus j and also grows more rapidly than the ventral wall throughout its extent, thus producing the convex greater curvature. The whole stomach becomes curved and its cranial end is displaced to the left by the enlarging liver (Fig. 168). This forms a ventral concavity, the lesser curvature , and produces the first flexure of the duodenum.


The rapid growth of the gastric wall along its greater curvature also causes the stomach to rotate upon its long axis until its greater curvature, or primitive dorsal wall, lies to the left, its ventral wall, the lesser curvature, to the right (Fig. 201). The original right side is now dorsal, the left side ventral in p)osition, 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 (cf. Fig. 138). The change in position progresses rapidly and is already completed early in the second month (12 to 15 mm.). The rotation of the stomach explains the asymmetrical position of the vagus nerves of the adult organ, the left nerve suppl>ing the ventral wall of the stomach, originally the left wall, while the right vagus supplies the dorsal wall, originally the right.


Gastric pits are indkaled in 16 mm. embryos, and, at 100 mm. (C R), gland cells of the gastric glands are differentiated. These undoubtedly arise from the gastric epithelium (Lewis). The cardiac glands are developed early (91 mm. (C R) fetuses), and, according to Lewis, there is no "evidence in favor of Bensley's conclusion that the cardiac glands are decadent ,fundus glands.

Fig. 177.— Median sagittal mni, human embryo t after Ingalls). X 14. show the dif!estive canal (modified

At 10 mm. the stomach wall is composed of three layers: the entodermal epilhdium, a thick mesenchymal layer, and the peritoneal mesothdium. At 16 mm. the circular muscle layer is indicated by condensed mesenchyma. At 91 mm. (C R) the cardiac region shows a few longitudinal muscle fibers, which become distinct in the pyloric region at 240 mm. (C R). In 17 mm. embryos the stomach has reached its permanent position, the cardia having descended through about ten segments, the pylorus through six or seven.

Intestine

In 5 mm. embryos (Fig. 177), 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 to form the cloaca. It is supported from the dorsal body wall by the mesentery (Fig. 178),

From 5 to 9 mm. the ventral bend of the intestinal loop becomes more marked and the attachment of the yolk stalk to it normally disappears (Fig. 179).

Fig. 178. — ReconstnjctioD of » 5 mm. human embryo showing the entodermal a (Hia in KollmBon). X 25.

The attachment of the yolk stalk may persist in later stages (12 to 14 mm. embryos, according to Keibel, EIze, and Thyng). Also in 2 per cent, of adult intestines a pouch 3 to 9 cm. long is found about 80 cm. above the coUc valve, where the yolk stalk was formerly attached. This pouch, the diverticulum o} the ileum or Meckel's diverticulum, is of clinical importance as it may cause intestinal strangulation in infants.


At the stage shown in Fig. 179, the dorsal pancreatic anlage has been developed from the duodenum, and, in the caudal limb of the intestinal loop, there is formed an enlargement, due to a ventral bulging of the gut wall, which marks the anlage of the cacum and the boundary line between the large and smail intestine. The cscal anlage gives rise later both to the adult caatm and to a more distal appendage, the vermiform process, which lags in development and remains small.


Succeeding changes in the intestine consist (1) in its torsion and coiling due to its 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, its caudal limb hes at the left and cranial to its cephalic limb (Fig. 179). At this stage the intestinal loop enters the coelom of the umbilical cord, thus causing a temporary umbilical hernia.


Fig. 179. — Diagram, in median sagittal section, showing the digestive canal of a 9 mm. human embryo dapted from MaU). X 9.


The small intestine soon lengthens rapidly and at 17 mm. (Fig. 180) forms loops within the umbilical cord. Six primary loops occur and these may be recognized in the arrangement of the adult intestine (Mall, Bull. Johns Hopkins Hosp., vol. 9, 1898), In embryos of 42 mm. the intestine has returned from the umbilical cord into the abdominal cavity through a rather small aperture; the coelom of the cord is soon after obliterated.


In embryos between 10 and 30 mm., vacuoles appear in the wall of the duodenum and epithelial septa completely block the lumen. The remainder of the small intestine remains open, although vacuoles form in its epithelium, VUIi appear as rounded elevations of the epithelium at 23 mm. Qohnson). They begin to form at the cephalic end of the jejunum, and at 130 mm. (C R) they are found throughout the small intestine (Berry), Inltstinai glands appear as ingrowths of the epithelium about the bases of the villi. They develop first in the duodenum at 91 mm. (C R). The duodenal ffands (of Brunner) are said to appear during the



Fig. 180. — Diagrammatic median sagittal section of a I7inm. human embryo, showing the digestive canal (modified after Mall). X 5.

fourth month (Brand). In embryos of 10 to 12.5 mm. the circular muscle layer of the intestine first forms. The longitudinal muscle layer is not distinct until 75 mm. (C R).


Iht impervious duodenum of the embryo may persist as a congenital anomaly, and the persistence of the yolkstalk, as ifecyte/'iftitrrfifu/um. has already been mentioned (p. 171).


The large intesline, as seen in 9 mm. embryos (Fig. 179), forms a tube extending from the cacum to the cloaca. It does not lengthen so rapidly as the small intestine, and, when the intestine is withdrawn from the umbilical cord (at 42 mm. C R), its cranial or cxcal end lies on the right side and dorsal to the small intestine (Fig. 181). It extends transversely to the left side as the transverse colon, then bending abruptly caudad as the descending colon, returns by its Uiac flexure to the median plane and forms the rectum.


Fig. 181.— Three successive stages shoniDg the development of the digestive tube and the nwseii' teries in tlic huirian fetus (Toumeux in Heisler): /, Stomach; 2, duodenum; j, small intestine; 4, colon; 5. yolk sialic; 6, cxcum; 7, great omentum; 8, mesoduodenum; 0, mesentery; 10, mesocolon. The arrow points to the orifice of the omental bursa. The ventral mesentery is not shown.


The caecum (Fig. 182) may be distinguished from the vermiform process at 65 mm. (C R) (Tarenetzky). The oecum and vermiform process make a U shaped bend with the colon at 42 mm. (C R) , and this flexure gives rise to the colic valve (Toldt). In stages between 100 and 200 mm. (C R) the lengthening of the colon causes the caecum and cephalic end of the colon to descend toward the pelvis (Fig. 181). The ascending colon is thus formed-and the vermiform appendix takes the position which it occupies in the adult.


Fia. 182,— The cxcum of a human fetus of SO mm. (Kollmann); A., from the ventral side; B, from the dorsal side.


The circular musde layer of the large intestine appears first at 23 mm., the longitudinal layer at 75 mm. (C R). In 55 mm. (C R) fetuses villi are present. The development of the entire digestive tract has been described by Johnson (Amer. Jour. Anat., vob. 10, 1910; 14, 1913; 16, 1914).


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, b known as meconium. At birth the intestine and its contents are perfectly sterile.


The Liver

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. 167 B). Its thick walls enclose a cavity which is continuous with that of the gut. This hepatic diverticulum becomes embedded at once in a mass of splanchnic mesoderm, the sepium transversum. Cranially, the septum will contribute later to the formation of the diaphragm; caudally, in the region of the liver anlage, it becomes the ventrar mesentery (Fig. 189). Thus, from the first the liver is in close relation to the septum transversum and later when the septum becomes a part of the diaphragm the liver remains attached to it.

In embryos 4 to 5 mm. long, solid cords of cells proliferate from the ventral and cranial portion of the hepatic diverticulum (Fig. 86). These cords anastomose and form a crescentic mass with wings extending dorsad lateral to the gut (Fig. 177). This mass, a network of solid trabeculse, is the glandular portion of the liver. The primitive, hollow diverticulum later differentiates into the gall bladder and the large biliary ducts.

Fig. 183. — Model of the liver anlage of a 4 mm. human embryo (Bremer). X 160. In., Intestine; Pa., pancreas; V., veins in contact with liver Ira


Referring to Fig. 88, it will be seen that the liver aniage lies between the vitelline veins and is in close proximity to them laterally. The veins send anastomosing branches into the ventral mesentery. The trabeculffi of the expanding liver grow between and about these venous plexuses, and the plexuses in turn make their way between and around the liver cords (Fig, 183). The vitelline veins on their way to the heart are thus surrounded by the liver and largely subdivided 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. 184). The transformation of the \'itelline veins into the portal vein and the relations of the umbilical veins to the liver will be treated in Chapter IX.


Fig. IM.— The trabeculse and sinusoids of the liver in section (afler Minot). X 300. Tt., Trabecule of liver celts; Si., sinusoids.


Fig. lf<5.— Hecon: human embryo (after ThynR), X 50; B, 10 mm. human embryo, X 33.


The glandular portion of the liver grows rapidly, and, in embryos of 7 to 8 mm,, is connected with the primitive hepatic diverticulum only by a single cord of cells, the hepatic duct (Fig. 185 A). That portion of the hepatic diverticulum distal to the hepatic duct is now differentiated into the terminal, solid gaU bladder and its cystic duct. Its proximal portion forms the ductus choledochus. In embryos of 10 mm. (Fig. 185 B) the gall bladder and ducts have become longer and more slender. 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. 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 31 mm. (Jackson). 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.


Fig. 186.— Diagrams of the a. Hepatic ^dc; d, portal side; b and r the portal vein (Mall).


The development of the ligaments of the liver is described on p. 192.

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 fclal life to some time after birth, blood cells arc actively developed between the hepatic cells and the endothelium of the sinusoids, Lumina bounded by five or six cells may be observed the liver traljccula? of 10 mm. embryos (Lewis), At 22 mm. hollow periportal duels develop, spreading inward from ihe hepatic duct along the larger branches of the portal 44 mm. (C R) fetuses. hUe capillaries with cuticular borders are present, the periportal ducts with which some of them connect. At birth, or shorlly after, the numbei of liver cells surrounding a bile capillary is reduced to two, three, or four. Secretion of the bile commeiict'S at about the end of the third fetal month.


The lobules, or vascular units of the liver, are fonned, according to Mall, by the peculiar and regular manner in which the veins of the liver branch. The primary branches of tbe portal vein extend along the periphery of each primitive lobule, parallel to similar branches of the hepatic veins which drain the blood from the center of each lobule (Fig. 186). 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 until thousands of Uver lobules are developed.


Until the 20 mm. stage the portal vein alone supplies the liver. The hepatic artery, from the cceliac 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 branches of the portal vein, and also supplies the capsule of the liver.

Anomalies

A common anomaly of the liver consbts in its subdivision into multiple lobes, .\bsence or duplication of the gall bladder and of the ducts may occur. In some animals (horse, elephant) the gall bladder is normally absent.

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 {Fig. 177). At7.5mm.it is separated from the duodenum by a slight constriction and extends into the dorsal mesentery (Fig. 185 -4). The ventral pancreas develops in- the inferior angle between the hepatic diverticulum and the gut (Lewis) 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. 187.— Two stages showing the development of the human pancreas: A, Embiyo of 8 mm.; fl, embryo of about 20 mm. (after Kollman).


Of the two pancreatic anlages, the dorsal grows more rapidly and in 10 mm. embrjos forms an elongated structure with a central duct and irregular nodules upon its surface (Fig. 185 5). The ventral pancreas is smaller and develops a short slender duct which 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. 185 and 187).


In embryos of 20 mm. the tubules of the dorsal and ventral pancreatic anlages interiock (Fig. 187 B). Eventually, anastomosis takes place between the two ducts, and the duct of the ventral pancreas plus the distal segment of the dorsal duct persist as the functional pancreatic duct of the adult. The proximal portion of the dorsal pancreatic duct forms the accessory duct, which remains pervious, but becomes a tributary of the adult pancreatic duct. The ventral pancreas forms part of the head and uncinate process of the adult gland. The dorsal pancreas takes part in forming the head and uncinate process and comprises the whole of the body and tail.


The ventral pancreas may arise directly from the intestinal wall (Bremer; Keibel and Elze), and paired ventral anlages also occur (Debeyre; Helly; Kollmann). 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 becomes of chief importance; in the pig and ox the dorsal duct.


In 10 mm. embryos the j)ortal vein separates the two pancreatic anlages and later they partially surround the vein. The alveoli of the gland are developed from the ducts as darkly staining cellular buds in fetuses of 40 to 55 mm. (C R). The islands characteristic of the pancreas also bud from the ducts (and alveoli, Mironescu, 1910) and appear first in the tail at 55 mm. (C R).


Owing to the shift in the position of the stomach and duodenum during development, the pancreas takes up a transverse position, its tail extending to the left. To its ventral surface is attached the transverse mesocolon.


The Primitive Coelom and Mesenteries

In the Peters embryo the primary mesoderm has already split to form the extra-embryonic coelom (Fig. 74 C). When the intra-embryonic mesoderm differentiates, numerous clefts appear on either side between the somatic and splanchnic layers of mesoderm. These clefts coalesce in the cardiac region and form two elongated pericardial cavities lateral to the paired tubular heart. Similarly, right and left pletiro- peritoneal cavities are formed between the mesoderm layers caudal to the heart. The paired pericardial cavities extend toward the midline cranial to the heart and commimicate with each other (Fig. 188). Laterally they are not continuous with the extraembryonic coelom, for the head of the embryo separates early from the underlying blastoderm. The pericardial cavities also are prolonged caudally until they open into the pleuro-peritoneal cavities. These in turn communicate laterally with the extra-embrj'onic ccelom. In an embryo of 2 mm. the coelom thus consists of a U-shaped pericardial cavity, the right and left limbs of which are continued caudatly into the paired pletiro-peritoneal cavities ; these extend out into the extraembryonic coelom.


When the head fold and fore-gut of the embryo are developed, the layers of splanchnic mesoderm containing the heart tubes are folded together ventral to the fore-gut and form the ventral mesentery between the gut and the ventxal body wall (Fig. 190), Owing to the position of the yolk sac, the caudal extent of the ventral mesentery is hmited. At the level of each side, where the vitello-umbilical trunk (Fig. 88) courses to the heart, the splanchnic mesoderm and the somatic mesoderm are united {cf. Fig. 110). Thus is formed the septum transversum, which incompletely partitions the ccelom into a cranial and caudal portion (Fig, 189). Cranial to the septum, the heart is suspended in the ventral mesentery which forms the dorsal and ventral mesocardia (Fig. 190 A). Into the ventral mesentery caudal to the septum grows the liver. This portion of the ventral mesentery gives rise dorsally to the lesser -Extra-embryotik «^m omentum of the stomach, and, where it fails to separate from the septum transversum, it forms the ligaments of the liver. Ventrally it persists as the falciform ligament (Fig. 190 B).


Fig. 188,— Diagrammatic model ot the tore-gut and crvlom [n an early human cm-

Dorsal to the gut, the splanchnic mesoderm of each side is folded together in the median sagittal plane to constitute the dorsal mesentery which extends to the caudal end of the digestive canal (Figs. 189 and 190 C). This suspends the stomach and intestine from the dorsal body wall and is divided into the dorsal mesogaslrium or greater omentum oi the stomach, the mesoduodenum, the mesentery proper of the small intestine, the mesocolon, and the mesorectum.


The covering layers of the viscera, of the mesenteries, and of the body wall are continuous with each other and consist of a mesothclium overlying connective tissue. The parietal lining is derived from the somatic layer of mesoderm and the visceral c<jvering from the splanchnic layer.


The primitive ccelom hos in the horizontal plane, as in Fig. 188. Coincident with the caudal regression of the septum transversum, the pericardial cavity is bent ventrad and enlarged (Fig. 191). The ventral mcsocardium attaching the heart to the ventral body wall disappears and the right and left limbs of the U-shaped cavity become confluent ventral to the heart. The result is a single, large pericardial chamber, the long axis of which now lies in a dorso-ventral plane


PUuro-petitoneal canal nladerm of gul brj-o. viewed from above and behind (modified after Robinson).


Ptruardwl cmity. Venlrult 0} ktart

Fig. 189. — Diagram showing the priniitive mesenteries of an early human embryo in DKdian sa^ttal aection. The broken lines {A , S, Mid O indicate the level of sections ^ , B, and C in Fig. 190.

Fig. 190. — Diagrammatic transveise sections. A , Through the heart and pericardial cavitiei

human embiyo; B, through the (ore-gut and liver; C, through the intestine and peritoneal cavity.

nearly at right angles to the plane of the pleuro-peritoneal cavities, and connected with them dorsally by the right and left pleuro-peritoneal canals.

The diviaon of the primitive coelom into separate cavities is accomplished by the development of three membranes which join in a <-shaped fashion (Figs. 194 and 195) : (1) the septum transserstim, which separates incompletely the pericardial and pleural cavities from the peritoneal cavities; (2) the paired pleuropericardial membranes, which complete the division between pericardial and pleural cavities; (3) the paired pleuro- peritoneal membranes, which complete the partition between each pleural cavity, containing the lung, and the peritoneal cavity, which contains the abdominal viscera.

The Septum Transversum

The vitelline veins, on their way to the heart, course in the splanchnic mesoderm lateral to the fore-gut. In embryos of 2 to 3 mm. these large vessels bulge into the coelom until they meet and fuse with the somatic mesoderm (Figs. 88 and 110). Thus there is formed caudal to the heart a transverse partition filling the space between the sinus venosus of the heart, the gut, and the ventral body wall, and separating the pericardial and peritoneal cavities from each other ventral to the gut. This mesodermal partitioD was termed by His the septum transversum. In Fig. 191 it comprises both a cranial portion (designated "septum transversum"), which is the anlage of a large part of the diaphragm, and a caudal portion, the ventral mesentery, into which the liver grows.


—Reconstruct on cut at the left of the median sagitut plane of a 5 mm. human embryo, sfaoning the body cavities and septum transversum (KoUmaui).

At first the septum transversum does not extend dorsal to the gut, but leaves on dther side a pleuro-perUoneal canal through which the [>ericardial and pleuroperitoneal cavities communicate (Fig. 191). In embryos of 4 to 5 mm. the lungs develop in the median walls of these canals and bulge laterally into them. Thus the canals become the pleural cavities and will be ilge laterally into them. termed hereafter.


On account of the more rapid growth of the embryo, there is an apparent constriction at the yolk stalk, and, with the development of the umbilical cord, the peritoneal cavity is finally separated from the extra-embryonic coelom. Dorsally, the pleural and peritoneal cavities arc permanently partitioned lengthwise by the dorsal mesentery.


The seplutn iransversum in 2 mm. embryos occupies a transverse position in the middle cervical region (Fig. 192, 2). According to Mall, it migrates caudally, its ventral position at first moving more rapidly so that its position becomes oblique. In S mm. embryos (Fig. 192, 5) it is opposite the fifth cervical segment, at which level it receives the phrenic nerve. In stages later than 7 mm. the septum migrates caudad. until at 24 mm. it is opposite the first lumbar segment. During this second period of migration its dorsal attachment travels faster than its ventral portion. Therefore, it rotates to a position nearly at right angels to its plane in 7 mm. embryos and its original dorsal surface becomes its ventral surface.


The Pleuro-pericardial and Pleuro-peritooeal Membranes

The common cardinal veins (ducts of Cuvicr) on their way to the heart curve around the pleural cavities laterally in the somatic body wall (Fifjs. 191 and \9X). In embryos of 7 mm. each vein, with the overlying mesoderm, forms a ridge which projects from the IxHiy wall mesialh' into the pleural canals. This ridge, the pulmonary ridge of Mall, is the anlage of both the plcuro-pcricardial and pleuro-peritoneal membranes. Later it broadens and thickens cranio-caudally (Fig. 193), forming a triangular structure whose apex is continuous with the septum transversum {Fig. 194). Its cranial side forms the pleuro-pericardial membrane and in 9 to 10 mm. embryos reduces the opening between the pleural and pericardial cavities to a mere slit. Its caudal side becomes the pleuro- peritoneal men^ane, which eventually separates dorsally the pleural from the peritoneal oavity (Fig. 195).


Fig. 192. — Diagram showing the change in position of the septum transversum in stages from 2 to 24 mm. (modiAed after Mall). The septum is indicated at different &tafei by the numerals to the left, the numbers corresponding to the length of the embr^'o at each stage. The letters and numbers at the right represent the segments of the occipital, cervical, thoracic and lumbar regions.


Fig. 193. — Recoostruciion of a 7 mm. human embof sbowing from the left side tbe pleuro-pericaidial membrane, the pleuro-peritoneal membrane and the septum transversum (after Mall). X 20. The phrenic ner\e courses in the pleuro-pericardial membrane. Arrow passes from pericudul to peritoneal canty through the pleuro-pericardial canal.

The two sets of membranes at first lie nearly in the sagittal plane and a portion of the lung is caudal to the pleuro-pcritoneal membranes (Fig. f93). Between the stages of 7 and 1 ! mm, the dorsal attachment of the septum transversum is carried caudally more rapidly than its ventral portion and its primary ventral surface becomes its dorsal side (Figs, !92 to 194). The plcuro-peritoneal membrane is carried caudad with the septum transversum until the lung lies in the angle between the pleuro-pcritoneal and pleuro-[X'ricardial membranes and is included within the ^herica] triangle which has been described above (Fig. 194). During this rotation the dorsal end of the pleuro-pericardial membrane lags behind and so takes up a position in a coronal plane nearly at right angles to the septum transversum (Figs. 194 and 195). In 11 mm. embryos the pleuro -pericardia] membranes have fused completely on each side with the median walls of the pleural canals and thus separate the pericardium from the paired pleural cavities.


Fig. 194.— Reconstruction of embryo to show the ss MaU). X 14.

By way of the pleuro-pcricardial membranes the phrenic nerves ruurse to the septum transversum fFig. 194).

The pleuro- peritonea! membranes arc continuous dorsally and caudally with the mesonephric folds; vcntniHy and caudally they fuse later with the dorsal pillars of the diaphragm or coronary appendage!: of the liver (Lewis) iFip. 196). Between the free margins of the membranes anct the mesentery a temporary i>p<.'ntng is left on each side, through which the pleural and peritoneal cavities communicate (Figs. 194 and 200).


Fig. 135 —Transverse section through a 10 mm. human embryo showing the pleuro-iwricardial n brane separating the pericardium fiom the pleural cavities. X 33.

Fig,, 196. — Transverse section through a 10 mm. human embryo showing the pleiuo-peritoneal n branes. X 16.

Owing to the caudal migration of the septum transversum and the growth of the lungs and liver, the pleuro-peritoneal membrane, at first lying in a nearly sagittal plane (Fig. 193), is shifted to a horizontal poation (Fig. 194), and gradually its free margin unites with the dorsal pillars of the diaphragm and with the dorsal mesentery. The opening between the pleural and peritoneal cavities is thus narrowed and finally closed in embryos of 19 to 20 mm.


Fig. 197.— Diagrams showing th« development of ihe lungs and ihe formation of the pericardial mem++++braw {modified after Robinson}. A, Coronal section; B, transverse section.


The Diaphragm and Pericardial Membrane

The lungs grow and expand not only cranially and caudally but also laterally and ventrally (Fig. 197), Room is made for them by the obliteration of the very loose, spongy mesenchj-me of the adjacent body wall (Fig. 196). As the lungs burrow lateraLy and ventrally into the body wall around the i>cricardiiii cavity, the picuro-pcricardial membranes enlarge at the expense of this tissue and more and more the heart conius to lie in a mesial position between the lungs (Fig. 197 B). The pleural cavities thus increase rapidly in size.


Fig. 198.— Diagram showing the origin of the diaphragm (after Broman): /, Septum transv-er sum; a, j, derivativ-es of mesenlerj-; j. 4, derivatives ot pleuro-pentoneal membrane; j, j, jarts deri\ed frtun the body wills; .1, aorta; fV, esophagus; KC, inferior x-ena cava.


At the same time the liver grows enormously, and on either side a portion of the body wall is taken up into the septum transversum and pleuro-peritoneal membranes. The diaphragm, according lo Broman, is thus derived from four sources (Fig. 198): (!) its ventral pericardial portion from the septum transversum; its lateral portions from (2) the pleuro-peritoneal membranes plus (3) derivatives from the body wall; lastly, a median dorsal portion is formed from (4) the dorsal mesentery. In addition to these, the striated muscle of the diaphragm, according to Bardeen, takes its origin from a pair of premuscle masses which in 9 mm. embryos lie one on each side opposite the fifth cervical segment.


Fig. 199. — DLagrammatic model of an embryo of 7 to 9 mm. showing the position of the le toneal sac. The embryo is represented as sectioned transversely, caudal to the liver, so that one liwlcs at the caudal surface of the section and of the liver, and cranially into the body cavities.

This is the level at which the phrenic nerve enters the septum transveisum. The exact origin of these muscle masses is in doubt, but they probably represent portions of the cervical myotomes of this region. The muscle masses migrate caudally with the septum transversum and develop chiefly in the dorsal portion of the diaphragm (Bardeen, Johns Hopkins Hospital Report, vol. 9, 1900).

Keith (Jour. .Anat. and Physiol., vol. 39, 1905) derives the muscle of the diaphragm also from the rectus and transversalis muscles of Ihe abdominal wall.

The cavities of the mesodermic segments are regarded as portions of the ccelom. but in man (hey disappear early. The development of ihe vaginal sacs which grow out from the inguinal region of the peritoneal cavity into the scrotum will be described in Chapter VIII.


The Omeatal Bursa or Lesser Peritoneal Sac

According to Broman, the omental bursa is represented in 3 mm, embryos by a peritoneal pocket which extends cranially into the dorsal mesentery to the right of the esophagus. A similar pocket present on the left side has disappeared in 4 mm. embryos. Lateral to the opaiing of the primitive lesser peritoneal sac, a lip-like fold of the mesentery


Fig. 200. — S. diagrammatic venlral view of iht middle tiiinl of a human embryo 12 lo 15 mm. lonR. The figure sliows the caudal surface of a section through the stomach and spleen, a ventral view of the stomach, the liver having been cut away to leave the sectioned edges of the lesser omentum and plica vena: ayx, and the caudal E>urface of the septum transvcrsum and pleuro-|K-ritnneal membrane. Upon the surface of the septum is indicated diaKrommalicalh' the attachment of the liver. (Based on figures of Mall and F. T. Lewis and model by H. C. Tracy.)


is continued caudally along the dorsal body wall into the mesonephric fold as the fiica vena cava, in which the inferior vena cava later develops (Fig. 109). The liver, it will be remcmberfd, grows out into the ventral mesentery from the foregut, and, expanding laterally and vcntrally, takes the form of a crescent. Its right lobe comes into relation with the plica vena- cava;, and, growing rapidly caudad, forms with the plica a partition between the lesser sac and the peritoneal cavity. Thus the cavity of the lesser peritoneal sac is extended caudally from a point opposite the bifurcation of the lungs to the level of the pyloric end of the stomach. In 5 to 10 mm. embryos it is crescent-shaped in cross-section (cf. Fig. Ill) and is bounded mesially by the gieater omentum (dorsal mesentery) and the right wall of the stomach, laterally by the liver and plica vena cavas, and ventrally by the lesser omentum (ventral mesentery). It communicates to the right with the peritoneal cavity through an opening between the liver ventrally and the phca venae cavx dorsally (Fig. 201). This opening is the epiploic foramen (of Winslow). When the dorsal wall of the stomach rotates to the left the greater omentum is carried with it to the left of its dorsal attachment. Its tissue grows actively to the left and caudally and gives the omentum an appearance of being folded on itself between the stomach and the dorsal body wall (Fig. 200). The cavity of the lesser peritoneal sac is carried out between the folds of the greater omentum as the inferior recess of the omental bursa.



Fig. 201. — .\n obliquely transverse section through a 10 mm. human embiyo at tbe level of the epipkuc foramen (of Winslow). X 33.


From the cranial end of the sac there is constricted off a small closed cavity which is frequently persistent in the adult. This is the bursa iiifracardiaca and may be regarded as a third pleural cavity. It lies at the right of the esophagus in the mediastinum.


When the stomach changes its position and form so that its mid-ventral line becomes the lesser curvature and lies to the right, the position of the lesser omentum is also shifted. From its primitive location in a median sagittal plane with its free edge directed caudally, it is rotated through 90° until it lies in a coronal plane with its free margin facing to the right (Fig. 194). The epiploic foramen now forms a slit-Uke opening leading from the peritoneal cavity into the vestibule of the omental bursa. The foramen is bounded ventrally by the edge of the lesser omentum, dorsally by the inferior vena cava, cranially by the caudate process of the liver, and caudally by the wall of the duodenum.

During fetal life the greater omentum grows rapidly to the left and caudad in the form of a sac, Battened dorso-vcntrally. It overUcs the intestines ventrally and contains the inferior recess of the omental bursa (Fig. 202). The dorsal wall of the sac during the fourth month usually fuses with the transverse colon where it overlies the latter (Fig. 202 B). Caudal to this attachment the walls of the greater omi-ntum may be fused and its cavity is then obliterated. The inferior recess of the omental bursa thus may be limitetl in the adult chiefly to a SI>ace between the stomach and the dorsal fokl of the greater omentum, which latter is largely fused to the peritoneum of the dorsal body wall. The spleen develops in the cranial portion of the greater omentum and that portion of the omentum which uxtends between the stomach and spleen is known as the gaslrolienic ligamenl (Fig. 200). The dorsal wall of the omentum Ix-tween the si>leen and kidney is the liciw-remil ligament.


Fig. 202.— Diagrams showing ihe development of the mesenteries (Hert«iR). A illustrates the beginniiig of the great omentum and its independence of ihc transverse mesocolon; in It the two come into contact; in C they have fused; :\, ftomach; B, transverse colon; C, small intestine; 1), duodenum; E, pancreas; F, greater omentum; G, greater sac; H, omental bursa.


Further Differentiation of the Mesenteries

Ligaments of the Liver. — We have seen (p. 179) that the cranial portion of the ventral mesentery forms the mesocardium of the heart. In the ventral mesentery caudal to the septum transversum the liver develops. From the first, it is enveloped in folds of the splanchnic mesoderm which give rise to its capsule and ligaments as the liver increases in size (Fig. 190 B). Wherever the liver is unattached, the mesodermal layers of the ventral mesentery form its capsule (of Glisson), a fibrous layer covered by mesothelium, continuous with that of the peritoneum (Fig. 190 B), Along its mid-dorsal and mid-ventral line the liver remains attached to the ventral mesentery. The dorsal attachment between the liver, stomach, and duodenum is the lesser omentum. This in the adult is differentiated mto the duodeno-hepatic and gastro-hepatic ligaments. The attachment of the liver to the ventral body wall extends caudally to the umbilicus and forms the falciform ligament.

In its early development the liver abuts upon the septum transversum, and in 4 to 5 mm. embryos is attached to it along its cephalic and ventral surfaces. Soon dorsal prolongations of the lateral liver lobes, the coronary appendages, come into relation with the septum dorsally and laterally. The attachment of the liver to the septum transversum now has the form of a crescent, the dorsal horns of which are the coronary appendages (Fig. 200). This attachment becomes the coronary ligament of the adult liver. The dorso-ventral extent of the coronary ligament is reduced during development and its lateral extensions upon the diaphragm give rise to the triangular ligaments of each side.

The right lobe of the liver, as we have seen, comes into relation along its dorsal surface with the plica vence cavce in 9 nmi. embryos (Figs. 199 and 200). This attachment extends the coronary ligament caudally on the right side and makes possible the connection between the veins of the liver and mesonephros which contributes to the formation of the inferior vena cava. The portion of the liver included between the plica venae cavae and the lesser omentum is the caudate lobe (of Spigelius).

In a fetus of five months the triangular ligaments mark the position of the former lateral coronary appendages. The umbilical vein courses in a deep groove along the ventral surface of the liver, and, with the portal vein and gall bladder, bounds the quadrate lobe.


Changes in the Dorsal Mesentery, — That part of the digestive canal which lies within the peritoneal cavity is suspended by the dorsal mesentery, which at first forms a simple attachment extending in the median sagittal plane between body wall and primitive gut. That portion of it connected with the stomach forms the greater omentum, the differentiation of which has been described (p. 191).


The mesentery of the intestine is carried out into the umbilical cord between the limbs of the intestinal loop. When the intestine elongates and its loop rotates, the cxcal end of the large intestine comes to lie cranially and to the left, the small intestine caudally and to the right, the future duodenum and colon crossing in dose proximity to each other (Fig. 179). On the return of the intestinal loop into the abdomen from the umbilical cord, the caical end of the colon lies to the right and the transverse colon crosses the duodenum ventrally and cranially (Fig. 203 A). The primary loops of the small intestine lie caudal and to the left of the ascending colon (Fig. 203 B). There has thus been a torsion of the mesentery about the origin of the superior mesenteric artery as an axis. From this focal point the mesentery of the small intestine and colon spreads out fan-like. The mesoduodenum is pressed against the dorsal body wall, fuses with its peritoneal layer, and is obliterated (Fig. 202). Since the transverse colon lies ventral to the duodenum it cannot come into apposition with the body wall ; where its mesentery crosses the duodenum it fuses at its base with the surface of the latter and of the pancreas. Its fixed position now being transverse instead of sagittal, the mesentery is known as the transverse mesocolon. The mesentery of the ascending colon is flattened against the dorsal body wall on the right and fuses with the peritoneum (Fig. 203). Similarly, the descending mesocolon is applied to the body wall of the left side. There are thus left free: (1) the transverse mesocolon ; (2) the mesentery proper of the jejunum and ileum, with numerous folds corresponding to the loops of the intestine; (3) the iliac mesocolon; (4) the mesorectum, which retains its primitive relations.


Fig. 203— Diagrams showing the development of the mesenteries in ventral liew (modified after Toumeui), * Cut edge of greater omentum; a. area of ascending mesocolon fused to dorsal body wall; b, area of descending mesocolon fused to dorsal body wall. Arrow in omental bursa.


Anomalies of the diaphragm and mesenteries are not uncommon. The persistence of a dorsal opening in the diaphragm, more commonly on the left side, finds its explanation in the imperfect development of the pleuro-peritoneal membrane. Such a defect may lead to diaphragmatic hernia y the abdominal viscera projecting to a greater or less extent into the pleural cavity.

The mesenteries also may show malformations, due to the persistence of the simpler embryonic conditions, usually correlated with the defective development of the intestinal canal. In about 30 per cent, of cases the ascending and descending mesocolon is more or less free, having failed to fuse with the dorsal p>eritoneum. The primary sheets of the greater omentum may also fail to unite, so that the inferior recess extends to the caudal end of the greater omentum.

A striking anomaly is siius viscerum inversus ^ in which the various visceral organs are transposed right for left and left for right as in a mirror image. An independent transposition of the thoracic or abdominal viscera alone may occur. The larger left great venous trunks are thought to be chiefly responsible for the usual positions of the viscera.



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Prentiss CW. and Arey LB. A laboratory manual and text-book of embryology. (1918) W.B. Saunders Company, Philadelphia and London.

Human Embryology 1917: The Germ Cells | Germ Layers | Chick Embryos | Fetal Membranes | Pig Embryos | Dissecting Pig Embryos | Entodermal Canal | Urogenital System | Vascular System | Histogenesis | Skeleton and Muscles | Central Nervous System | Peripheral Nervous System | Embryology History
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Pages where the terms "Historic Textbook" and "Historic Embryology" appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms and interpretations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)
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Cite this page: Hill, M.A. 2017 Embryology Book - A Laboratory Manual and Text-book of Embryology 7. Retrieved December 18, 2017, from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_A_Laboratory_Manual_and_Text-book_of_Embryology_7

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© Dr Mark Hill 2017, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G