Book - A Text-book of Embryology 11

From Embryology

Chapter XI. The Development of the Digestive System

Heisler JC. A text-book of embryology for students of medicine. 3rd Edn. (1907) W.B. Saunders Co. London.

Heisler 1907: 1 Male and Female Sexual Elements - Fertilization | 2 Ovum Segmentation - Blastodermic Vesicle | 3 Germ-layers - Primitive Streak | 4 Embryo Differentiation - Neural Canal - Somites | 5 Body-wall - Intestinal Canal - Fetal Membranes | 6 Decidual Ovum Embedding - Placenta - Umbilical Cord | 7 External Body Form | 8 Connective Tissues - Lymphatic System | 9 Face and Mouth | 10 Vascular System | 11 Digestive System | 12 Respiratory System | 13 Genito-urinary System | 14 Skin and Appendages | 15 Nervous System | 16 Sense Organs | 17 Muscular System | 18 Skeleton and Limbs

Early Draft Version of a 1907 Historic Textbook. Currently no figures included and please note this includes many typographical errors generated by the automated text conversion procedure. This notice removed when editing process completed.

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The adult digestive system consists of the mouth with its accessory organs, the teeth, the tongne, and the salivary glands ; of the pharsmx, the esophagus, the stomach, and the small and the large intestine, including also the important glandular organs, the liver and the pancreas. Notwithstanding the apparent complexity of its structure, the alimentary tract may be regarded as a tube, certain regions of which have become specialized in order to adapt them to the performance of their respective functions, tiie salivary glands, the liver, and the pancreas being highly differentiated evaginations of its walls. While in man and in the higher vertebrates the tube is thrown into coils by reason of its excessive length, in the lower-type animals it is much more simple in its arrangement. For example, in certain fishes and in some amphibians the alimentary tract has the form of a slightly flexuous tube, the deviations from the simple straight canal being few and insignificant, and the stomach being represented by a local dilatation of the tube.

The simple condition obtaining in the representatives of the animal kingdom referred to above suggests the likewise simple fundamental plan of the human embryonic gut-tract. There is, in fact, a period in development when the gut-tract of the human embryo has the form of a simple straight tube. The processes incident to the formation of this tube mark the earliest stages of the development of the alimentary system, the tube itself acquiring definite form simultaneously with the production of the body of the embryo.

The first indication of the alimentary canal appears at a very early period of development, being inaugurated in fact by those important alterations that serve to differentiate the blastodermic vesicle into the body of the embryo and the embryonic appendages. It will be remembered that, after the splitting of the parietal plate of the mesoderm into its two lamellae, and the union of the outer of the layers with the ectoderm and of the inner with the entoderm to form respectively the somatopleure and the splanchnopleure, these two double-layered sheets undergo folding in different directions. Before the folding occurs, the germ is a hollow sphere whose cavity is the archenteron and whose walls are the somatopleure and the si)lanchnopleure.^ While the somatopleure in a zone corresponding with the margin of the embryonic area becomes depressed and is carried under that area to form the lateral and ventral body- wall of the embryo (Plate II., Figs. 2, 3, and 4), and also more distally folds up over the arcji to produce the amnion and the false amnion, the splanchnopleure, likewise in a line corresponding with the ivriphery of the embryonic area, is depressed and carried in^-^r^i fr^>m all sides toward the position of the future wimWlk**!?^ This folding in of the splanchnopleure effects t>K^ <^iviMi>n of the archenteron into two parts, a smaller v^x-itv t^Uiuj; within the body of the embryo, which latter iv "SNJnvi^nf «it x\\o same time, and a larger extra-embryonic ^\s*^Yv^itiiw'^t. whioh is the yolk-sac or umbilical vesicle. The cavity is the gut-tract. The constricted ,ss4*,**w*s>ji^v»\^^ Uiw^MMi the two is the vitelline duct. While »K s;',v''^^ v^^^ is still a nither wide aperture, the anterior k^isi 'Ns<*v.-«NSi jv^rts of its intestinal orifice are designated vv^NNv*\v^\ vKs^ i^Wrtor and the posterior intestinal portals.

Vv'*K^ ^NWiMvv^vKsm* oK>st*s in around the vitelline duct, it v^^a. iW NV.4H wt' iho abdomen, the opening left, which is • *.o V <nnI '^^ » ^K^ vi\K U Iviutf the umbilical aperture.

timt the lining of the gut-tract is 

v.. i.s.AxI N> ihs* »uhonmv»t giTm-layer, the entoderm, and

  • Iw * i ^\ - V jmlu l»ul clcnunits art* consequently of entodermic

M^ »4 Mk loKliu^ ill of the splanchnopleure begins at

I'v'n iii^ . uvl vU ihv^ <%HHMul week, and is so far advanced nm« nml the splanchnopleure are not (ut«k bul lU«» |mK*v88es go on at the same time.

no. W.—RHUtisl ructions nf humaq emhrro of ibuut MvuuWen dayi (Ula) ; dd, optic and u(, otic veakloi: ne, nr', nolochord : Mg. hcod-gut; g. mid-gut ; A0. hludKUt; It, Titclllne aBi:; 1. liver: V, lu, ijrlmUlve vcnlrlde and truncu* >HeHcwu>; ni, da. TCDtral and diiraal aortie; on. Hnrlic aruhes; jv, prluitlva jugular vein; cv, cardinal vein; dc. duct of CuvIef; iii'. h>i, umbilical vein and artery: ol, allanUli:

before the end of the third week that the arehenteron nitcly divided int^i the gut-tract and the yolk blastodermic vesicle into the body of the embryo and the embrj'onic apiwndages. It ^v■i^ be remembere<l that, after the splitling of the parietal plate of the mes<xiemi into its two lamellje, and the union of the outer of the layers with the ectoderm and of the inner with the entoderm to form respectively the somatoplenre and the spUnchnopleure, these two double-layered sheets undergo folding in different directions. Before the folding occurs, the germ is a hollow sphere whose cavity is the archeutcrun and whose walls are the somatopleure and the splanchnopleure.' AVhilc the somatoplenre in a zone corresponding with the margin of the embryonic area becomes depressed and is carrie<l under that area to form the lateral and ventral body-wall of the embryo (Plate II., Figs. 2, 3, and 4), and also more distally folds up over the area to produi-e the amnion and the false amnion, the aplanchnoplenrc, likewise in a line corresponding with the periphery of the embryonic area, is (iepressed and I'arried inward from all sides toward the position of the futnre umbilicus. This folding in of the splauchnopleure effects the division of the archenteron into two jjarte, a smaller cavity falling within the body of the embryo, which latter is forming at the same time, and a larger extra-cnibryonic compartment, which is the yolk-sac or nmliilical vesicle. The intra-embryonic cavity is the gut- tract. The constricted commnnioatiou between the two is the vitelline duct. While the vitelline duct is still a rather wide aperture, the anterior and posterior parts of its intestinal orifice are designated respectively the anterior and the posterior intestinal portals.

As the somatopleure closes in artjund the vitelline duet, it forms the wall nf the abdomen, the opening left, which is traversed by the dnct, Iwirig the mnhllical aperture.

It is evident therefore that the lining of the gut-tract is constituted by the innermost genn-layer, the entoderm, and that all its epithelial elements are consequently of entodermic origin. The folding in of the splanehnopleurc begins at about the end of the second week, and is so far advanced

' Strictly Hi«nkin(t. llie somntopleurc and the splanchnopieiire ire not formed bf/ore Ibe foldmg ocrurti, but the proceues ){o on M Ihe »aaic time.

In its earliest definite form, then, the ^t-tract is a tube extundiug from one end of the embryonic body to the otiier, whii;h o[)ens widely at the middle uf its ventral aspect into the vitelline duct, but which is closed at both ends. It is nsuul U} speak of the primitive gnt-tract as consisting morphihlogically of three jmrts, the head-gut, which is the region on the headward side of the orilice of the vitelline duct; the hind-pit, wliich is the part near the tail-end of the embryo; and the mid-gut or interveniug thinl portion (Fig.' 90).

The closed head-end of the gtit-tuhe corresponds with the floor of the primitive moutb-cavity, the two spaces being separated by a thin veil of tissue, which consists of the enfotlern) and the ectoderm and is called the pharysKeal ffiembrane (Fig. 91). A considerable pniportioii of the so called head-gilt constitutes the primitive pharynx. This region of the tube has a relatively large enliher. and presents on its lateral and ventral walls the serios of recesses or evaginations known as the throat-pockets or pttarTngeal pouches (Fig. 71).

Pm. tl —Median ncUon Hiroueh Ihe haad nf an embryo nbbit S mm. loDC (kfUtr Mlbklkovical ; rh, membrane bclween alomiKliEUm and rnrc-eut. pharfngeal membrane (Baaheiihiiiit) ; hp, place bam whicb the b^papbysis in iJcvelotidl : A. heart: M, lumen of fore-gut; th. ehonla; v, ventricle or the eerobruui: r", tbird Tenlriele, tliatoflhebclween-braiii (ihalamencephalou) : r<. n>nrlli vcntritle, Ihat of the hlnd-braln and afler-braln (epencephalon aniJ ineteowphalon, or medulla (iblongala): (*, cential canal or the iplnat cord.

While llic inner, entodermic layer of tlic gut-tiilie l)ecomes the inteatinal mucosa, tli<? outer, mesodermic stratum |inulu( the muscular and the connecttTe-tiBBne p:u-t^ of tlii! Imwelwall, the must superficial layer uf tliL- latt*;r with ila mesothelial or ciidotholial ooIIh furmiiig the visceral l&yer of the peritoneum. Since the lueHodermic layer of the gplnnclmupleurc of each side ia coiitiDuoua with the corresponding mcsodemiic layer of the tiomatoplrure on either side of the embryonic axis, the primitive intestinal canal has a broad area of attachment with the dorsal wall of the body-cavity (Fig. 92). Tiio ventral wall is Iikewirie connected with the

Fig. Hi— TtanBvcras seollon of

ventral body-wall throughout the anterior or upper part of its exteut by the continuity of the splnnchnopleuric mesoderm of each side with the somatopleuriu rucsoderm of the fjame side. As development advances, the body-cavity increases in caliber more rapidly than does the intestinal tube, BO that the interval between the two is augmented, in consequence of which the masses of connective tissue uniting the dorsal and the ventral surfaces of the gut with the

Hpouiling woUs of the bod>-caMh become drawn out so as to oontttitute m each case a median vertical foM consibting of two closely approximated lasers of serous membrane with a little toiinective tissue between them These folds are the dorwl and the vsntral msBenteries (Fig 93) While the

Fig. Ml.— Uluinunmallr crofis-sectlons of the body of the embrro In the region of lliu linart nl IbvcI of fliture diaphrtgni : a. eaophagenl Be^nnent of KUt-lnct ; b, (liirHl inuHntury: r, mt'Bocardliiin poaterlua ; d. meioordluiu BnleriuB: c. be^nnliiii iif H-iiliiiii truiiavvrtnni. conlalnlnu vlt«11tne and allamotc veins ; /, aeptum traiiavnnuiii ; a- tlioraclv proliiagation n( abdominal cavity ; nc, neural canal,

(loroul ni(!ri<-nt<^ry extends throughout the entire length of the (Hiiml, till- vt'tirral fold in present only at its anterior or upper IHirt, iiiirn-HjHinding in the extent of its attachment to the dijii'^livc tiilie to that jwrtion representing the future stomach and iip]KT jHirl of the duodenum (Fig. 94). The ventral niirwnU-rv iil tirNi in ]ir('sent throughout the entire extent of the caiinl, Init very early undergoes obliteration except in the Hituiiliiiii iibovi' imU'd. ( '(infeniiiig the reason and the method of ilH diHiippi'iii-iini'i' uotliing is definitely known.

'I'hi- intcMliiiid lulu-, iit a romiKiratively early stage, pre(K'ntH on iN ventral Mirliice near the posterior or cuudat end a smatl evaginiitioii that eidni^s to form the aU&ntolB (see p. 89). While a jKirt of the intra-cnibryonic portion of the allaiitriis diliilon luid develo[>K into the bladder, the part between this latter anil the intestine is known as the urogenital alnuB. Tli(' jtiirt <)f the jrut-tiibe posterior to, caiidad of, the origin of the allantois, is a blind ]ioneli known as the cloa,ca. The latter is, tlicrefori-, the (Mnnnion termination of the urinan,and the intestinal tnicis.

To repeat, we have now, in the thini week of development, the alimentary canal repn'scntwi by a single straight tube

(compure Fig. 90 J, idoried at each eiiJ, bin willi moiitli-cavity an<l anus bolh Judicated, the tube lying within a larger tnhe, the body-cavity, with the walla of which latter it is connected by the dorsal and ventral mesenteries. Along the dorsal wall

Fig. !M,~-R«oi>iutrucUuii iif baiiinn embryo of ■ bout leventeon days (sRer Hli]: 00. optic and nf.otic valcles; ne, uotaehori: hdg, head-gut: p, mid-gut: Aj7. hiadgot: vt, vKollIiio hut; <, liver; e, prInilllTe Tenlrtcle: va, da. renlral and donal «orl«! Jir, primitive Jugblar vein; n. canlinal vein; 'IC. duot of Cuvlcr; iin, ho. umbilical vein aud artery: a', allanloii: Hi. umbillca) cord: dn. dureal mesGUlery; tm, ventral miiaentvry (modified from HIb).

of the body-cavity, dorsad to the parietal peritoneum, pass the two primitive aortic, and later, the single aorta which results from the fueinn of these two. Between the two folds of the dorsal mesentery pass the blood- vessels that nourish the walls of the gut. Within the ventral mesentery are the vitelline veins, which bring the blood from the yolk-sac and VAnwi^y it to the primitive heart. On the ventral wall of the gilt in the wide aperture of the vitelline duct. Farther (JiUHlud, uIho on the ventral surface of the bowel, is the orifice of the iillantoiH. These conditions may be better understood by reference to Figs. 90 and 94. Before tracing the further diivrlopiiicnt of the abdominal part of the alimentary system. It will be pro[)er to note certain very important processes |Htrtiiining to its anterior or head-extremity, and also to conNldor the formation of the anus.

The Mouth

The development of the mouth, the tongue, the teeth, and ilio lUiUvAnr glands has been fully described on pages 134llil. Ill thin connection, therefore, it will be necessary to mil iit((*iiii()ii to only a few of the salient features of their itvoliifloii,

Tim oral oaylty is produced by a folding in of the surfaceiMftodnnn, the fortsa thus formed becoming deeper until it liMMifN llin hcad-itiid of the gut-tract. From the walls of this limMii (Im« NAlivAry glands are developed as evaginations, in the liMiiiiMM' iilnuidy <h»Hcribed, while the teeth are specialized gi'owfliM of itH iM'toderinal lining and of the underlying mesoiJMriii (vlih' p. I.*J7). The first intimation of this infolding U ii|ipiir(Mif at (ho twelfth <lay in the form of a localized ihlitkiMiliiK <» the HiirfjMuwells on the ventral surface of the ImmIv oI* \\\\\ oiiibrvo nrar the head-end. The thickened area in the orftl plato, whieli Hp<»edily becomes depressed, producing tint oral pit or fossa. By the third week, the oral fossa or Nteiuodaum Ih a well-marked pit of pentagonal outline, its buiiiidiirMiM brin^ the niiHolVontal process above, the maxillary proneHHort hitenilly, and the mandibular arches below. The origiiiul oral plate, having n»<M»d(Hl farther and farther from the Hiiriiieo and ioriiiiiig the posterior limit of the mouthmvity, iinw H««piiniteH that cavity from the pharyngeal region of the gut-tube aiui eoines into contact with the anterior wall ot* tlhi hitter. It \h (Milh*d the phanrngeal membrane (Fig. 91). ItH dimippeaniiKu* tM'ciiiN at Home time during the fourth week.

by which event the gut-tube is brought into communication with the mouth.

The exact position of the pharyngeal membrane is not easily definable. It is certain, however, that it falls farther back than the posterior limit of the adult oral cavity, since the primitive mouth includes the anterior part of the adult pharynx. For example, the diverticulum that gives rise to the anterior lobe of the pituitary body belongs to the primitive mouth, yet its vestige, the pharyngeal bursa ^ or Rathkd's [K)cket, is found in the pharynx of the adult. The primitive oral cavity, by the growth of the palate, becomes divided into the adult mouth and the nasal cavities. The hard palate is completed in the ninth week and the soft palate in the eleventh week.

The Pharynx

The pharynx is represented in the embryo by the expanded cephalic end of the primitive gut-tract. It is of greater relative length in the earlier stages of development than later, including as it does, almost half the length of the gut-tube in the fourth and fifth weeks. The primitive pharyngeal cavity is widest at its anterior or cephalic extremity and narrowest at the opposite end, tapering here into the esophagus. Until the breaking down of the pharj^igeal membrane, which takes place in the fourth week, this structure marks the anterior limit of the pharynx and separates it from the oral cavity.

The pharsrngeal poncheB or tliroat-pockets have been referred to in connection with the visceral arches on page 111. They are out-pocketings or evaginations of the entodermal lining of the pharynx, there being four furrows on each lateral wall, and they pass from the ventral toward the dorsal wall of the cavity, each pouch lying between two adjacent visceral arches. The entoderm of the pouches comes into close relation with the ectoderm of the outer visceral furrows (Fig. 71). The mesodermic stratum being

^ It has been shown recently. (Killian) that the pharyngeal bursa is not identical with Rathk^'s pocket, but is an independently formed evagination.

Fig. Wi. Section Ihrougb *Dlage of tonsil of a human fetus (Tourneui) : 1, tonsillar pit, continuous with mouthcsvlly: 2. BeoondBry diverticula: 3. Bolld epltbelial buds ; 4, striped muscular flber.

an evagination of the lateral wall of the pharynx. In the third month the lateral pharyngeal wall jwuches out to form a little foasa (Fig. 95, 1) which is i^itiinted Iwtween the second and thir<l visceral arches, the fossa Ixiing lined witli .tratificd pqiiamous epithelium continnons with that of llie i>lmryngeal cavity. Little solid epithelial buds (Fig. 95) proceed from this diverticulum into the surrounding connective tissue, the buds subsequently becoming hollowed out. Wandering lenkocytes from the neighboring blood-vessels — or, according to some authorities, fnim the mesenchyme cells or from epithelial sources — infiltrate the connective tissue around the young crypts, and these cells becoming aggregated into condensed and isolated groups give rise to the iTmphoidfoUicleB peculiar to the tonsil. The separate and well-differentiated condition of the follicles is not attained until some months after birth. The place of origin of the tonsil lietween the second and third visceral arches explains the position of the adult organ between the anterior and posterior palatine arches, since the latt«r structures represent the deep extremities of the former,

The Anus

The early stages of the development of the anus are similar to those of the mouth. The so-called anal membrane is produced by the growing together of the ectoderm and the entoderm, the mesoderm being crowded aside. The site of the aual membrane, or anal plate, is in the median line of the dorsal surface of the embryonic body, at its posterior or caudal extremity. It makes its appearance in the third week. Since the tissue immediately in front — that is, head- ■ ward, of the anal plate projects and develops into the primi

IrHiiEt'inri'-'l '-liii'fly ifil'* t\u: iiriiinry lilmlilt-r. Imt it ^ivc:i r)w iil«i, liy it" |»rf>«imal <-xtnmit_v, to :i i-hort wide duct. lli« iiroKanlt^I nlnuii, wliidi !■> iiti avi-iiiic of 'ffiiiiiiiiiiitntiDn ttllli III'- 1«>«.-I. 'Dm- )rtir1 of tliff (;iil on tlit- caudal side ot' lli>- ii)H'rliir<' Iff rill' iiroff'-iiitiil kIiiiim i^ llio closca, whieli is

||ii. i-oirir I I'Tiiiiiiittiori, X\>i-TfU<ri; of the p>tiit<Mirinur\ nyiilriii iind of lln- iiili-xtiiiid 'iiriul.

'I'll.' ■iirliiir ilcjir<*«iofi n-fi-rri-il In atiuvft an llic; anal pit in often called the doacal depression during the time that the cloaca is present. In the lowest mammals, the monotremes, as also in the Amphibia, in reptiles, and in birds, the cloaca is a permanent structure. By the breaking down of the membrane between it and the cloacal depression, it acquires an outlet, through which the feces, the urine, and the genital products find egress. In all higher mammals, however, including man, the cloaca suffers division into an anterior or ventral passage-way, the urogenital sinus, and a posterior canal, the rectum and canal of the anus. This division is eff*ected by the growth of three ridges or folds, of which one grows from the point of union of the urogenital sinus and the gut, while the other two proceed, one from each lateral wall of the cloaca. The three folds coalesce to form a perfect septum. The division is complete at about the end of the second month (or, according to Minot, at the fourteenth week). The cloacal depression or anal pit shares in this division, so that at about the tenth week, it is separated into the anal pit proper, or the proctodeum, and the orifice of the urogenital sinus. The newly-formed septum continues to thicken, especially near the surface of the body, until it constitutes the pyramidal mass of tissue known as the perineal body, or perineum.

The anal pit deepens, the anal membrane being thereby approximated to the end of the bowel, and in the fourth month the anal membrane breaks down and disappears. Persistence of the anal membrane after birth constitutes the anomaly known as imperforate anus.

The Differentiation of the Alimentary Canal into Separate Regions

The fourth week marks the beginning of certain important changes in the simple straight alimentary tube. The reader is s^in reminded that this tube is connected with the dorsal body-w^all by the dorsal mesentery and with the ventral wall, for a part of its extent, by the ventral mesentery ; that the canal is, as yet, without communication with the exterior ; and also that the vitelline duct and the allantois are connected with its ventnil surface (Fig. 94). The umbilical vesicle having reached the limit of its development in the fourth week and having begun to shrink, the vitelline duct likewise begins to retrograde and very soon becomes an inconspicuous structure.

The dorsal wall of the tube at a point nearer the head-end begins to bulge toward the dorsal body-wall, forming a somewhat spindle-shaped enlargement (Figs. 97, 98). This di

Afidd/e lobe

of thyroid gland.

Thymus gland.

Lateral lobe

of thyroid gland.

Trachea. Lung.

Right lobe of liver.

Vitelline duct.

Pharyngeal pouches.



Left lobe of liver.

Small intestine. Large intestine.

Fio. 97.— Scheme of the alimentary canal and its acceiisory orj^nB (Bonnet).

latation is the beginning of the future stomach. The part of the mnal on the cephalic side of the stomach lags behind somewhat in growth, corresponding in this respect with the relatively smaller size of the adult esophagus. The esophagus begins to lengthen in the fourth week. At this time, also, the beginning of the liver is indicateil by a small diverticulum which pouches out from the ventral wall of the intestine just posterior to (below) the stomach — the future duodenal region therefore — and which grows into the ventral mesentery. Very soon after the appearance of the hepatic evagination, a similar out-pouching from the dorsal wall of

Fig. 98.— Outline of alimentary canal of human embryo of twenty-eight days (His) : p5, pituitary fossa ; tg^ tongue ; Ix^ primitive larynx ; o, esophagus ; tr, trachea; Ig, lung ; «, stomach ; p, pancreas ; Ad, hepatic duct ; rd, vitelline duct ; a/, allantois ; hg, hind-gut ; W'd, Wolffian duct ; t, kidney.

the future duodenal region of the intestine indicates the beginning of the development of the pancreas.

In the latter part of the third week or in the beginning of the fourth, the esophagus presents a longitudinal groove on the inner face of its ventral wall. This groove increases in depth and caliber and finally becomes constricted off from the esophagus, with which it retains connection only at its pharyngeal end. The tube or tubular sac thus formed is the first step in the development of the lungs and the trachea.

It may be said then that the gut-tract has now, in the fourth week, reached the stage of differentiation into the pharTux, the esophagus, the Btomach, and the intesttne, with the liver, the pancreaB, the respliatoiy sTstem, and the allsntoia fairly begun.

As lieretofore pointed out (p. 90), the allantois — which grows directly from the primitive gut-tract, and which con

alimentury caiial nf huTTinn pmbryo or thIrty-flTe dayi (HIh) : Jib, picuibiry fiiwu: tg. tOQKUv: U. priDiitivv larynx: ", eeo|ib(>KUB : Ir, (rachea; Iq, liing; «, Jlomai-h; ji, pancri.-B*; ftif. hcpalk duel ; e. conira; c(, cloaca; It, kldnuy ; a, anus ; -Tp, genlUt ominonco ; I. cauilal piocesB.

sists therefore of the entoderm and the visceral meaodenn — althoiigli destinetl to produce in part the permanent bladder, functionates for a time, after its union with the false amnion to form the chorion, an an organ of respiration ; while the permanent respiraU)ry pystem, as we have seen, likewise developH from the entodermal epithelium of the guttract The entoderm, therefore, sustains an important relation to the nutrition of both the embryonic and the adult oi^nism. Increase in I^ength and Further Subdivision. —

The intestinal canal grows in lengtli much more rapidly than <loes the embryonic body. It is in consequence of this disproportionate growth that the tube becomes bent and thrown into ooiU or convolutions. During the fifth and sixth weeks a conspicuous flexure appears at some distance below the stomach. Here the bowel assumes the form of a U-shaped tube, the claseil end of the U projecting toward the ventral body-wall (Fig. 100), In other words, the redundant portion

Fig. lOCb— iDtmtniil omi] at Iiuman embryo

of the gut is pulle<] away, as it were, from the dorsal wall of the body-cavity and, as a consequence, the dorsal mesentery is Icngthenetl in this region to a corresponding extent (Fig. 100). The vitelline duct is attached to the piirt of the bend nearest the ventral wall (Fig. 98). At a point on the lower limb of the U the bowel abruptly acquires increased caliber. This dilated part is the beginning of the cacnm or head of the colon, and its appearance initiates the distinction between the large and the small intestine, since the part' of the bowel on the distal side of the point in question becomes also of lai^r caliber and forms the colon.

During the succeeding week or fortnight, the character of the colon and of the csecum becomes better established. The remaining part of the lower limb of the U-loop, with all of the tube included between the loop and the stomacb, is the small intestine, wliicli presents a (iltght dorual flexure at it£ proximal extremity. The stomach meanwhile ha» increatied in size and has almost attaineil its characteristic shape. By the end of the sixth week, then, the alimentary canal has not only increased in length but lias so far dilfereulialed as to have aci)uireil stomacli, dnodetiiun, Bmall intestiiie, cscmn, and Tectnin,

Alteration in the Relative Position of Parts, and Further Development. — The nmst iniportsuit ntudiliciition of llie uliinfutary tube a^ it exists at the end uf the sixth week is effected by certain changes of position of some of its parts. The stomach and the large intestine are the jjortions of the tract most conspicuously affected. The lower limb of the U-segment of bowel, which consists chiefly of the radimentary ctecum and a [Mirt of the colon, is lifted, as it were, over the npfwr limb and comes to occupy a position above it (Fig, 101, ,1], the wecum assuming a posilion in the right

Fig. 101,— Three sucreulre staires ■hnwlng tln> fli'velopnieiil ol the dlgcatlTe tnboand Ihe meaeutcrietln the human IbIiiR{modincdIhnnTounivUii: I.buiduihi l,i1uiH]enum; S, anikll 1ntral1ne:4, colon; 5, vlleUine dnol: B,CKOiim; 7, great Dmentum : 8. mnioiluodcnuni : 9, mrw nt^r? : >", meaocolun. The nrraw polols to the oiiflcc of \h>r nmcntHl buna. The veDlral mespntory In not shown, hyjK vers L

hypochondriac region, and the colon paasinf; thence transversely acros-s the abdomen ventrad to the diiwlenum. This shifting of position on the part of the colon brings about important complications in the arrangement of the mesentery, since the part of the dorsal mesentery that pertains to the upper part of the colon correspondingly alters its position and line of attachment, becoming adherent to the i)eritoneum on the ventral surface of the duodenum. The part of the mesentery in question becomes the transverse mesocolon (Fig. 101, B), The large intestine, after this change of position, presents csecum, transverse colon, descending colon, and rectum, the ascending colon being still absent.

The vermiform appendix in the third month has already acquired the form of a slender curved tube projecting from the cflecum. At the time of its first appearance and for some weeks afterward, the appendix has the same caliber as the caecum. Subsequently the c«ecum outstrips the appendix in growth, the latter appearing in the adult state as a relatively very small tube attached to the much larger csecum.

The caecum soon begins again to change its position, gradually moving downward toward the right iliac fossa (Fig. 101). The downward migration of the caecum is (hie to the growth of the colon in the same direction. In this manner the ascending colon is gradually produced, it having developed to such an extent in the seventh month that the caecum lies below the right kidney, while in the eighth month it passes the crest of the ilium.^ Corresponding with the growth of the ascending colon, the mesentery shifts its parietal attachment and increases in extent until the ascending mesocolon is produced ; and with the descent of the caecum, the terminal part of the small intestine necessarily alters its position to a like degree.

The stomach, up to the third month, is a localized dilatation of the intestinal tube, bulging most in the dorsal direction and having its long axis parallel with that of the body (Fig. 100). In the third month, however, it undergoes an important alteration in position, rotating about two axes. First, it turns about a longitudinal axis, whereby the left side comes to face toward the ventral surface of the body (anteriorly) and the right surface looks toward the spinal column. In addition to the longitudinal rotation, the stom

  • According to Treves, the caecum lies under the liver until the fourth

month after birth.

aoh also rotatt^ ui>oii a dorsoventral (anteroposterior) axis, bv which the lower or pyloric extremity moves somewhat iipwTinl ami to the rijrht, and the cardiac end goes tailward ^dowHx^nnh and to the left (Fig. 101). By this double rotatuMi tho stoniavh is made to assume approximately its adult jHv^iuoiu Tlu^ lonjritudinal rotation of the stomach, in which «bo U^xwr jK^rtion of the esophagus takes |>art, explains the t^lMi «t ^« TaffUB nenres in the adult. The nerves, before U^^* rx^i^li^MK Ho one on each side of the esophagus and stom?^^^, bul ^in*v tlu» loft surfaces of both turn forward and the \k\^\\\ MUlJuHw turn Imokward, the left vagus lies on the t^t^v tivM' ^urfa*^* of the esophagus and of the stomach, while vKo M.^hl uorvo Is in rt^lation with their posterior surfaces.

V\w x^dkUimtL of the mesogastrium are influenced in an im|K«tiiOi( uuuuior by the rotation of the stomach. As long as {\w ^\\^\\us\'\\ itiains its original position and relations, with W^ \S,^\^sWv \'\{V\i\U\\v facing dorsad (or |)osteriorly), the mesoyiua^uiu i« a voriiivil mesial fold of peritoneum (Fig. 100), wlulv* iho wntnd mesoutory similarly connects the future or vent ml surface of the stomach with the w b\Hl\ wall At the very beginning of the process of ua-Uhm, tho nir«ogt»ster luHMmies somewhat redundant and

uU'. » t^'waid llio Irl^ O'^K* ^^^y -^)* -^^ ^'^'^ increases in \ slrm, \U\\v \^ ronnod, between the stomach and the dorsal U»mI\ Nsall, a |u»ni^h or |MH*ke(, the omental bursa, whose o|)enu\i\ ti u»\\anl tlh' v\^\\i ( Kig. 101). In the third and fourth iuuulli"« llir ori^riniil unv*4i>gjister, lengthening more and more, i\\\\\ lu'iiu? JilVo*'hHl by the inertMising torsion of the stomach, Hui|i»l"i lu tho a sae considerably below the level of Clii. ^liiumoli, in fwwxt of (ventral to) the small intestine and {\w l»iiu-.\ri>ir oolt»n. It ultimately becomes the great omeniuur riio nirio^MMiriunu fn)m having been a vertical mesial told, i^ miN\ become a tnmsvers** fold, so nnlundant as to be Ibldrd M|iou it.nrir and to constitute a bag.

\\\ \\\\v nuiuht^r (he ventral mesentery (Figs. 94 and 102), whirli roiuimli llh« anterior or ventral surface of the stomach with tlu' Nrnind iMulv-wall, and in which the liver develops, i^ ulUTfil tVoiu a medial ft)ld to a transverse fold by the rotatiou of the. rttiunai'h. As the liver migrates to a position above the stomach, the part of the ventral mesentery which connects the liver with the body-wall becomes its falciform ligament and coronary ligament, while that portion of this mesentery that connects the originally ventral surface of the stomach, now its lesser curvature, with the liver is the lesser or gastrohepatic omentum. The lesser omentum, therefore, is the anterior or ventral boundary of the orifice of the omental bursa referred to above.

The small intestine begins to exhibit flexures as early as the fifth week, and by the end of the sixth w^eek the duodenum is well indicated as a segment of the gut-tube passing from the pyloric end of the stomach toward the dorsal bodywall. From this time the development of the small intestine, aside from its liistological characters, consists chiefly in increase in length with consequent modification of its mesentery. A striking feature of human development is that, with the growth in length of the small bowel, it is gradually extruded from the abdominal cavity into the tissues of the umbilical cord. The extent to which extrusion takes place increases until the tenth week, after which period the intestine is gradually withdrawn into the abdomen. In the fourth month it lies entirely within the abdominal cavity. Failure of complete restoration of the gut to the cavity of the abdomen constitutes congenital umbilical hernia.

The histolog^ical alterations incident to tiie development of the alimentary tube, from the beginning of the esophagus to the end of the rectum, consist in the differentiation of the constituent elements of its walls from the two strata, the entoderm and the visceral mesoderm, which compose the walls of the early gut-tube. As an initial step in the process, the cells of the mesodermic stratum undergo multiplication and arrange themselves in a narrow loose inner zone and a thicker outer lamella. The inner layer subsequently becomes the submucosa of the fully formed state, while the cells of the outer layer undergo differentiation into unstriped muscular tissue, and constitute the muscular coat of the canal. In the case of the esophagus and stomach, at least, this muscular tunic, in the fourth month, exhibits the distinction between inner circular, and outer longitudinal, layers. The surface-cells oC the mc.':Jodennic stnituiu of tlio primitive stonuich and bowel become the endothelium of the serous coat.

The glands of the entire canal are products of the inner, entoilermic stratuni, and therefore they are intimately related genetically, as well as histologically, with the mucous membrane.

The glands of the Btomach, both the peptic and the pyloric, originate from small cylindrical cell-masses that have been produced by local multiplication and aggregaii<jn of entodeniial cells. By the hollowing out of the cylinders and the branching of the tubes thereby formed, the two varieties of gastric glands are evolvetl. Both sets make their appearance in the tenth week. Until the fourth month the peptic glands contain cells of but one tyi>e ; at this [leriod, however, certain cells of these glands becurae altered bv the gradual accumulation of griinules within their protoplasm, by which they arc trnnsfurmcd into the ehurac (eristic acid or parietal calls of these glands.

The glands and villi of the intestine are likewise products of the entodermal lining f>f the gut. Their evolution begins in the second month, and they are fairly well formed by the tenth week. As in the case of the gastric glands, the glands of the bowel develop from cylindrical masi^es of entodermal cells which are at first solid, hut which later become hollowed out to form tubular depressions or follicles. In the regioii corresponding to the upper part of the small intestine many of these follicles branch to give rise to the gl&nds of Bmnner, while tmbranched, simple, tubular depressions distributed throughout the entire length of the bowel become the glands of LieberklUui. While the surface entoderm is thus growing into the underlying mesodermic tissue to form the glands, it becomes elevate<l into nnnnte projections between the months of the gland-ducts, forming the villi of the intestinal mucosa. The connective-tissue core of the villus is derived from the »ndprlyini» mcsmicrmic tissue, the cells of which, proliferate tng, grow forth into the entoderm. The villi at first are present throughout the large and the small intestine alike, being well developed by the fourth month. While the villi of the small bowel continue their development, those of the large intestine, after the fourth month, begin to retrograde. At the time of birth they are still discernible, but at the end of the first month after birth they are completely obliterated.

Meckel's Diverticulum

The vitelline duct, it will be remembered, is the avenue of communication between the early gut-tube and the umbilical vesicle. In the sixth week the umbilical vesicle has already begun to retrograde, and the vitelline duct is attached to the ventral extremity of the U-loop of the bowel present at this stage. The vitelline duct in most cases suffers complete obliteration in the later stages of fetal life. In some instances, however, its proximal extremity persists in the form of a small blind tube varying in length from one to several inches, which is known as Meckel's diverticnlunu Since the site of attachment of the vitelline duct is not far from the termination of the small intestine,- Meckel's diverticulum, when present, is connected with the lower part of the ileum, at a point from one to three feet from its termination. Should this tube remain attached to the umbilical aperture and retain a patulous orifice, there would result a congenital fecal fistula.^

The Development of the Liver

The essential features of the dev^elopment of the liver will be more easily apprehended if the reader will not lose sight of the fact that the organ is a compound tubular gland, and if, further, he will recall the method by which glands in general are developed — that is, as evaginations of the wall of the cavity or organ to which they pertain.

The first step in the evolution of the liver is the growth of a diverticulum from the ventral wall of the gut-tube at a point corresponding to the region of the future duodenum. This occurs in the third week, since His found the diver

  • Meckel* 8 diverticulum is of interest clinically, since by contracting

adhesions to adjacent coils of intestine or by entanglement, it may produce acute obstruction of the bowel.

abundant cell-proliferation. The numerous branches into which they divide are not tubes, but solid cylinders of cells, the hepatic cylinders. The secondary branches of these cylinders unite with corresponding branches of adjacent systems, producing thereby a network of inosculating cell-cords, the meshes of which are occupied by young connective-tissue cells and the developing bloodvessels. The connective and vascular tissue of the liverridge, thus surrounding and permeating the epithelial cellcords, produces all the connective-tissne parts of the liver, while the liver parenchyma — the proper hepatic cells — and the epithelium of the bile-dncts originate from the primitive entodermic evagination. The cords of cells are in part hollowed out to form the bile-ducts and bile-capillaries, and in part become the cells of the lobules. The cylinders that are to produce the bile-ducts acquire their lumen by the fourth week.

Until the middle of the fourth month, the right and left lobes of the liver are of equal size, but after this period the right lobe outstrips the left in growth. The liver grows very rapidly and is relatively of much greater size in the fetus than in the adult, almost filling the body-cavity at the third month. In the later months of pregnancy it reaches almost to the umbilicus, while at birth it makes up oneeighteenth of the body- weight.

The £fall-bladder develops as an evagination from the original diverticulum. It is present in the second month. The pedicle of this evagination lengthens somewhat and becomes the cystic duct. The stalk of the hepatic evagination itself becomes the ductus communis choledochus.

The ligaments of the liver, save the round ligament, are simply folds of the peritoneum which connect the organ with the abdominal wall. Falling into the same category, though not usually designated a ligament, is the gastrohepatic omentum, which connects the liver with the stomach. These various peritoneal folds may be looked upon as parts of the ventral mesentery. Since the liver evagination grows between the two layers of the ventral mesentery to reach the septum transversum, the liver will be found, iti the early stages of its development, embedded between the lamellte of tliis mesentery, which in a riiedian vertical fiild of jxritoneum (Fig. 102). The liver is therefore enclosed iu the peritoneum and is connected below, by a part of the ventral mesentery, with the lesser curvature of the stumiich, which still lies in the median plane of the body, and above and in front, with the diaphragm and the ventral body-wall by the upper and anterior part of the same structure. The latter fold is somewliat modified by the intimate asi^ociation of the early stage of the liver with the primitive diaphragm, the liver having develi)i>ed within a ]H>rtiori of the septum transversum, the liver ridge. As development advances, a partial separation of the liver and the diaphragm is effected, the peritoneum, as it were, growing between the two from both the ventral and the dorsal edges of the liver. The region which is not invaded by the peritoneum represents the nonperitoneal surface of the adult liver between the lines of reflection of the two layers of the coronary ligament. Since the peritoneum on the under surface of the diaphragm is reflected from that muscle, both in front of and behind this area of contact, to liecome continuous with the peritoneum on the convex surface of the liver, there are formed two transverse, parallel, but separated, folds which constitute the coronary ligament of adult anatomy. The lateral prolongations of these foldp to the lateral wall of the abdomen constitute the lateral ligaments of the liver.

The nitation of the stomach to assume it^ i)ermanent relations alters the position of the fold that ci>nnect8 its lesser curvature with the liver, bringing this fold into a plane parallel, approximately, with the ventral wall of the abdomen. This fold is now the lesser or gastrohepatlc omentmn.

The ronnd ligament of tlic mlult represrnls the impervious vestige of the umbiliiid vein. This vessel, entering the fetal body at the umbilicus and pa.ssing to the under surface of the liver, diverges from the abdominal wall to reach that organ and. in doing so, carries with it the piirietal iK'ritoneiim, The fold thus formed is the falciform or suspensory ligament.

The special system of blood-vessels belonging to the liver is described in the chapter on the Vascular System, p. 177.

The Development of the Pancreas

Until recently it was believed that the pancreas developed from an ovagination of the dorsjil wall of the gut-tube in the region of the future duodenum, opposite the site of the hepatic diverticulum. Later investigations have shown, however (Stoss, Hamburger, Brachet, and others), that three evaginationSy one dorsal and two ventral, appear upon the wall of the duodenal region of the gut-tube, the method of development being strikingly similar in mammals, birds, fishes and ampliibia.

Fig. 103.— Reconstruction of duodenum with pancreatic diverticula (after Uamburger) : A^ Five weeks' embryo ; It, six weeks' embryo ; D, duodenum ; D.chol., common bile duct; V.P, ventral pancreas; D.P, dorsal pancreas; JT, point of fusion of the two ; .S, stomach.

In the sheep a dorsal evagination appears in a 4-mm. embryo (Stoss), and somewhat later two ventral outpouchings apj)ear in close proximity to, if not in actual connection with, the hepatic diverticulum, the stalk of which latter becomes the common bile-duct. The dorsal diverticulum penetrates between the two layers of the mesogastrium (Fig. 103) and gives off lateral branches, remaining attached to the dorsal wall of the duodenum by its stalk or duct. Eventually tliis system of branching epithelial tubes, the dorsal pancreas, becomes the body and tail of the adult pancreas.

The rigid and left ventral evaginations become confluent and form the vential pancreas. According to some authorities the left diverticulum atrophies, the right alone persisting to form the ventral |)ancreas. In either case the stalk or duct of this ventral fundament bw;oinc^ tnorgfxl into the eommoJCQ bile-duot — if not jireviously eouaected with it — so thatit i^ in effect, a bramli of that dnct. The ventral pancreas grow to the left, in front of the portiil vein, this change beinj favored by the rotation of the duotlcniim on its long i penotmtos between the kvei-a of the nicsogastritini and fuses with the dorsal pancreas (Fig. 103), hecomiiig the head of the adult oi^n. Thin onioo occurs in the sixth week in man (Hamburger). Willi the union of the two |iortions their respeetive ducts— the dorsal duet or duct of Santorini and the ventral or duct of Wiranng — acquire auaslomosefi with each other, after wliieh event the terminal or intestinal part of tl duct of Santorini atrophies and disappcais, the duct of Wi: suug being heneefiirth the avcune by wich the later-estate ]ishe<l .'ieeretinn enters the dmiflenurn. Occasionally entire duct of Santorini jiersisls to adnlt life, entering t duodenum upon its dorsal wall. In the cow and pig I ventral duct atrophies, the duct of Santorini alone per&istin^a while in the horse and dog both duets persist.

What has been said above applies to the origin of thftl epithelial ])ai'ts of the gland ; the coimectlTe-tissne and i lar elements are of mesnderniic origin.

At six weeks the long axis of the pancreas nearly c sponds with that of the body of the fetus. With the rota-^ tion and change of position of the stomach and the alti tionft in the mesogastrium, it moves to the left, acquiring its 1 permanent relations with the lert kidney and the spleen. It ] continues to be an intraperitoneal organ until the fifth month, when, by the disappearance of the tlorsal ]iart nf its investment, it becomes retr(i[>eritoneui (Fig. lOfi, ,1 and H).

The Development of the Spleen

Although the sjileen dm-s not belong to the digestive system, it may convenienliy lie eonsidcrid here because of its position and relations.

This organ is differentiated from the meaodermic tissue ( nchy ma) found between the layers of the mesogastrium in close proximity to the developing pancreas (Fig. 102).

Primitively, therefore, it is situated behind the stomach. The first step in its development, recognizable at about the end of the second month, is the accumiilation of numerous lymphoid cells with large granular nuclei. The origin of these cells has been a matter of dispute. It has been asserted (Maurer, Kupfer) that they come from the epithelium of the gut-tube, but this is denied by most authorities. The findings of Laguesse in fish-embryos, demonstrating the origin of the spleen aniage from mesencliyma in close relation with the branches of the later portal vein, are possibly significant in view of the relationship between the spleen and one of the largest tributaries of the porta\ vein, namely, the splenic vein. Tonkoff's observations on birds and mammals (1900), confirmed by Hochstetter, reaffirm the mesenchymal origin of the spleen.

The mass of cells is augmented by the addition of cells immediately beneath the peritoneal surfaces of the mesogastrium, which cells elongate imtil they are spindle-shaped and then become aggregated into fusiform masses. Bloodvessels penetrate the fundament in the third month and become surrounded by cells of the same spindle-shaped type. From both the cells surrounding the blood-vessels and from those of the fusiform aggregations, processes grow out and unite with each other, and from the network thus formed the trabecular framework of the organ is ultimatelv evolved. Accumulations of small nucleated cells, forming dense masses along the arteries, furnish the chief constituent of the pulp. The delicate intercellular substance which makes up the remainder of the pulp is filled with blood-corpuscles. The Malpighian corpuscles appear before the end of the fourth month. By the sixth month, the spleen attains its characteristic shape and the fibrous capsule is clearly indicated.

The spleen undergoes a change of location coincident with the rotation of the stomach and the alteration of the mesogastrium. The organ being from the first embedded within the mesogastrium, it follows that peritoneal fold to the left side of the abdominal cavity. Here it lies close to the cardiac end of the stomachy between the two layers of the mesogastrium, but projecting toward the left. The part of th(» mcsogastrium which intervenes between the spleen and th(* stomach is the gastrosplenic omentum ; while the part that pass<;H from the spleen to the posterior wall of the abdomen, r(»prcs<»iiting the parietal attachment of the mesogastrium, (^onstitiitcH the phrenicosplenic omentum.

The Evolution of the Peritoneum

The arrangement of the peritoneum being subordinate to thc! pcmition and relations of the vis(»era contained within the iilidonion, th(! development of this complex membrane can be pro|M?rly <lcwTib(Hl only by tracing the growth of the digestive HyHlnii. Am the formation of the early gut-tul>e by the infolding of tlu; splanchnopleure has been pointed out (pp. I HO iiiid IH8), w<» may begin at once with the i)eriod when fhi' IriM'l luiH ilir form of a straight tube connected with the doi'.-iil niid iIm' vontPid IxKly-wall respectively by the dorsal iiiid I III' ventral mesentery (Fig. 104). Covering the tube as a roih'shtiiriit purl of its wall, is the splanchnic or visceral layer 111' llir iiiiv<^od('rm, while the somatopleuric or parietal layer ol' ilii« lallrr lificM th(» wall of the bodv. Obviously these Iwo liiiiirllip of llic incsfMlcrm are continuous with each other tliroiipfli \\\i* iiM'diiim of tlu* mesenteries mentioned above (ll^i. lO/i, .1 Mild //). The space thus enclosed by the mesodrniih* nlnitn Im the body-cavity or co»loin or pleuroperitoneal nu\M\. TIm' ^iirfiUM'-cells of both strata flatten and assume llin rliiinirtiT of mcsothclial, the later endothelial, cells. If, at thin ntit^e, one iN'gins at any point to trace the mesothelial lining, of the body-<'avity, that lining is found to form praclirallv oiiti ctMitinuous sheet.

Tliin hiiiiple nrrangenuMit of the primitive peritoneum is (ninntonnrd into the (Mimpli(*ated membrane of the adult, prinmrily, by tin* inerejiHe in length and cons(»quent tortuosity id' tht) ulinii'iitiirv tube; and, sec<mdarilv, bv the fact that iH^i'tain oppoM'd portions of the s<tous membrane, which have been brought int<» eontaet by the altered relations of the bowel and tlu* stomach, undergo con<»reseence or fusion with each (itlior. Siiiiiihancoiialy wilh ihcsc alterations, [he original pIeuroperi1»neal cavity sufl'ci's division into tlic abdominal or peritoneal cavity anil the thoracic part of the body-cavity by the devclupinent of the (lia[»luiigni. This is described on p. 175.

Fig, im —Recoiislruplion of Immun embryo of »boiilaeTtnteen(l»yB(«flerHW: OP, opl[c and of, otic vinlclcs: nr. imtuchord : Mg, head-gut: g. mid-gut: lig, hindgut; T,, vitelline aac; I, liver; v, primitive venlrtcle; m. da. ventral and doraal «i)iniE:Jp. primitive Jugular vein; ™, cardlual vein; rfC, dnct of CuvlBr; m, no, umlilUcal vein and artery; at, allaniaii; ua. umbiilcal eord; dm. doiMl meKDtery; vn, vcntnl meneiitvry Imudilled (mm Hlsi.

Tho first modification of the original arrdngemeDt is effected by tiie development of the stomach a^ a spiiidlfi-shaped <lilatation of lh{' gul-tiil>e, ditferentiatiiig the tube int*) the stomach and the intestine, and the c-omniuD dorsal mesentery

into the meBoeastrinin and lite iatestmal mesentei?. The drawing out of the U-shu])cd liwp of ihc inti'.-^tiiic from.the dorsal body-wall, which is the prolirniiiary siep to the distinctioD between the small iulcstinp and the colon, iiiL-reaspa the length of the iiitcHtinal raesenlerv to a corresjwnding extent (Fig, 10'>, C). As heretofore indicated, the lower limb of the liKip presents an enlargement which is the beginning of the development of the lai^e intestine.

Km. los -A. B. two tniuvene sectlona, A Ihniiii Intl regjao: C aogiltal aecKon (Toumcux) : l, clone lerr; H, mewKardlum poalerlui: 4, inesocanlliiia anterliu: !>, lumer omentum ^ 6, ■UBpensory ligament uF tlvcr; 7. eaophague; 8. lungs: H. bi»rli ID, puncnu; 11, itnmnch: 12, llrer; l:i, apleen: 11, loop of lulcatlno vilh Tltclllue duct: IS.ciccum; IB. trecbcL

An important stage in the evolution of the jieritoneum is tnarkod by the rotation of the stomach and by the migration of the proximal part of the largo intestine to a new location. The eliange of position on the part of the colon may perhaps be best expressed by saying that the U-Ioop of intestine rotates upon an obllqne dorsoventral axis, whereby the lower limb of the loop, in other words, the termination of the small bowel and the l>eginning of the colon, is carried to a positioD above, cephaliid to, the nppor limb (Fig. 101, A), This rotation brings the beginning of the colon into the riglit hyjK)chondriac region of the abdomen, from which point the transverse colon passes across the abdominal cavity, ventrad to the proximal end of the small intestine or duodenum. As a consequence of the altered situation of the transverse part of the colon, its mesentery shifts its area of attachment by fusing with the peritoneum of the dorsal wall along a horizontal line and also with that of the ventral surface of the duodenum. The descending colon having meanwhile moved to the left, its mesentery likewise acquires a new area of attachment by concrescence with the parietal peritoneum of the dorsal wall of the abdomen on the left side. During the progress of these alteraticms, the small intestine increases in length, and its mesentery becomes correspondingly more voluminous both in the extent of its intestinal border and in length. The convolutions of the small intestine now occupy the space below the transverse colon and its mesentery.

The duodenum, which in the early stage shares with the gastro-intestinal tube in the possession of the common dorsal mesentery, loses its mesenterial connection with the abdominal wall and becomes thereby a fixed part of the intestine. Mention was made above of the fusion of the transverse mesocolon with the peritoneum of the ventral surface of the duodenum. At about the same time, the duodenal mesentery (Fig. 101, A) fuses with the parietal peritoneum of the posterior abdominal wall, the result being that the lower layer of the transverse mesocolon, as it passes downward, is now continuous with the parietal peritoneum, there being no longer any serous membrane between the transverse part of the duodenum and the abdominal wall (Fig. 106, J5). This part of the duodenum therefore becomes retroperitoneal, there being an investment of serous membrane only on its anterior or ventral surface.

The second modifying DEietor in the complication of the peritoneum, the rotation of the stomach, initiates alterations in its mesogastrium. The latter membrane, it will be remembered, is a vertical median fold of peritoneum continuous with the mesentery of the duodenum (Fig. 105, C).

As the stomach tcovca about its two axes of rotation, the mesogttstriuni begins to eaj^ toward the left (Fig. 101), sothat now it constitutes a pouch or foasa, the omental bursa, situaated between the stomach and the dorsal body-wall, the opening of wliich looks towartl the right side of the body (Figs. 100, ^1 and 107). Wilh the rapidly increasing rednndaney of the niesogastriTnii, the omental bun-a become.s

Fig. V

more and more capacious. In correspondence with the progressive rotation of the stomach, what was at first the left surface of the mc>sogastriiim comes Into contact with the peritoneum of the dorsal abdominal wall and fuses with it, thus changing its area uf parietal attachment fnim a'median vertical Hue to a transverse one. This change is (Mimpleted by the time the :«tomach has attained its normal adult jmsition. The omental bursa now has the position and relations shown in Fig. 106, A, 8. A still further increase in the size of the bursa results in its protrusiim downwanl in front of, ventrad to, the transverse colon and the small intestine. Kefence to Fig. 106, B will show that the dependent part of the lnirs;i very nearly corresponds with the fully formed great omeDttiro. It will be seen, however, that the deeper layer of the bursa, the layer nearer the intestines, may be traced above the transverse colon and its mesentery to the dorsal wall of the abdomen, where its two lamellie separate to enclose the pancreas, one lamina passing over the ventral surface of the pancreas to become continuous with the parietal peritoneum, while the other layer passes between the pancreas and the abdominal wall. The latter layer is in continuity here with the i»arietal peritoneum, which almost immediately leaves the abdominal wall to form the upper layer of the transverse mesocolon.

The further alterations necessary for the attainment of the completed condition consist in the coD<3«Bcence of certain opposed peritoneal snrfaceB. As a conspicuous example of such concrewcnce, the deeper lamella of the layer of the omental bursa just dcicribed fuses with the ventral jxritoneal surface of the transverse colon and with the upper layer of the transverse mesocolon (Fig. 106, A), after v^hich event this deeper lamella is practically continuous with the lower layer of the mesocolon, while the superficial lamella is in continuity with the upper layer of the mesocolon (Fig. 106, B), Thus the transverse colon appears as if enclosed between the two lamcllic of the deeper layer of the great omentum, while its mesocolon is constituted by a part of the same structure. In other words, the adult transverse mesocolon includes not only the primitive membrane of that name but also a part of the early mesogastrium. Similarly, the opposed surfaces of peritoneum between the pancreas and

iUt*iltirm\ iilHlcmiinalwall undergo fusion (Fig. 106), the effect of wlii<*|i, after the concrescence of the mesocolon with the <l<tif|Mfr lay^'r of the omental bursa, is to make the lower layer of iUt*, in(*M<K*f)lon continuous, over the transverse part of the <iM#HliffMitii, with the parietal peritoneum.

I ii« ffreat omentum of descriptive anatomy, resulting from Mi<< downwardly projecting process of the omental bursa, i'^malMn originally of two layers of membrane, each one liuvin^ two wTous surfaces. At the time of birth these iwtf UtyovH uro, ntill separate — the permanent condition in ^*ini' tMuiMfiialH— l)ut during the first year or two after birth imy \u*iuniu» adherent, the great omentum thus coming to iunii\ir\tui but a ningle layer.

it rttitHiuiti to note the metamorphosis of the ventral mesen tWVi H bii'h, prior to the rotation of the stomach, is a vertical

liMfdiaii fold ronnecting the lesser curvature of that viscus

with i\m vt'Ofnil ulHlominal wall. Since the evagination of

tUii |iiif-(iibi; (hut given rise to the liver grows between the

liiytii'D of \\w. \'i*ulvii\ ni(?sentery to reach the septum trans Vi^reMnii (Ih^ liv<*r in not only enclosed by the mesentery, but

}H roiiiH'rh'd by it with the stomach and with the ventral

wall of the lilNlonii'ii and also with the primitive diaphragm

(Kij/. lorj). l^y tin* rotation of the stomach, the vortical

nM'diaii (old whirh connrrts that orgjin with the liver becomes

ho allrnd in pohitinn an to \\r in a plane approximately par alhJ with lh<^ vi'nlrnl hurfarr of the body. This fold is now

till' gatttrohapatic or lesser omentum. As referenc'c to Fig.

lOtI will hhow, il iri (hf anterior boundary, above the position

of thii ht4»nnu*h, of tint nac deH<Tilwd above as the omental bursa.

That pari of thi^ ventral incHentery that connects the liver with the abdominal wall antl with the diaphragm, while originally orfupyiiig the inetlian plane, is modified by the relation of the <h»v<«loping liver to the primitive diaphragm. thene organs are intiniati*ly united with each other (p. 175) in the early stjige <»f tlifir growth, l)Ut with their completion a Hi'pamtion takrs place. V\h)u the two separated surfaces, except in a region near the dorstd wall, the cells assume the endothelial type, the opposed surfaces thus acquiring the characters of serous membrane. The peritoneum on the under surface of the diaphragm is continuous with that on the upper surface of the liver, both in front of and behind the non-peritoneal area of contact. Therefore, in the completed condition of the liver and the diaphragm, these two structures are connected by two layers of peritoneum separated from each other by a region containing only areolar tissue. These layers constitute the coronary ligament of the liver. If now Fig. 106 is inspected, it will be seen that the posterior layer of the lesser omtMitum, and the upper layer of the transverse mesocolon, together with that part of the peritoneum with which they are in direcjt continuity, enclose a sac which is the so-<^alled lesser bag of the peritoneum or the lesser peritoneal cavity. All other parts of the peritoneum taken together constitute the greater peritoneal cavity. The communication between the two, the foramen of Winslow, situated behind the free right border of the lesser omentum^ is the constricted orifice of the early omental bursa.

The position of the kidneys and the ureters as retroperitoneal structures and the relations of the bladder and of the uterus to the peritoneum, encroaching as they do upon the parietal layer of this membrane, and being, therefore, invested by it to a greater or less extent, are easily accounted for when it is recalled that all these organs develop from the somatic or outer layer of the mesoderm.

The peritoneum does not acquire all the characteristic features of a serous membrane until about the third month. The histological alterations begin in the fourth week, from which time until the sixth week the superficial cells, the mesothelium, pass through various phases of transition to reach the condition of somewhat flattened elements. By the eighth week they have acquired the form of true endothelium. It is not, however, until the third month that the subjacent tissue has attained to the condition of a fullyformed basement membrane.