PART IV. MORPHOLOGY OF THE HUMAN CAECUM AND VERMIFORM APPENDIX 237

I. Development of the Caecum and Appendix 237

II. Changes in the Position of the Caecum and Appendix during normal Development, depending upon the Rotation of the Intestine and the subsequent Descent of the Caecum 239

III. Variations of Adult Caecum and Appendix 244

_A._ Shape of Caecum and Origin of Appendix. Types and Variations of Adult Caecum and Appendix 245

_B._ Position and Peritoneal Relations of Appendix 250

_C._ Ileo-Caecal Folds and Fossae 260

INTRODUCTION.

In considering the anatomy of the human abdominal cavity and peritoneum in the following pages the explanation of the adult conditions encountered is based upon the development of the parts, and the successive human embryonal stages are illustrated by the examination of the lower vertebrates presenting permanent adult structural conditions which appear as merely temporary embryonal stages in the development of the higher mammalian alimentary tract.

For the sake of clearness and brevity all discussion of the _theories of peritoneal development_ has been designedly omitted. The assumption of peritoneal _adhesion_, and consequent obliteration of serous areas, offers many advantages in considering the adult human abdominal cavity, especially from the standpoint of comparative anatomy. The same has consequently been adopted without reference to divergent views and theories.

In studying the descriptive text and the diagrams the student should remember that the volume offers in no sense a complete or detailed account of the development of the abdominal cavity and its contents. The purpose is not to present the embryology of this portion of the vertebrate body, but to _utilize_ certain embryological facts in order to _explain_ the complicated adult conditions encountered. To avoid confusion, and to bring the salient points into strong relief, the majority of the diagrams illustrating human embryonal stages are purely schematic.

Moreover, in order to avoid confusing and unnecessary details it is often desirable to disregard developmental chronology entirely. Many of the diagrams combine several successive developmental stages, showing different degrees of development in different portions of the same drawing. Again it is frequently necessary, for the sake of brevity and clearness, to actually depart from known embryological conditions. If, for example, the stomach and liver are treated as if they were from their inception abdominal organs, the student of systematic embryology will recall the fact that this position is only _obtained after_ their primitive differentiation by growth and migration.

Again the mesenteries are treated here as if they formed definite and well-defined membranes from the beginning--without reference to the abdominal organs with which they are associated. We speak of the liver as growing into and between the layers of the ventral mesogastrium, because this conception offers the opportunity of more clearly explaining the adult condition. Actually, however, the membrane develops, as a new structure, after the first differentiation of liver and stomach, as these organs descend into the abdominal cavity.

Similar discrepancies between fact and schema are encountered throughout. Consequently, while the purpose of the volume is to facilitate the study and comprehension of the _adult_ peritoneal cavity and its contents, the reader should guard against receiving the developmental illustration as a correct successive and detailed account of the _embryology_ of the parts concerned.

In like manner the comparative anatomical facts adduced form in no sense even approximately a complete serial morphological account of the vertebrate alimentary tract.

To the student of human anatomy the zoological position of the forms which help him to understand complicated human structural conditions is immaterial. He can draw on all the vertebrate classes independently of their mutual relations. Hence neither ontogeny nor phylogeny are here introduced, except as aids to the study of adult human anatomy. The following pages offer neither an embryology nor a comparative anatomy of the alimentary tract, but an attempt has been made in them to illustrate the significance of the complicated anatomical details presented by the adult human abdominal cavity by reference to the simpler antecedent conditions encountered during the early developmental stages of the higher forms and permanently in the structure of the lower vertebrates.

While, as just stated, a complete presentation of the development of the abdominal cavity is not required, yet the student will find it of advantage to rehearse the main facts of vertebrate embryology, for the purpose of bringing a clear understanding of the manner in which the vertebrate body is built up to bear upon the problems which the special organs and structures of the body-cavity present for his consideration. This purpose can be accomplished by a very brief and condensed consideration of the cardinal facts.

The entire vertebrate body is the product of developmental changes taking place after fertilization in a single primitive CELL, the EGG or OVUM (Fig. 1).

In structure the ovum corresponds to other animal cells. On account of their special significance during development the different component parts of the egg-cell have received special distinctive names. The _cell-body_ is known as the _vitellus_ or _yolk_. It is composed of two substances, the _protoplasm_ or formative yolk and the _deuteroplasm_ or nutritive yolk, which vary in their relative proportions in the ova of different animals.

The protoplasm represents the material from which in the course of development the cells forming the body of the individual are derived, while the deuteroplasm serves for the nutrition of the ovum during the earliest stages of development.

The _nucleus_ of the egg-cell is distinguished as the _germinal vesicle_, and its _nucleolus_ as the _germinal spot_.

The _cell-body_ or _vitellus_ is surrounded by a condensed portion of the cell contents to which the name of the vitelline membrane has been applied, which in turn is enclosed by a transparent and elastic cover, the _zona pellucida_, presenting a radially striated appearance.

The ovum is contained in the cortical portion of the ovary, enclosed in the _Graafian follicle_, a vesicle 4-8 mm. in diameter, whose fibrous walls are lined by several layers of epithelial cells, which surround the ovum, forming the _discus proligerus_.

After impregnation the egg-cell, by a process of repeated division or cleavage, undergoes _segmentation_, the cell-body being divided successively into two, four, eight, sixteen, thirty-two, etc., _cells_, called _blastomeres_ (Figs. 2 and 3). The mass of cells finally resulting from this process of segmentation forms the ground work of the future body. A vertebrate ovum in this stage of complete segmentation is called the _morula_ from its resemblance to a mulberry (Fig. 4).

After segmentation is completed a cavity filled with fluid and surrounded by the developing cells is gradually formed in the interior of the mass. This cavity is known as the _segmentation-cavity_. The egg is now called the _blastula_, _blastosphere_ or _blastodermic vesicle_ and the cellular membrane enclosing the segmentation-cavity forms the _germinal membrane_ or _blastoderm_ (Figs. 5 and 6). The cells of the blastoderm become aggregated at one point on the circumference of the vesicle (dorsal pole of blastosphere) forming, when viewed from above, a thickened biscuit or disk-shaped opaque area. This is known as the _germinal area_, or _primitive blastoderm_ or _embryonic shield_ (Figs. 7 and 12).

This is the first indication of the coming division of the entire egg-cell into the _embryo proper_ and the _vitelline_ or _yolk-sac_ (Figs. 8 and 9). The entire future individual develops from the cells of the germinal area. This area comprises both the embryo proper and the region immediately surrounding it.

The remainder of the ovum, serving temporary purposes of nutrition and respiration, gradually becomes absorbed and disappears.

Transverse sections at right angles to the long axis of the embryonic area show that the single layer of cells composing the primitive germinal membrane becomes differentiated first into two (Fig. 10) and subsequently into three layers of cells (Fig. 11). At the margins of the germinal area these layers are of course continuous with the rest of yolk-sac wall. From their position in reference to the center of the cell the three layers of the blastoderm are described as--

1. The outer, Epiblast or Ectoderm. 2. The middle, Mesoblast or Mesoderm. 3. The inner, Hypoblast or Entoderm.

The central nervous system (brain and spinal cord) is derived from the ectoderm by the development of a groove in the long axis of the embryonic area (Figs. 13, 14, 16 and 17), and by the subsequent union in the dorsal midline of the ridges bounding the groove to form a closed tube (Fig. 18). (Medullary groove, plates and canal.)

The following changes in the ventral aspect lead to the formation of the alimentary canal and body-cavity:

The developing embryo at first lies flat on the subjacent yolk-mass, and subsequently becomes gradually separated more and more from the rest of the blastoderm by grooves or furrows which develop along the sides and at the cephalic and caudal extremity of the embryo. The folds resulting from these furrows indent the yolk more and more as development proceeds and tend to approach each other at a central point, the future _umbilicus_.

In the meanwhile changes in the region of the mesoderm have led to conditions which produce a differentiation of the ventral portion of the embryo into two tubes or cylinders, the _alimentary_ or _intestinal canal_ and the _general body-cavity_, the former being included within the latter.

Early in the course of development a number of spaces appear in the mesoderm on each side of the axial line of the embryo. These spaces soon unite to form two large cavities, one on each side. Taken together these cavities constitute the _coelom_ or _body-cavity_, which becomes subdivided in the adult mammal into the pleural, pericardial and abdominal cavities.

As these coelom cavities develop in the mesoderm the cells lining them become distinctly epithelial. This mesodermic epithelium lining the coelom is called the _mesothelium_.

The development of the coelom space divides the mesoderm on each side into an outer leaf, the _somatic_ or _parietal mesoderm_, and an inner leaf, the _splanchnic_ or _visceral mesoderm_ (Figs. 18 and 19). The former is closely applied to the ectoderm, forming with it the _somatopleure_ or _body-wall_. The latter, in close contact with the entoderm, forms with it the _splanchnopleure_ or wall of the alimentary canal. In the dorsal median line both somatic and splanchnic mesoderm become continuous with each other and with the axial mesoderm (Fig. 20).

The folds of the splanchnopleure, indenting the yolk-sac, form a gutter directly connected with the yolk, the _primitive intestinal groove_ or _furrow_, whose margins gradually approach each other (Fig. 20). In this way the primitive alimentary canal becomes separated from the yolk. At first this separation is ill-defined, and the channel of communication between the primitive intestine and the yolk is wide (Figs. 13, 16, 17 and 19). The folding of the splanchnopleure completes, at an early period, the dorsal and lateral walls of the embryonic gut, but ventrally, toward the yolk, the tube is incomplete and widely open.

By union and coalescence of the splanchnopleural folds, proceeding from the caudal and cephalic ends towards the center, this primitive wide channel gradually becomes narrowed down, until the communication between the yolk-sac and the intestine is reduced to a canal, the _vitello-intestinal_ or _omphalo-mesenteric duct_. The intestinal gutter is thus converted into a closed tube except at the point of implantation of the vitelline duct during the persistence of this structure. In the meanwhile the somatopleural folds forming the body-walls grow more and more together from the sides, approaching the vitello-intestinal duct. Finally touching each other they coalesce to form the ventral body wall, in the same manner as the splanch[n]opleural folds met and united to form the alimentary tube.

At the same time the vitello-intestinal duct and the remnant of the yolk-sac, to which it was attached ("umbilical vesicle"), normally become obliterated and disappear.

After the intestinal tube and the body cavity have thus become closed the embryo straightens out and the alimentary canal appears as a nearly straight cylindrical tube extending from the cephalic to the caudal end of the embryo. This primitive alimentary tube at first terminates at its cephalic extremity in a blind pouch, while at the caudal end in the early stages the intestine is connected with the nerve-tube by a channel called the _neuro-enteric canal_, forming in the earliest embryos a communication between the ectoderm lining the bottom of the medullary groove and the entoderm (Figs. 22 and 26). In man this stage is encountered very early, in embryos of 2 mm. before the formation of either heart or provertebrae.

At the point where the canal develops the primitive groove presents a thickened circumvallate spot, marking the beginning perforation of the medullary plate from the ectoderm to the entoderm. The canal exists only for a short period during the earliest stages of embryonal life. It becomes rapidly closed, the neural and intestinal tubes henceforth remaining permanently separated from each other.

The embryonal caudal end of the primitive alimentary canal is not the final adult termination of the tube. When the anal aperture is formed in a manner to be presently detailed, the opening is situated cephalad of the portion connected with the nerve-tube by the neuro-enteric canal. Hence this terminal portion of the early embryonic alimentary canal is called the "post-anal gut" (Fig. 21).

The post-anal gut and the neuro-enteric canal are better developed in the embryos of the lower than in those of the higher vertebrates. But in all vertebrates of the present day both of these structures undergo regressive changes and finally disappear altogether. They serve to recall conditions which existed in bygone ages, and, while they have a long and significant phylogenetic history, they have lost among living vertebrates all physiological importance.

After closure of the neuro-enteric canal and obliteration of the post-anal gut the alimentary tube ends, during a short period, both cephalad and caudad in a blind pouch. Very soon, however, the ectoderm becomes invaginated at both extremities and finally perforates into the lumen of the intestine, thus establishing the oral and anal communications with the exterior. The anal ectodermal invagination (proctodaeum) (Fig. 21), is smaller than the oral (stomadaeum) (Fig. 27), but the intestinal tube forms an extensive pouch in the anal region which descends to meet the ectodermal invagination of the proctodaeum. The details of the embryonic processes leading to the final establishment of the adult condition are of great interest on account of the pathological importance of abnormal or arrested development in these parts. Failure of the caudal intestinal pouch to establish a communication with the anal invagination, or failure of development in either anal invagination or intestinal pouch, leads to the condition known as atresia ani or imperforate anus, of which there are several varieties.

Before the anal opening forms the primitive caudal intestine receives from above the stalk of the _allantois_, while the Wolffian duct, the canal of the embryonic excretory apparatus, also opens into it. The renal bud on the Wolffian duct in Fig. 28 indicates the beginning development of the permanent kidney (metanephros), and the proximal portion of the allantoic stalk is destined to form by a spindle-shaped enlargement the future urinary bladder (Fig. 28). The caudal gut has as yet no anal opening. Ventrad of the tail end of the embryo the ectoderm presents at this time a depression (Fig. 21). The ectoderm lining the bottom of this anal fossa or depression is separated by a little mesoderm tissue from the entodermal lining of the blind pouch of the caudal gut. Ectoderm and entoderm in this region with the intervening mesodermal layer form the _cloacal membrane_ (Fig. 21).

=Development of Cloaca.=--The entodermal pouch or prolongation sent down from the end-gut to meet the anal invagination enlarges and dilates to form a short wide piece of the intestinal tube into which open on the one hand the urinary and sexual ducts of the genito-urinary system, while it receives on the other the termination of the _end-gut proper_ (Figs. 28 and 29).

This is the permanent condition of the terminal openings of the alimentary and genito-urinary tracts in the lower vertebrates. It is found in certain fishes, in all amphibia, reptiles and birds, and occurs also in one order of mammals, the monotremes. In man and mammals generally the anal orifice is separated from the genito-urinary opening, lying dorsad of the same and provided with special sphincters. Only in the monotremes do the anus and the genito-urinary tract open into a common cloaca surrounded by a sphincter common to the anal and genito-urinary openings (sphincter cloacae). In birds, reptiles, amphibia and many fishes (especially the Plagiostomata) this cloacal formation is the rule. In many fishes, especially the Teleosts, the anus and the genito-urinary openings are separate, as in mammals, but their position is reversed, the anus being ventral, while the genito-urinary opening is placed dorsally.

Fig. 23 shows the cloaca in a female specimen of _Iguana tuberculata_. The ventral wall of the cloaca has been divided to the left of the median line and turned over to the right, carrying with it the cloacal opening of the bladder. The termination of the alimentary canal opens into the cloaca from above.

A transverse fold of the mucosa separates this upper compartment of the cloaca (_coprodaeum_) from a lower space (_urodaeum_) which receives in its dorsal wall the openings of the two oviducts and immediately above them--upon two papillae--the openings of the ureters, while the ventral wall contains the cloacal opening of the bladder.

The right ovary has been removed--to show the abdominal opening of the right oviduct--by dividing the mesovarian peritoneal fold.

Fig. 24--taken from a preparation of the hen--shows the typical arrangement of the female genito-urinary tract and cloaca in the birds.

The terminal portion of the alimentary canal, in entering the cloaca, forms an expanded upper cloacal compartment for the accumulation of the excreta, called the _coprodaeum_.

It is separated by a prominent mucous fold from the central compartment, or _urodaeum_ which receives the terminations of the two ureters and of the single (left) oviduct. A second fold forms the distal limit of the urodaeum and separates it from the lowest cloacal compartment, the _proctodaeum_.

Fig. 25 shows the male genito-urinary tract and the cloaca in the monotreme, _Platypus anatinus_. The cloaca is a spacious sac formed by the confluence of the rectum and the genito-urinary sinus.

The penis, consisting of two large cavernous bodies, is contained in a fibrous sac which arises from the junction of the genito-urinary sinus and the cloaca, and is continued into the ventral wall of the cloaca near its termination by an opening through which the penis can pass into the cloaca and beyond the external cloacal aperture.

The semen enters the penis at its root through a narrow opening situated close to the junction of genito-urinary sinus and cloaca.

For a short period, therefore, the human embryo and the embryos of the higher mammalia present conditions which correspond to the permanent structure of the parts in these lower vertebrates. In human embryos of 11.5 mm. cervico-coccygeal measure (32-33 days) (Fig. 28), the cloaca appears as a short sac continuous dorsad with the intestine, ventrad with the rudiment of the urinary bladder. The larger portion of the caudal gut (postanal gut) has disappeared, having been reduced to a thin epithelial strand which gradually becomes entirely absorbed. Only the proximal portion of the end-gut is used for the development of the cloaca, which, however, at first has no external opening (Fig. 28).

The tail end of the embryo becomes more extended and between it and the umbilical cord an interval appears in which the genital protuberance develops. Behind this point the ventral cloacal wall is formed by the cloacal membrane.

A considerable interval also develops between the points of entrance into the cloaca of the intestine proper and of the allantoic stalk (urinary bladder). The growth of the mesoderm pushes the intestine against the sacral vertebrae, while the stalk of the allantois with the rudimentary urinary bladder is forced against the ventral abdominal wall. These changes prepare the way for the first appearance of the _genito-urinary sinus_. The neck of the embryonic bladder elongates and receives the ducts of the urinary and genital glands (Fig. 29). In embryos of 14 mm. cervico-coccygeal measure (36-37 days) (Figs. 29 and 30), the genito-urinary sinus perforates the cloacal membrane on the ventral aspect of the genital protuberance, forming the _uro-genital cleft_. The rectum remains closed for a few days longer. The perforation is preceded by the formation of a transverse ectodermal reduplication, producing a depression called the _transverse anal fissure_. This depression increases in depth until a distinct anal invagination results, known as the _proctodaeum_, which grows as a funnel-shaped fossa toward the blind termination of the endgut. In embryos of 25 mm. cervico-coccygeal measure (81/2-9 weeks) the intestine still ends in a blind pouch. The anus is, therefore, independent of the end-gut in its development. It is derived from the ectoderm and its production is analogous to the formation of the oral cavity by means of the ectodermal invagination called the _stomadaeum_.

Finally the cloaca is converted into a ventral tube from which part of the urinary bladder, the urethra and genito-urinary sinus develop, and a dorsal tube from which the _rectum_ is derived. This double disposition of the cloaca is accomplished by gradual changes in the entoderm and mesoderm. The entoderm proliferates until a partition is formed which separates the two divisions of the cloacal tube from each other, and the mesoderm likewise increases, surrounding the newly formed entodermal tubes with tissue from which the muscles, connective tissue and blood vessels of the parts are derived (Figs. 28 and 29).

This partition, the _septum uro-rectale_, develops symmetrically on each side, appearing first as paired folds on the right and left sides called the _internal perineal folds_ (Figs. 28 and 29). When these folds have reached the cloacal membrane they complete the separation of the cloaca into two adjacent canals. Each of these canals is still closed caudad by its respective portion of the cloacal membrane, now divided into an _anal_ and _uro-genital_ segment. These two portions of the original cloacal membrane become perforated separately, the uro-genital before the anal. Hence the external opening of the uro-genital sinus is the first to appear, to be followed by the anal perforation. The internal perineal folds are supplemented by the formation of similar external folds, ridges of mesoderm tissue which surround the anal orifice in the form of a low wall and thus deepen the anal ectodermal invagination into the fossa of the proctodaeum.

These developmental stages in the formation of the end-gut are of importance because they offer the explanation of the pathological conditions which result from an arrest of development and from the failure of either the uro-genital or anal opening to form in the usual manner. These malformations must date back to an early stage, and probably have their inception in disturbances occurring in the normal development between the 15th and 23d day (embryos of 3-6 mm.). Perhaps in some cases of atresia there may be a secondary obliteration of a previously formed opening. In Fig. 31 the proctodaeum persists but the perforation of the anal membrane into the end-gut has not occurred. The ectoderm of the anal fossa and the intestinal entoderm remain separated by a transverse mesodermal partition. Different degrees of this malformation are observed. The layer separating the skin from the blind end of the rectum may be so thin that the meconium contained in the latter can be felt through it. On the other hand the rectum may terminate high up in a blind pouch, which is separated from the skin by a distance of several centimeters.

We may now briefly consider the genetic, histological and mechanical conditions which the above-outlined course of development imposes on the alimentary tract.

The ectoderm forms the superficial covering of the embryo and in the dorsal axial line develops the medullary groove which subsequently becomes converted into the cerebro-spinal axis by closure of the medullary plates and inclusion of the neural tube within the surrounding mesoblast (Fig. 18). The entoderm forms the epithelial lining of the interior of the alimentary canal and its appendages and derivatives (Fig. 19). The mesoderm furnishes the skeletal, muscular and vascular systems. At first single, like the two remaining layers of the blastoderm, the mesoderm splits early on each side of the chorda dorsalis into two layers, including between them spaces which after coalescence form the _primitive pleuro-peritoneal_ or _body-cavity_ (Fig. 20). One of these mesodermal layers bounding this space becomes closely connected with the ectoderm, forming the _somatopleure_ or body wall, while the other joins the entoderm to complete the wall of the alimentary canal, forming the _splanchnopleure_. In the course of further development the edges of these two layers approach each other ventrally in the median line and finally fuse.

The products of this fusion are two epithelial tubes, one included within the other, with walls reinforced by tissue derived from the two layers of the mesoderm. The internal or entodermal tube is of much smaller diameter than the outer or ectodermal tube, but much longer. The walls of the two tubes are placed in contact with each other by their mesodermal elements dorsally in the axial line, but elsewhere are separated from each other by the body-cavity (except in the region of the ventral mesogastrium).

The splanchnopleure is not so wide as the somatopleure. As it closes in the ventral median line it includes the deepest or entodermal layer. It now forms a tube whose walls are composed superficially of mesoderm (splanchnopleure) while the lumen is lined by epithelium derived from the entoderm. This tube is the _primitive enteric_ or _alimentary canal_. The somatopleuric layers bounding the body cavity take a wider sweep and after they have united ventrally in the median line they embrace a much more extensive space, the _primitive body cavity_ or _coelom_. The walls of this space are largely made up of the skeletal and muscular elements developed from the mesoderm of the somatopleure, covered superficially by the common ectodermal investment of the body. It will be seen that the enteric tube thus becomes included within the wider and more capacious coelom cavity.

Both the somatic and the splanchnic leaf of the mesoderm consist at first solely of a layer of flattened epithelial cells, the mesothelium. But very early this tissue is increased to form a massive layer by direct development from the mesothelium. The new mesodermal cells thus produced constitute the _mesenchyma_, which includes the whole of the mesoderm of the embryo except the mesothelial lining of the coelom. The cells of the mesenchyma, connected with each other and with the mesothelial cells by protoplasmic processes, are not as close together as in an epithelium and do not form a continuous membrane. By migration and multiplication a large mass of mesodermal tissue is produced which fills the entire space between the mesothelium and the primary germ layers. The mesenchymal tissue between the mesothelium and the ectoderm forms the mass of the skeletal, muscular and vascular systems. The mesenchymal tissue between the mesothelium and the entoderm forms an important constituent of the alimentary canal and of its appendages. The entoderm furnishes the internal epithelial lining of the tube upon which the performance of the specific physiological function of the entire apparatus depends. This epithelial tube is covered from without by the splanchnic mesoderm. The mesodermal elements thus added to the enteric entodermal tube consist of connective tissue and muscular fibers. The latter, arranged in the form of circular and longitudinal layers, control the contractility of the tube and regulate the propulsion of the contents. The connective tissue of the splanchnic mesoderm appears as an intermediate layer uniting the epithelial lining and the muscular walls. Situated thus between the mucous and muscular coats of the intestine this layer is known as the _submucosa_. It contains, imbedded in its tissue, the glandular elements of the intestine derived from the entodermal epithelium, and the blood vessels, lymphatics and nerves. The second chief function of the splanchnic and somatic mesoderm is the production of the serous membrane investing the body cavity and its contents from the mesothelium lining the primitive coelom. This mesothelial tissue, differentiated as a layer of flattened cells, lines the interior of the body cavity and covers the superficial aspect of the enteric tube. By subsequent partition of the common coelom the great serous membranes of the adult, the pleurae, pericardium and peritoneum, are developed from it.

The entodermal enteric tube is, as already stated, closely attached at an early period along its dorsal surface to the axial rod of mesoderm containing the chorda dorsalis immediately ventrad of the neural canal. In the earliest stages, just after the splanchnopleure and somatopleure have closed to complete the alimentary tube and body cavity, the remnant of these layers extends between the ventral abdominal wall and the ventral surface of the intestine forming a partition which divides the body into a right and left half. (Fig. 32, _A._) For the most part this primitive connection between the ventral abdominal wall and the intestinal tube is lost very early. The stomach, however, is always connected by a ventral mesogastrium, from which the lesser omentum is derived, to the ventral body wall. The disappearance of the ventral mesentery caudad of this point establishes the condition indicated in Fig. 32, _B._ The entodermal tube and the surrounding splanchnic mesoderm forming the intestinal canal is attached along its dorsal surface to the axial mesoderm of the dorsal mid-line. The primitive mesothelial peritoneum is reflected along this line from the internal surface of the body wall upon the ventral and lateral surfaces of the intestine. The coelom of one side communicates ventrad of the intestine with the coelom of the opposite side. Hence by the disappearance of the ventral mesentery caudad of the stomach the paired body-cavities have become fused into a single abdominal cavity--while cephalad the original division into right and left halves is maintained by the portion of the ventral mesentery which attaches the stomach to the ventral abdominal wall. The mesodermal tissue which at this time attaches the alimentary tube along its entire extent to the dorsal wall of the coelom carries the primitive embryonic arterial vessel, the aorta. This vessel supplies a series of small branches to the intestine, which reach the same by passing ventrad imbedded in the mesoderm connecting the tube to the dorsal body wall.

With the further development of the alimentary canal a gradual elongation of this connecting band of mesoderm and of the contained vessels is observed, the tube itself gradually receding from the vertebral axis. The early broad attachment is replaced by a narrower stalk into which the mesoderm is drawn out. With this narrowing in the transverse and elongation in the sagittal direction the connecting tissue assumes the character of a thin membrane with two free serous surfaces, including the intestinal vessels imbedded between them. Coincident with this elongation of the enteric attachment and its narrowing in the transverse direction the primitive intestine becomes more completely invested by the serous lining membrane of the coelom cavity. In this stage we can speak of the double-layered membrane attaching the tube to the dorsal body wall and carrying the intestinal blood-vessels as the primitive dorsal mesentery. The intestinal canal itself is invested by serous membrane except along a narrow strip of its dorsal border where the mesentery is attached and where the vessels reach the intestine. We can now distinguish the serous lining membrane of the abdominal cavity, derived from the mesothelium of the splanchnic and somatic mesoderm as the _peritoneum_. The membrane presents the following topographical subdivisions:

1. _Parietal Peritoneum_, lining the inner surface of the abdominal walls.

2. _Visceral Peritoneum_, investing the external surface of the intestine and its derivatives.

3. _Mesenteric Peritoneum_, connecting these two, carrying the intestinal blood vessels and lymphatics and acting as a suspensory support to the alimentary canal.

The dorsal mesentery in fishes, amphibia and reptiles contains smooth muscular fibers derived from the mesoderm. These bands of smooth muscle fibers are also encountered, though less well developed, in the mesentery of birds and mammals. The so-called "suspensory muscle of the duodenum" belongs to this category. It consists of a few strands of unstriped muscular and fibrous tissue which passes from the praeaortal tissue around the origin of the superior mesenteric artery and coeliac axis to the duodeno-jejunal angle. Fasciculi from this band may penetrate into the root of the mesentery (Gegenbaur).

Similar muscular fasciculi have been observed in the peritoneal folds of the ileo-caecal junction (Luschka) and in the mesorectum--forming in the latter situation the recto-coccygeal muscles of Treitz, and in the female the recto-uterine muscles.

In its earlier stages the primitive common mesentery forms a membrane which carries the intestinal blood vessels between its two layers, surrounds the embryonic alimentary canal and attaches the same to the ventral aspect of the chorda dorsalis and aorta. This is the permanent condition in many of the lower vertebrates in which the intestinal tube is suspended by a simple dorsal mesentery, a condition which is repeated by the embryos of man and the higher vertebrates. From this primitive common mesentery are derived, by further development, displacement and adhesion, all the other mesenteries, omenta and peritoneal folds of the adult. The character and degree of these subsequent changes is determined by the increase in length and change in position of the intestine and the growth of large organs, like liver, spleen and pancreas. Many portions of the intestinal canal, at first suspended by the mesentery and freely movable within the abdominal cavity, become later, by secondary adhesion, firmly connected with adjacent portions of the tube or with the abdominal parietes.

In certain of the lower vertebrates (fishes) large sections of the intestine lie entirely free within the abdomen, their only connection with the parietes being afforded by the blood vessels. This condition depends upon _absorption_ of the original mesentery. A similar process, though much more circumscribed, is observed in the omenta of many mammals, which appear perforated at several points.

=Derivatives of the Entodermal Intestinal Tube.=--The entodermal epithelium is physiologically the characteristic element of the alimentary canal. Besides lining the entire internal surface of the tube it gives rise by budding and protrusion from the intestinal canal to a series of organs which from the mode of their development must be regarded as diverticular or derivatives of the alimentary canal (Figs. 33, 34, and 35). These organs, proceeding in order cephalo-caudad, are the following:

The salivary glands. Thymus and thyroid. The lungs. Pancreas. Liver.

The epithelium of all these structures is derived from the primitive entoderm of the intestinal tube, except the epithelium of the salivary glands, which, being derived from the stomadaeal invagination, is ectodermal in character. We have previously noted the general history and appearance of the yolk-sac and its connection by means of the vitello-intestinal duct with the intestine. In contradistinction to the adult organs just noted the yolk-sac or umbilical vesicle is merely a temporary embryonal appendage to the alimentary canal. It also differs from them in the fact that it is not an extension or budding from the completed intestinal tube, like the liver and pancreas, but indicates, by the implantation of the duct (Fig. 21), the last point at which closure of the intestinal canal takes place, when after obliteration of the duct the separation of the intestine from the yolk-sac is completed.

The segment of the primitive alimentary canal cephalad of the attachment of the vitello-intestinal duct gives rise to the pharynx, oesophagus, stomach, proximal portion of small intestine proper and its derivatives, the liver and pancreas.

The portion situated caudad of the duct produces the rest of the small and all of the large intestine (Figs. 33 and 35). At times in man and other mammals (cat) the vitello-intestinal duct does not become absorbed, but persists and continues to develop as a part of the small intestine, forming the blind pouch or appendage known as _Meckel's diverticulum_ (Figs. 37 and 38). This diverticulum may vary in length from 1.5 to 15 cm. It either projects freely into the abdominal cavity as a pouch arising from the convex border of the small intestine opposite to the mesenteric attachment, or else it reaches the abdominal wall at the umbilicus and is attached to the same. In a few instances it has not terminated in a blind pouch, but has remained open at the umbilicus, in which case the aperture discharges intestinal contents. Sometimes the process of obliteration which normally leads to the absorption of the vitello-intestinal duct extends to the adjoining segment of the small intestine, resulting in obliteration of the intestinal lumen and consequent obstruction at this point.

The intestinal opening of the diverticulum is situated at a varying distance above the ileo-colic junction, ranging from 27.5 cm. to 290 cm., with an average of 107 cm.

While the obliteration and complete absorption of the duct is normal in nearly all vertebrates, a remnant persists in some birds, in which a short caecal pouch (_diverticulum caecum vitelli_) is found at about the middle of the small intestine. A portion of the vitello-intestinal duct thus persists throughout life in some wading and swimming birds. Figs. 39 and 40 show this condition in the small intestine of _Urinator lumme_ and _imber_, the red-throated loon and the great northern diver. In other birds, however, such as birds of prey, song birds, etc., the duct is absorbed and disappears completely.

In order to complete the embryological history of the alimentary canal it is necessary to take brief account of another structure derived from it, namely the _allantois_. Its significance to the adult organism is seen in connection with the genito-urinary tract, the urinary bladder being formed by its persistent portion. In the embryo, however, it has important nutritive and respiratory functions. In the embryos of the higher vertebrates nutrition depends only in the earliest stages upon the yolk-sac of the ovum, over which a vascular network extends.

Very soon the caudal portion of the primitive intestine develops a vascular sac-like outgrowth (Figs. 21 and 41). This pouch forms the _allantois_. It is intimately connected with embryonal respiration, and probably also forms a reservoir which receives the secretion of the primitive kidney. This foreshadows the final destiny of the proximal intra-abdominal portion of the allantoic sac which persists and is converted into the urinary bladder of the adult.

The allantois is present in Amphibia but is very small. In Amniota[1] it is large and grows around the embryo. In those of the higher vertebrates which are developed within an egg (reptiles, birds and monotremes) the sac of the allantois comes to lie beneath the egg-shell and acts as a respiratory organ. In the higher mammalia, developed within the uterus, the allantois becomes attached by vascular villi to the uterine wall and establishes a vascular connection between the foetal and maternal blood vessels. In this way the _allantoic placenta_ is formed (Fig. 41). The placenta, as just stated, is absent in the monotremes and is only slightly developed in marsupials, in which animals the foetus develops to maturity in the marsupial pouch after leaving the uterus. These animals are therefore distinguished as _Aplacentalia_ from the remaining higher mammals in which the allantoic placenta develops and which are hence called the _Placentalia_.

[1] In the embryos of reptiles, birds and mammals folds of the somatopleure arise externally to the constricting furrows by means of which the embryo is gradually separated from the yolk-sac, with the resulting formation of the intestinal and abdominal walls. These folds, situated at the head, tail and on the sides, grow upwards and finally meet and unite to form a membranous sac called the _amnion_. Hence these higher vertebrates (reptiles, birds and mammals) are called _Amniota_, in contradistinction to fishes and amphibia who have no amnion and are hence known as _Anamnia_.

=Summary.=--To recapitulate, therefore, the intestinal tube gives origin to two kinds of appendages or derivatives:

1. Organs of the adult body, derived by budding from the alimentary entodermal epithelium, in the form of pouch-like diverticula which follow the glandular type of development and become secondarily associated with mesodermal elements. These organs are again of two kinds:

(_a_) _Organs which retain their original connection with the lumen of the digestive canal:_

The salivary glands,} The liver, } Connected by their ducts with the digestive The pancreas, } canal. The lungs, }

which open by means of the trachea and the laryngeal aperture into the pharyngeal cavum.

(_b_) _Organs which lose their primitive connection with the alimentary canal._

Thymus and Thyroid Gland.

2. Embryonic appendages of the alimentary tract.

(_a_) The vitello-intestinal or omphalo-mesenteric duct and the yolk-sac or umbilical vesicle. This structure does not form as an extension from the intestinal tube after the same has been closed by coalescence of the splanchnopleure in the ventral mid-line, but is the result of the folding in of the layers of the embryonic germinal area, by means of which the body-rudiment is constricted off from the yolk-sac. The reduced channel of communication forms the vitello-intestinal duct. In the vast majority of vertebrates this disappears completely by absorption in the course of further development. It may persist in part abnormally as Meckel's diverticulum. In a few birds its proximal portion remains normally as a small blind pouch attached to the free border of the small intestine.

(_b_) The allantois. This is a hollow outgrowth from the embryonic intestinal canal of the higher vertebrates, performing important functions in connection with the early nutrition of the embryo. In the course of subsequent development its proximal portion, situated within the abdominal cavity, becomes converted into the urinary bladder. In mammals it loses its original connection with the intestinal canal and is assigned entirely to the genito-urinary tract. In some of the lower vertebrates, amphibia and reptiles it retains its connection with the ventral wall of the cloaca throughout life. (See Fig. 42, genito-urinary tract of _Iguana tuberculata_.)

After the intestinal canal has become separated from the yolk-sac it forms at first a straight tube, running cephalo-caudad beneath the chorda dorsalis. In most forms, however, the intestine grows much more rapidly in length than the body-cavity of the embryo in which it is contained. Hence the intestine is forced to form coils or convolutions.

The entire alimentary canal, from the mouth to the anus, can be separated into the following divisions and subdivisions:

=I. Foregut=, including

1. The oral cavity. 2. The pharynx. 3. The oesophagus. 4. The stomach.

=II. Midgut=, closely associated at its beginning with the liver and pancreas.

It extends between the pyloric extremity of the stomach and the beginning of the last segment, the endgut, frequently separated from both by ring-like aggregations of the circular muscular fibers and corresponding projections of the mucous membrane (pyloric and ileo-colic valves).

The midgut is usually the longest portion of the intestinal tube.

=III. Endgut=, the last segment of the intestinal canal, courses through the pelvic portion of the body cavity. From this short end-piece are developed: (1) The colon, sigmoid flexure and rectum; (2) the cloaca with the uro-genital sinus and the duct of the allantois.