Book - Quain's Embryology 7
DEVELOPMENT OF THE ALIMENTARY CANAL AND ORGANS ARISING FROM THE HYPOBLAST
The whole alimentary canal, from the fauces to the anus, together with the rudiments of certain organs associated with it in their commencement, viz., the thyroid gland, lungs, trachea and larynx, the liver, and pancreas, as well as the allantois, owe their origin more immediately to inflections of the hypoblast layer of the germinal membrane, which supplies the epithelial lining of their principal cavities ; but in all these organs parts of their structure are supplemented, and some other organs, such as the mesentery and spleen, are wholly formed from the mesoblast, whence proceed the vascular, fibrous, and parenchymatous elements, and also the serous coverings of the organs, where these exist.
ALIMENTARY CANAL,
The primary digestive cavity of birds and mammals, as it extends from one end of the embryo to the other below the vertebral axis, presents at first a manifest division into three parts. One of these^ occupying the part of the embryo which is enclosed by the cephalic fold, and which may be named the foregut, comprises the rudiments of the pharynx and gullet, the stomach and duodenum. The posterior division, which is comparatively short, occupies the caudal fold of the embryo, and corresponds mainly to the lower part of the colon and rectum. Both of these parts have from the first a tubular form, and are closed respectively by the Inflection of the whole blastodermic layers at the anterior and posterior extremities of the body. The middle division has primarily the form of a long and wide groove, lying close below the corresponding part of the vertei)ral bodies, leading at its opposite ends into the cephalic and caudal portions of the gut, and is freely open throughout on its ventral aspect into the cavity of the yolk-sac, with the blastodermic walls of which, as formerly described, the constituents of the intestinal walls are directly continuous (see fig. 576).
The mouth, as elsewnere explained, is no part of the primitive alimentary canal, but is formed by involution of parts of the face, and receives, therefore, its lining membrane from epiblast. It is separated for a time from the pharynx, which is the foremost part of the primitive alimentary canal, by the reflection of the layers of the blastoderm, and the communication which is later established between the mouth and pharynx at the posterior arch of the fauces, is due to a solution of continuity in these layers, which occurs in the chick on the fourth day of incubation, and has been traced at a corresponding period of development in several mammals. The aperture has at first the form of a vertical slit, which widens later as it becomes the opening from the pharynx into the common cavity of the nose and mouth. The diverticulum of the pituitary gland, it will be remembered, occupyingthe place which becomes the top of the permanent pharynx, is formed in connection with the epiblastic or buccal, and not the hypoblastic or pharyngeal division of the alimentary passage (see fig. 535, A and B, py).
The hypoblastic layer of the germinal membrane, from which is derived the epithelial lining of the whole alimentary canal and passages communicating witli it, is at first extremely thin and simple, and is composed of flat cells ; but as soon as this layer comes to form a part of the. inflected alimentary tube, its character is completely altered, its cells become cylindrical, and it attains a great proportional thickness, which it preserves for a considerable time.
Fig. 576. — Early form OF THE Alimentary' Canal (from Kolliker after Bischotf).
In A a front view, and in B an antero-posterior section are represented.
a, four pliaryngeal or visceral plates ; h, the phai-ynx ; c, c, the commencing lungs ; d, the stomach ; /, /, the diverticula connected with the formation of the liver ; g, the yolk-sac into which the middle intestinal gi'oove opens ; h, the posterior part of the intestine.
The outer surface of the inflected hypoblast of the alimentary tube is more or less in contact with the splanchno-pleure division of the mesoblast. In the head no marked separation of the splanchnopleure and somatopleure divisions of the mesoblast takes place, but the elements of the former are no doubt combined with the hypoblast in the walls of the pharynx, and the formation of the tympano-eustachian and following pharyngeal clefts is therefore due to the perforation of both epiblastic and hypoblastic layers with intervening mesoblastic tissue, just as occurs in the formation of the opening of the fauces. But in the thorax and abdomen, the primitive alimentary canal is brought into relation with the pleuro-peritoneal cavity, and receives in various parts a serous investment from the lining membrane which becomes developed in that space. In the thorax the right and left cavities remain distinct as the two pleura, while a central portion is separated for the formation of the pericardium, and thus the gullet, as well as the lungs, is brought into relation with the pleura, and receives partial covering from them. The formation of the diaphragm, which does not at first exist, and which grows down from the vertebral column as a partition between the thorax and abdomen, leads to the ultimate separation of the peritoneum from the pleurte. Some examples of diaphragmatic hernia may be considered as arising from the persistence of the original connection between the two cavities. In the abdomen, also, the right and left peritoneal cavities are at first distinct, but when the intestine assumes a tubular form, the right and left cavities are thrown into one across the middle plane of the body.
As the development of the alimentary canal proceeds, the middle open part shortens, more and more of it being converted into the tubular intestiue, and at last, as before explained, there remains only the narrow opening by which the gradually lengthening ductus vitellointestinalis leads into the umbilical vesicle. The middle part of the intestinal canal has, when first produced, more or less the form of a straight tube lying close to the vertebral column ; but as it increases in length, it is thrown into the shape of a loop bent downwards to the umbilicus, — a change which is accompanied by the formation of the mesentery. The latter structure is undoubtedly entirely due to splanchnopleure mesoblastic elements, which, extending themselves between the proto-vertebral masses and the elongating gut, become developed into the vascular and other parts of the mesentery, as was long ago shown by Yon Baer. But the mesoblast, also, by its splanchnopleure division, furnishes the contractile fibrous, vascular, and connective tissue elements of the intestinal walls. The extent to which the glandular elements of the alimentary canal are supplied by the hypoblast, to which their origin was entirely attributed by Remak, or furnished rather by mesoblast from the proto vertebral mass, as held by Schenk, is not yet determined.
Fig. 577. — Human Embryo of TniRxr-FiVE DAYS SEEN FROM BEFORE (from Kcilliker after Coste).
3, left external nasal process ; 4, superior maxillary process ; 5, lower maxillary jirocess ; z, tongue ; b, aortic bulb ; b', first perrmmeut aortic arch, which becomes ascending aorta ; b" , second aortic arch ; b"' , third aortic arch or ductus Botalli ; ?/, the developing pulmonary arteries ; c, the superior cava and right azygos vein ; c', the common venous sinus of the heart ; c", the common stem of the left vena cava and left azygos ; o', left auricle of the heart ; v, right v', left ventricle ; a e, lungs ; c, stomach ; j, left omphalo-mesenteric vein ; s, continuation of the same behind the pylorus, which becomes afterwards the vena portaj ; x, vitello-intestinal duct ; a, right omphalo-mesenteric artei-y ; m, Wolffian body ; ?', rectum ; n, umbilical artery ; Vi umbilical vein ; 8, tail ; 9, anterior, U', posterior limb. The liver has been removed.
As development proceeds in the forepart of the alimentary canal, a change in its form manifests itself, by which one part, becoming dilated, forms the commencement of the stomach, while the others remain of smaller diameter as gullet and duodenum ; and in connection with different parts of these 'the rudiments begin to appear of the lungs, liver, and pancreas.
When the tubular parts of the gut have attained to some length, a change of position gradually accompanies their further development. While the oesophageal part remains comparatively straight, the dilated portion of the tube which forms the stomach turns over on its right side, so that the border, which is connected to the vertebral cohminby the membranous fold (or true mesogastrium) comes to be turned to the left, — the position of the tube being still vertical, like the stomach of some animals. By degrees it becomes more dilated, chiefly on what is now the left border but subsequently becomes the great curvature, and assumes first an oblique and finally a transverse position, carrying with
Fig. 578. — Outline of the Form ,and Position of the Alimentary Canal in Successive Stages of its Development.
A, alimentary canal, &c., in an emln-yo of five weeks ; B, at eight weeks ; C, at ten ■weeks ; D, at twelve weeks ; I, tbe primitive lungs connected with the pharynx ; s, the .stomach ; d, duodenum ; /, the small intestine ; i', the large ; c, the ciecum and vermiform appendage ; r, the rectum ; c /, in A, the cloaca ; a, in B, the anus distinct from s /, the sinus uro-genitalis ; r, the yolk-sac ; v i, the vitello-intestiual duct ; u, the urinary bladder and urachus leading to the allantois ; g, the genital ducts. In B, and 0, the thickness of the colon is erroneously represented as greater than that of the ileum.
it the mesogastrium, from which the great omentum is afterwards produced. A slight indication of the pylorus is seen at the third month. Upon the surface of the part of the canal which immediately succeeds the stomach, and which forms the duodenum, the rudiments of the liver, pancreas, and spleen are simultaneously deposited, in the manner to be stated in the description of the development of these organs.
The place of transition from the small to the large intestine, which is soon indicated by the protrusion of the cfECum, is at a point just below the apex or middle of the simple loop already mentioned, as accompanying the first elongation of the tubular gut. As the small intestine grows, the part below the duodenum forms a coil which at first lies in the commencing umbilical cord, but retires again into the abdomen about the twelfth week ; aftei'wards it continues to elongate, and its convolutions become more and more numerous.
The large intestine is at first less in calibre than the small. In the early embryo there is at first no csecum. This part of the bowel gradually grows out from the rest, and in the first instance forms a tube of uniform calibre, without any appearance of the vermiform appendix: subsequently the lower part of the tube ceases to grow in the same proportion, and becomes the appendix, whilst the upper portion continues to be developed with the rest of the intestine. The caecum now appears as a protrusion a little below the apex of the bend in the primitive intestinal tube, and, together with the commencing colon, and the coil of small intestine, is at first lodged in the wide part of the umbilical cord W'hich is next the body of the embryo. The ileo-cascal valve appears at the commencement of the third month. When the coils of intestine and cecum have retired from the umbilicus into the abdomen, the colon is at first entirely to the left of the convolutions of the small intestines, but subsequently the first part of the large intestine, together with the meso-colon, crosses over the upper part of the small intestine, at the junction of the duodenum and jejunum. The ca3cum and transverse colon are then found just below the liver ; finally, the ca3cum descends to the right iliac fossa, and at the fourth or fifth month the parts are in the same position as in the adult. At first, villous processes or folds of various lengths are formed throughout the whole canal. After a time these disappear in the stomach and large intestine, but remain persistent in the intermediate portions of the tube. According to Meckel, the villous processes are formed from larger folds, which become sen-ated at the edge, and divided into separate villi.
The formation of the hinder part of the gut is complicated with the development of the allantois, which arises as a projection or outgrowth of the hypoblast and mesoblast from the lower wall of its terminal portion. This part rapidly buds out in the pleuro-peritoneal space, having from a very early period a rich network of the branches of the umbilical vessels in its outer layer. The anal or cloacal portion remains behind the allantoid pedicle, and by the fifth day in the chick the whole of the tissues which close the terminal fold thin rapidly away, and become perforated so as to form the primitive anal, or rather the cloacal opening. The separation of the permanent anus fi'om the urogenital orifice is the result of a later process of development.
The mode of development of the alimentary canal explains, in some measure, the complicated folds of the peritoneum. The stomacii being originally more nearly in the line of the alimentary canal and mesial in position, the small omentum aad gastro-phrenic ligament are originally parts of a mesial fold with the free edge directed forwards, which afterwards forms the anterior boundary of the foramen of Winslow. Thus the anterior wall of the sac of the omentum, as far as the gi'eat curvature of the stomach, may be considered as formed by the right side of a mesial fold, while the peritoneum in front of the stomach belongs to the left side of the same, and a sac of the omentum is a natural consequence of the version and disproportionate growth of the tube between the duodenum and the cardiac orifice of the stomach. It is obvious that the view of the omental sac, according to which its posterior layers are held to return to the duodenum and posterior wall of the body before proceeding to form the transverse meso-colon (p. 484) is more consistent with the phenomena of development now described than that which would make them directly enclose the colon. On the other hand, the farther elongation of the omental sac and the whole disposition of the peritoneum, with respect to the colon, must be regarded as having taken place after the assumption by the great intestine of its permanent position.
Fig. 579. — Sketch op the Human Embryo of the Tenth Week, showing THE Coil of Intestine in the Umbilical Cord. (A. T.)
the amnion and villous chorion have been ojiened and the embryo drawn aside from them ; r, the umbilical vesicle or yolk-sac placed between the amnion and chorion, and connected with the coil of intestine, i', by a small or almost linear tube ; the figure at the side represents the first part of the umbilical cord magnified ; i, coil of intestine ; v i, vitello-intestinal duct, alongside of which are seen omphalo-mesenteric blood-vessels.
The occurrence of umbilical hernia in its various degrees may be referred to the persistence of one or other of the foetal conditions in which a greater or less portion of the intestinal canal is contained in the umbilical cord ; and it has been shown that the most common form of abnormal diverticula from the small intestine is connected with the original opening of the ductus vitello-intestinalis into the ileum.
