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Position of the various parts of the alimentary canal at different stages. A. Embryo of five

weeks; B. of eight weeks; C. Of ten weeks. (Allen Thomson.) 1. Pharynx with the lungs; s. Stomach; i. Small intestine ; 2°. Large intestine; g. Genital

duct; u. Bladder; c. Cloaca ; c. Cæcum ; vi. Ductus vitello-intestinalis ; si. 'Urogenital sinus ; v. Yolk sac.


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Longitudinal section of a foetal sheep. (Cadiat.) a. Pericardium; b. Commencement of diaphragm ; c. Heart; d. Branchial arches ; e. Pha

rynx; f. Origin of lung; g. Liver.

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Fig. 293.

of tissue connected with the primitive intestine. The ductus vitello-intestinalis accordingly becomes obliterated, and thus the mid-gut is closed at its lower aspect.

The primitive intestine placed at the inferior aspect of the embryo, just below the protovertebræ, is lined by hypoblast and covered by mesoblast. The cephalic or anterior extremity of the canal is formed by ancleft mesoblast ; the rest of the canal is formed by the splanchnopleural layer.

A dilatation of a part of the fore gut gives origin to the primitive stomach; this is quite straight at first, lying below the vertebral column, with which it is connected by mesoblast. After a time the stomach becomes turned to the right side, so that the left surface of the organ lies anteriorly and the right posteriorly, the mesoblast connecting it with the vertebral column, being developed into the peritoneal processes of the organ. The lower part of the fore gut is of much smaller calibre than

the dilated portion forming the stomach ; it becomes the duodenum, in connection with which arise two important viscera, the liver and the pancreas. The mid-gut and hind gut form the small and large intestines, these being at first one straight tube, of which the small intestine has the larger calibre. The small intestine, as it grows, falls into folds, and the mesoblast connecting it to the vertebral columns forms the mesentery.

The large intestine is at first a Diagram of the alimentary canal of

straight tube lying to the left of the a chick at the fourth day. (Foster and Balfour, after Götte.)

embryo; it becomes bent, and part lg. Diverticulum of one lung. St. of the tube is directed toward the Stomach. 1. Liver. p. Pancreas.

right side ; this develops another flexure, the portion of intestine below which grows downward. That part remaining on the left side forms the rectum, the sigmoid flexure, and the descending colon ; while that between the flexures becomes the transverse colon, and that on the right side the ascending colon.

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The cæcum is developed from the ascending colon, the ileocæcal valve shutting off one part of the intestinal canal from the other. The vermiform appendix originates from the inferior extremity of the cæcum, which, owing to its feeble growth, is of much smaller calibre than the upper part.

The epithelial lining of the intestines is derived from the hypoblast, and the muscular, vascular, connective tissue and serous coverings are mesoblastic in their origin.

The liver is developed from two diverticula of the duodenum, in connection with which arise cylinders of cells. The hypoblast develops into the liver cells and the cells lining the ducts, the mesoblast furnishing the vascular and connective tissue parts of the organ. The two diverticula are connected by a transverse piece, and form the right and left lobes of the liver.

The process connecting the liver to the duodenum forms the common bile duct, and from this the gall bladder is developed as an outgrowth.

The vessels of the embryo, which are in relation to the liver, will be described under the vascular system.

The pancreas arises as an outgrowth from the duodenum, its constituent parts originating in a manner similar to those of the liver.

The spleen is derived from the mesoblast, and is developed in one of the peritoneal processes of the stomach.

The lungs are developed in connection with the esophagus, of which they are early outgrowths.

The canal of the fore gut at a certain point becomes laterally constricted, its transverse section presenting an hour-glass shape, consisting of an upper and lower dilated portion, united by a central constricted neck. The lower of these cavities becomes subdivided by the outgrowth of the lateral portions and the upgrowth of a part of the lower wall which forms a central septum, so that the fore gut is composed of an upper undivided tube, giving off two appendages.

These appendages consist of hypoblastic tissue, and as they grow into the surrounding mesoblast they divide and subdivide, until at last they consist of very minute lobules, which terminate in dilated extremities. The undivided canal forms the permament trachea, the appendages the main bronchi, while their minute subdivisions are the bronchioles, which terminate in the dilated alveoli.

The hypoblast forms the delicate lining membrane of the air passages, and the mesoblast gives rise to the supporting tissue holding them together, to the blood vessels, the muscular, cartilaginous and connective tissue of the bronchial tubes.

The pleure surrounding the lungs are, like the other serous membranes, mesoblastic in their origin.

GENITO-URINARY APPARATUS. In the interval between the protovertebræ and the cleavage of the mesoblast into its somatopleural and splanchnopleural layers,

of cells arranges itself into a longitudinal ridge. This ridge, which lies beneath the epiblast, becomes hollow, and thus a tube is produced, called the Wolffian duct.

From this tube diverticula arise, which extend into the sur

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Transverse section through the embryo of a chick on the second day, where the medullary

canal is closed. (Kölliker.) mr. Medullary canal. h. Epiblast. uwh. Cavity of protovertebra uw. ung: Wolffian duct.

mp. Mesoblast dividing. kpl. Somatopleure. df. Splanchnopleure, sp. Pleuroperitoneal cavity. dd. Hypoblast. ch. Notochord.

rounding mesoblast; they are tubular, and communicate with the central duct. The processes become twisted, and at their extremities the neighboring mesoblast undergoes differentiation, and forms vascular capsules corresponding in structure to the Malpighian corpuscles. This part of the Wolffian duct, which has acquired a glandular structure, is the Wolffian body or primitive kidney of the embryo, while the Wolffian duct corresponds to the primitive ureter.

The epithelium lining the interval between the somatopleure and splanchnopleure (pleuroperitoneal cavity) becomes columnar in character close to their origin from the uncleft mesoblast. It receives the name of the germinal epithelium. An involution of this takes place into the mesoblast, just below the somatopleure, and becomes shut off, forming a hollow cylinder.

FIG. 295.





Section of the inner part of the pleuroperitoneal cavity through the origin of the genito.

urinary organs. (Waldeyer.) L. Somatopleure. m. Splanchnopleure. a. Germinal epithelium. C, o. Primitive ova.

E. Mesoblast forming the ovary. WK. Wolffian body. y. Wolffian duct. a'. Epi. thelium giving rise to the duct of Müller z.

By this means a second duct is formed in close relation to the first; this is the Müllerian duct. This duct is developed from before backward.

According as the embryo is a male or a female, so one or other of these ducts develops. In the male the Wolffian duct remains as the vas deferens, and the Müllerian duct becomes atrophied.

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