Book - The Elements of Embryology - Mammalian 5
The Alimentary Canal and its Appendages
It is convenient to introduce into our account of the organs derived from the hypoblast, a short account of certain organs connected with the alimentary canal such as the mesentery, stomodaeum, etc., which are not hypoblastic in origin.
The origin of the hypoblast, and the process of folding by which the cavity of the mesenteron is established have already been described. The mesenteron may be considered under three heads.
1. The anterior or respiratory division of the mesenteron. The pharynx, thyroid body, Eustachian tube, tympanic cavity, oesophagus, trachea, bronchi, lungs and stomach are developed from this portion, and their development in the Mammal so closely resembles that in the Chick that it is unnecessary for us to add to the account we have already given in the earlier part of this work.
This section of the alimentary canal, as in the Chick, is distinguished in the embryo by the fact that its walls send out a series of paired diverticula which meet the skin, and, after perforation has been effected at the regions of contact, form the visceral clefts.
2. The middle division of the mesenteron, from which the liver and pancreas are developed, as in the Chick, forms the intestinal and cloacal region and is at first a straight tube. It remains for some time connected with the yolk sack.
The Cloaca appears as a dilatation of the mesenteron which receives, as in Aves, the opening of the allantois almost as soon as the posterior section of the alimentary tract is established. The eventual changes which it undergoes have already been dealt with in connection with the urinogenital organs.
The intestine. The posterior part of this becomes enlarged to form the large intestine, while the anterior portion becoming very much elongated and coiled forms the small intestine, and moreover gives rise anteriorly to the liver and pancreas.
From the large intestine close to its junction with the small intestine an outgrowth is developed, the proximal part of which enlarges to form the ccecum, while the distal portion in Man forms the vermiform appendix.
3. The postanal division of the mesenteron atrophies at an early period of embryonic life. In the Chick and lower types it communicates for a short time with the hind end of the neural canal.
Splanchnic mesoblast and mesentery
The mesenteron consists at first of a simple hypoblastic tube, which however becomes enveloped by a layer of splanchnic mesoblast. This layer, which is not at first continued over the dorsal side of the mesenteron, gradually grows in, and interposes itself between the hypoblast of the mesenteron, and the organs above. At the same time it becomes differentiated into two layers, viz. an outer epithelioid layer which gives rise to part of the peritoneal epithelium, and an inner layer of undifferentiated cells which in time becomes converted into the connective tissue and muscular walls of the mesenteron. The connective tissue layers are first formed, while of the muscular layers the circular is the first to make its appearance.
Coincidently with the differentiation of these layers the connective tissue stratum of the peritoneum becomes established.
The mesentery is developed as in the Chick (p. 172). In the thoracic region it is hardly if at all developed. The primitive simplicity in the arrangement of the mesentery is usually afterwards replaced by a more complicated disposition, owing to the subsequent elongation and consequent convolution of the intestine and stomach.
The layer of peritoneal epithelium on the ventral side of the stomach is continued over the liver, and after embracing the liver, becomes attached to the ventral abdominal wall. Thus in the region of the liver the body-cavity is divided into two halves by a membrane, the two sides of which are covered by the peritoneal epithelium, and which encloses the stomach dorsally and the liver ventrally. The part of the membrane between the stomach and liver is narrow, and constitutes a kind of mesentery suspending the liver from the stomach : it is known to human anatomists as the lesser omentum.
The part of the membrane connecting the liver with the anterior abdominal wall constitutes the falciform or suspensory ligament of the liver. It arises by a secondary fusion, and is not a remnant of a primitive ventral mesentery (vide p. 264).
The mesentery of the stomach, or mesogastrium, enlarges in Mammalia to form a peculiar sack known as the greater omentum.
The anterior section of the permanent alimentary tract is formed, as in the Chick, by an invagination of epiblast, constituting a more or less considerable pit, with its inner wall in contact with the blind anterior extremity of the mesenteron.
From the epiblastic liniog of this pit are developed the pituitary body and the salivary as well as the other buccal glands.
A palate grows inwards from each of the superior maxillary processes (Fig. 141), which, meeting in the middle line, form a horizontal septum dividing the front part of the stomodaeum into a dorsal respiratory section, containing the opening of the posterior nares, and a ventral cavity forming the permanent mouth. These two divisions open into a common cavity behind. This septum on the development within it of an osseous plate constitutes the hard palate. A posterior prolongation in which no osseous plate is formed constitutes the soft palate. An internasal septum (Fig. 141 e) may more or less completely divide the dorsal cavity into two canals, continuous respectively with the two nasal cavities.
The teeth are special products of the oral mucous membrane. They are formed from two distinct organs, viz. an epithelial cap and a connective tissue papilla, which according to most authors give rise to the enamel and dentine respectively.
The proctodaeum. The cloacal section of the alimentary canal is placed in communication with the exterior by means of a shallow epiblastic invagination constituting the proctodseum.
The Elements of Embryology - Volume 2 (1883)
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