. Chordate morphology. Morphology (Animals); Chordata. bile duct. spleen. Figure 9-4. (Dide/phys). large intestine Ventral view of the digestive tract of the opossun dorsal body wall, or lies retroperitoneally; it is attached to the large intestine in this area as a result of the rotation of Meckel's tract. A good part of the large intestine is usually involved in a rotated loop, in the bat and the tree shrew. Embryological development In mammals the lining of the digestive tract and its glands is formed of entoderm. The bulk of the wall and a part of the glands are formed from mesoder
. Chordate morphology. Morphology (Animals); Chordata. bile duct. spleen. Figure 9-4. (Dide/phys). large intestine Ventral view of the digestive tract of the opossun dorsal body wall, or lies retroperitoneally; it is attached to the large intestine in this area as a result of the rotation of Meckel's tract. A good part of the large intestine is usually involved in a rotated loop, in the bat and the tree shrew. Embryological development In mammals the lining of the digestive tract and its glands is formed of entoderm. The bulk of the wall and a part of the glands are formed from mesoderm. In the early embryo the formation of the head fold produces a forward-projecting pocket of entoderm, the foregut; the similarly produced tail fold gives rise to the hindgut (Figure 9-6). The embryo is gradually separated from the yolk sac by constriction of the yolk stalk. The fore- gut gives rise to pharynx, lung buds, esophagus, stomach, duodenum, and the liver and pancreatic diverticula. The midgut forms much of the small intestine, while the hind- gut forms the large intestine. These are arbitrary assign- ments for rather vague regions. The liver and pancreas arise behind the stomach region, which has expanded and arched to the left (Figure 9-7). The liver appears first; it arises as a ventral diverticulum. This diverticulum branches many times giving rise to a mass of cell cords embedded in mesoderm. These cell cords produce the secretory cords of the definitive gland. Another diverticulum appears from the main duct of this gland which grows out posteriorly and gives rise to the gall bladder. The pancreas arises from dorsal and ventral diverticula. The first opposite the duct of the liver and the second just to the left of the liver duct or from that duct to the left of the gall bladder evagination. In the process of development the ducts of the dorsal and ventral pancreatic outgrowths cross. Frequently the dorsal pancreas comes to drain through the ventral duct, but so
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