622 THE DIGESTIVE SYSTEM 



(d) The pyloric area is the last segment of the stomach and is joined to 

 the intestine. It has numerous glands, producing a mucus-like secretion. 



The pig's stomach resembles closely that of the human. 



If we compare the general morphogenesis of the stomach in the pig or human 

 with that of the shark, frog, chick, or the cow, the following diflferences exist. 



The shark stomach is composed mainly of fundic and pyloric segments (fig. 

 279C). The stomach of the frog closely resembles that of the pig (fig. 280F). 

 Unlike the pig, however, the frog is able to evert the stomach by muscular 

 action projecting it forward through the mouth to empty its contents. In the 

 chick (fig. 28 IE), an area of the esophagus expands into a crop which func- 

 tions mainly as a food-storage organ. A glandular stomach (proventriculus), 

 comparable to the fundus of the pig, is formed posterior to the crop, while, 

 still more caudally, a highly muscular gizzard or grinding organ is elaborated. 



In the cow or sheep, an entirely diflferent procedure of development pro- 

 duces a greatly enlarged, distorted, esophageal portion of the stomach. This 

 esophageal area of the stomach comprises the rumen, the reticulum or honey- 

 comb stomach, and the omasum (psalterium) or manyplies stomach. The distal 

 end of the stomach of the cow or sheep is the abomasum or true stomach, 

 comparable to that of the human or pig described above (fig. 292D). 



d. Morphogenesis and Histogenesis of the Hepato-pancreatic Area 

 The hepato-pancreatic area of the digestive tract is a most important one. 

 Its importance springs not only from the development of indispensable glands 

 but also from the relationship of the liver to the developing circulatory system 

 (Chap. 17) and the division and formation of the coelomic cavity. (See 

 Chap. 20.) 



1) Development of the Liver Rudiment. The liver begins in all vertebrates 

 as a midventral outpushing of the primitive metenteron, immediately caudal 



Fig. 294. Development of the liver and pancreatic rudiments. (Diagrams C-E, re- 

 drawn from Lillie, 1930, The development of the chick. Holt, N. Y. F redrawn from 

 Thyng, 1908, Am. J. Anat.) (A) Developing liver rudiment in 10 mm. embryo of 

 the dogshark, Squalus acanthias. (B) Developing liver in tadpole of Rana pipiens. 

 (See also Figs. 221, 223, 225, 280.) (C) Developing liver rudiments in the 3rd-day 

 chick. (D) Developing liver in early 4th-day chick. (E) Developing liver in late 

 4th-day chick. (F) Hepatic evagination in 7.5 mm. human embryo. (G) Relation of 

 the fully developed liver to associated structures in various vertebrates. (Gl) Squalus 

 acanthias. The liver is suspended from the posterior surface of the septum transversum 

 by the coronary ligament. (G2 and G3) Frog, Rana pipiens. G2 transverse view; G3 

 sagittal view. (G4 and G5) 16-20 day chick. Callus doinesticus. G4 transverse view. 

 Observe that the liver lobes and peritoneal cavity have grown forward on either side of 

 the heart and have separated the heart and pericardial cavity from the ventro-lateral 

 body walls. G5 is a left ventral view of the heart, pericardial cavity, and liver. Left lobe 

 of the liver is removed. Observe that the septum transversum is applied to the posterior 

 wall of the parietal pericardium. G6 Mammal. The septum transversum has been com- 

 pletely displaced by developing diaphragmatic tissue. The liver is suspended from the 

 caudal surface of the diaphragm by the coronary ligament. 



