STRUCTURE OF THE VERTEBRATES 195 



testine. The pancreas also functions as a gland of internal secre- 

 tion (Chapter XIX). 



2. The liver is a ventral outgrowth, almost opposite the pan- 

 creatic ducts. The homologous hepatic caecum of Amphioxus is 

 similar in development, but does not become highly glandular. 

 The embryonic hepatic caecum of the vertebrates grows an- 

 teriorly, the distal end dividing to form a mass of tubular 

 glands. The liver becomes attached to the diaphragm anterior 

 to the stomach, but the large duct, which is the original out- 

 pocketing, empties into the duodenum. A comparison with Am- 

 phioxus will explain the relationships in the vertebrate. 



The spleen is frequently considered with the digestive system 

 although it is mesodermal in origin, and functionally a part of 

 the vascular system. It has no duct leading from it. Its function 

 is (1) a blood forming organ in the lower vertebrates, and (2) a 

 blood corpuscle destrojung organ in the higher. 



B. Development of Mesodermal Visceral Structures 



The student should refer to the diagrams of the embryology 

 of Amphioxus and the reptile to understand the relationships of 

 the digestive organs in the body, and the origin of their muscular 

 walls and mesenteries. Recall that as the hypomere of the meso- 

 dermal somite grows distally from the notochord it divides inta 

 two layers, (1) the somatopleure (the outer or body layer) and 

 (2) the inner visceral layer, or splanchnopleure. The cavity be- 

 tween is the coelomic cavity. As the splanchnopleure pughes in 

 toward the median line the archenteron is separated from the 

 notochord and forced ventrally. The right and left sides of the 

 splanchnopleure enclose the gut and thicken to form the heavy 

 wall of smooth muscle with its peritoneal covering. Dorsally the 

 layers join to form the thin, strong mesentery which suspends 

 the gut from the dorsal wall of the body cavity. The thin layer 

 of mesodermal epithelium lining the coelomic cavity and cover- 

 ing the viscera is the peritoneum. 



It is evident that any outgrowth from the digestive tract will 

 push the mesodermal tissues out with it and become surrounded 

 by connective tissues and peritoneum. Therefore, although struc- 

 tures appear to lie in the coelomic cavity, they are technically 



