204 THE ENDODERMAL DERIVATIVES 



are connective tissue, blood, and lymph vessels, all of mesodermal 

 origin. 



The Liver. 



The liver originates very early as a single median ventral endoder- 

 mal diverticulum which is directed posteriorly between the heart rudi- 

 ment and the yolk mass. The diverticulum enlarges slowly and its 

 anterior wall will thicken, become folded, and finally branched, to 

 form the liver proper. The ultimate lobes of the liver will retain their 

 tubular connection with the original diverticulum as the hepatic duct. 

 The original diverticulum will elongate as the bile duct leading to 

 a terminal vesicle, the gallbladder, which becomes very large in the 

 tadpole. All of these derivatives become invested with connective 

 tissue and blood from the splanchnic mesoderm, but some of the ad- 

 joining yolk cells become the true hepatic cells. 



The Pancreas. 



The pancreas arises as three rudiments at about the 9 mm. stage 

 in a manner somewhat similar to the liver. The first rudiment appears 

 as a single posteriorly directed ventral diverticulum of the bile duct 

 at its point of entrance into the foregut. This diverticulum soon 

 divides into two and the cellular elements grow around the bile duct 

 to fuse into a single mass of spongy tissue anterior to the bile duct. 

 Subsequently a third mass of similar tissue arises from the dorsal 

 wall of the gut, and attains junction with the original masses, all three 

 to form the much-lobulated pancreas. These three pancreatic rudi- 

 ments then use the single pancreatic duct which retains its original 

 connection with the gut, just posterior to the liver or hepatic duct. 

 It marks the boundary between the foregut and the midgut. The 

 cells of the islet of Langerhans arise early from endoderm. 



The Oesophagus and Stomach. 



After hatching, the undifferentiated portion of the foregut between 

 the glottis and the bile duct elongates to become the oesophagus. Dur- 

 ing early larval development this part of the gut becomes temporarily 

 occluded by an oesophageal plug of cells whose origin is unknown. 

 Its function during its brief appearance may be to help direct any 

 water and food from the mouth out over the gills. These early larvae 

 require no external source of food as they are supplied with abundant 



