350 COMPARATIVE ANATOMY 



its greater part usually lying in the fold of the duodenum. In 

 some cases it remains embedded within the wall of the gut (i'.g. 

 Protopterus, cf. Fig. 247). Amongst Teleosts it may be in part 

 surrounded by the liver : in part, however, it does not form a com- 

 pact gland, but has the form of scattered lobules extending 

 throughout the mesentery. It is unrepresented in Amphioxus. 



In Petromyzon, a pancreas is developed in the embryo, and is 

 embedded in the wall and spiral fold of the gut and in the dorsal 

 portion of the liver. In Myxine and Bdellostoma also a glandular 

 organ can be recognised in the neighbourhood of the bile-duct, 

 into which its lobules open independently. 1 



1 The histological structure of this organ in Cyclostomes resembles that of 

 the peculiar " intertubular cell-masses" or "islets of Langerhans" present 

 amongst the ordinary pancreatic tubules of other Vertebrates. These are of 

 epithelial origin, but have no ducts : they probably pour their secretion into the 

 surrounding lymph-vessels and blood-vessels, and may be included under the 

 category of " glands with internal secretion" (cf. p. 247). It has been suggested 

 that the pancreas may represent phylogenetically two distinct glands, the more 

 primitive of which is alone present in Cyclostomes, while in other Vertebrates it 

 has been largely replaced by the pancreas proper. 



