556 



STRUCTURE OF VERTEBRATA 



in Birds), or double (as in some Mammals and Amphibians), or diffuse 

 (as in Bony Fishes). Only in the larval lamprey does it retain its 

 original connection with the pharynx, and is then a true gut-gland. 



As to its morphological nature, its mode of origin suggests com- 

 parison with the hypobranchial groove in Amphioxus and the eudostyle 

 of Ascidians. 



The thymus arises as a dorsal endodermic thickening where the 

 outgrowths which form the gill-clefts meet the ectoderm. It may 

 be associated with a variable number of clefts — seven in the shark 

 Heptanchiis, five in the skate, four in Teleosteans, three in the lizard, 

 one in the chick, and one (the third) in Mammals. In the young 

 lamprey there are said to be no fewer than twenty-eight thymus rudi- 

 ments. In Mammals it often seems 

 to degenerate after youth. In the 

 rabbit it has its maximum weight in 

 the fourth month, and thereafter 

 begins to be rapidly reduced. As it 

 has from its first origin a distinct 

 lymphoid nature, and apparently 

 forms leucocytes, it has been inter- 

 preted (Beard) as a structiore adapted 

 for the phagocytic protection of the 

 gills from bacteria, parasites, and the 

 effects of injury. If this be so, we 

 can understand its diminishing im- 

 portance in Sauropsida and Mam- 

 malia, where its place may be to some 

 extent taken by the palatal and 

 pharyngeal tonsils, which are beUeved 

 by some (Stohr, Killian, Gulland) to 

 Fig. 3i5.-Origin of lungs, have a similar phagocytic function, 

 liver, and pancreas in the 

 chick. — After Goette. 



The mesoderm is shaded ; the endo 



derm dark. 

 Ig., One of the lungs ; St., stomach 



/., liver ; p., pancreas. 



The pharynx leads into the 

 gullet or cESophagus, which is a 

 conducting tube, and this into 

 the digestive stomach, which is 

 followed by the digestive, ab- 

 sorptive, conducting intestine, ending in the rectum and anus. 

 From the oesophagus the air- or swim-bladder of most 

 Fishes, and the lungs of higher Vertebrates, grow out. 

 The air-bladder usually lies dorsally and is almost always 

 single ; the lungs lie ventrally and are double, though 

 connected with the gullet by a single tube. 



The beginning of the intestine gives origin to the liver, 

 which regulates the composition of the blood and secretes 

 bile, and to the pancreas, which secretes digestive juices. 

 The pancreas has often a multiple rudiment. 



