THE ALIMENTARY SYSTEM. 



451 



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. It is not certain that these 

 outgrowths are homologous, though the air bladder of 

 Dipnoi acquires the functions of a lung. 



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. 



From the hindmost region 

 of the gut, the allantois grows 

 out in all animals from Am- 

 phibians onwards. In Am- 

 phibians it is represented by 

 an unimportant bladder ; in 

 the higher Vertebrates it is a 

 vascular fcetal membrane con- 

 cerned with the respiration 

 or nutrition of the embryo, 

 or both. 



Cilia are very common on 

 the lining of the intestine in 

 Invertebrates, but they are 

 much rarer in Vertebrates. 

 Yet as they occur in Amphi- 

 oxus, lampreys, many fishes, 

 Protopterus, some Amphi- 

 bians, and in embryonic 

 Mammals, it seems not unlikely that the alimentary tract 

 was originally a ciliated tube. 



Speculative. — The primitive gut was probably a smooth straight tube, 

 but the rapid multiplication of well nourished cells would tend to its 

 increase in diameter and in length. But on increase in both directions 

 the slower growth of the general body would impose limitations, and in 

 this we may find the immediate growth-condition determining the origin 

 of folds, crypts, CKca, and coils, which would be justified by the increase 

 of absorptive and digestive surface. There are regular longitudinal 

 folds in Myxine, cross folds traversing these would form crypts, which 

 may be exaggerated into the pyloric c^ca of Teleosteans and Ganoids, 

 while other modifications would give rise to "spiral valves " and the 

 like. In the same way it may be suggested that the numerous important 



■/ 



Fig. 144. — Origin of lungs, 

 liver, and pancreas in the chick. 

 (After GcETTE.) 



The mesoderm is shaded ; the endo- 

 derm dark. 



Ig., One of the lungs ; st.^ stomach ; 

 /., liver; /., pancreas. 



