26o 



CHORDATE ANATOMY 



abandoned the water for a land life, they had acquired an organ, the 

 air bladder, which would serve as a substitute for gills. 



While some uncertainty remains in regard to the origin of the lungs, 

 the facts on the whole seem to accord with the gill-pouch hypothesis. 

 If it is assumed that the crossopterygian air bladder is a pair of modified 

 gill pouches, the rest of the problem of the history of the lungs is easily 

 solved, since there are among living vertebrates all intergradations in 



Fig. 241. — Diagrams of stages in the phylogenesis of the lungs. The respiratory 

 surfaces are stippled, and conductory passages cross-hatched. Embryological stages 

 corresponding with the comparative anatomical series shown in A-E occur in the 

 ontogenesis of lungs in mammals. See Fig. 239. (Redrawn after Huntingrton.) 



complexity between the simple air bladder of Polypterus and the mam- 

 malian lung. (Fig. 240) The evolutionary changes which occur involve 

 chiefly a great increase in the lung surface effected through the branching 

 and subdivision of the primary lobes. The facts of embryology and com- 

 parative anatomy are in complete agreement. An evolutionary series 

 based upon evidence from comparative anatomy is shown in Fig. 241. 



In birds a special modification of the lungs occurs. (Fig. 242) Air 

 sacs grow from the bronchi into the abdominal cavity, the thorax, the 

 neck, and even into some bones. Since they have very few blood-vessels in 



