238 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



they seem to be the direct descendants of the swim- 

 bladder of fishes, which we have already traced from 

 its position as a dorsal sac in Amia and Lepidosteus 

 to a completely divided ventral sac in Protopterus. 



PI) 



PD 



PD 



Fig. 101. Diagrams to illustrate the Development of tbe Lungs. 



PD, Primitive intestine; ss', lung sacs; t, trachea; b, bronchus (.in A, B, and 

 c); L0 (in D), primitive ; i,g (in a), secondary pulmonary vesicles. (After 

 Wiedersheim.) 



Comparable with these changes in the coarser details 

 of its anatomy are the modifications suffered by its 

 internal surface, which becomes more and "more spongy 

 and broken up into internal spaces ; and the changes 

 which bring its blood-vessels into direct relation with 

 the heart. (See page 203.) 



A similar set of changes affects the lungs, either 

 as we trace them through the ascending scale of the 



