346 



COMPARATIVE ANATOMY 



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 



Fig. 290. — Air sacs and canals of pigeon, c^—c^, intertransverse canals; da^-da-, 

 axillary sac and its ventral diverticulum; dc, canal for ribs; dot, infraclavicular canal; 

 ds, subscapular sac; dsl, sternal canal; pc, preacetabular canal; sad. sas, right and left 

 abdominal sacs; sc, cervical sac; sia, sip, anterior and posterior intermediate sacs. 

 (From Kingsley's "Comparative Anatomy of Vertebrates," after Bruno Miiller.) 



solved, since there are among living vertebrates all intergradations in 

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

 malian lung. 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. 289. 



