THE RESPIRATORY SYSTEM 



345 



more, it is obvious that if a gill pouch were to fail to reach the skin and 

 were to grow backwards into the body cavity it would assume the relations 

 of a lung. 



On the other hand, supporters of the air bladder hypothesis emphasize 

 the fact that the air bladder of such a fish as the Nile bichir (Polypterus) 

 develops like the lung as a median ventral outgrowth of the pharynx. 



Fig. 289. — 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. (Redrawn after Huntington.) 



Its bilobed adult form is secondary, as is also its vascular connexion with 

 the sixth aortic arch. Basing the homology of air bladder and lung upon 

 their similar development as median ventral outgrowths from the pharynx, 

 the supporters of this view are skeptical of the attempt to compare a. 

 median organ with paired structures such as gill pouches. 



To meet this difficulty, it may be pointed out that the transformation 

 of a paired organ into a median one is not unknown. For example the 

 thyroid gland in all vertebrates develops as a median ventral outpocketing 

 of the pharynx, yet all morphologists agree in homologizing the thyroid 

 with the endostyle of amphioxus. The endostyle, however, in amphioxus 

 develops from a pair of gill pouches. Likewise it is sometimes assumed 

 that the median and unpaired pineal organ of the brain arose by the 

 union of paired diverticula of the brain. 



