THE RESPIRATORY SYSTEM 



259 



two divergent opinions by asserting that the air bladder is itself derived 

 from a pair of modified gill pouches. 



Goette (1875) was the first to suggest that lungs are modified gill 

 pouches, on the ground that in some amphibian embryos the lungs develop 

 from a pair of posterior endodermal pouches in series with the gill pouches. 

 (Fig. 238) A number of observers 



have confirmed this observation and 

 reached the same conclusion. In 

 support of Goette's hypothesis is the 

 fact that the pulmonary arteries 

 develop from the sixth pair of aortic 

 arches. Furthermore, 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. 

 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 com- 

 pare 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. 



It may be doubted whether we have any adequate explanation of the 

 substitution of lungs for gills as respiratory organs. The fact that lungs 

 are much better adapted to the needs of land animals than gills, which tend 

 to dry in air, does not explain their origin. It is to be noted, however, 

 that in this change of life animals have "played safe." Even before they 



Fig. 240. — Diagrams of air bladder 

 in fishes. A, Physostomous fishes; B, 

 Lepidosteus and Amia; C, Erythrinus; 

 D, Ceratodus. The air bladder of the 

 crossopterygian fish, Polypterus, is, like 

 the lungs of amphibians, bilobed and 

 connected with the floor of the pharynx. 

 (From Kingsley's "Comparative Anat- 

 omy of Vertebrates," after Dean.) 



