. Chordate anatomy. Chordata; Anatomy, Comparative. 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 s
. Chordate anatomy. Chordata; Anatomy, Comparative. 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 n
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