BIBLIOGRAPHY 651 



rounding mesenchyme gives origin to the cartilage, muscle, and connective 

 tissue present in these structures. 



6) Ultimate Position of the Mammalian Lung in the Body. See Chapter 20. 



3. Development of Air Bladders 



It is difficult to draw a clear distinction between air bladders of Pisces and 

 the lungs of Tetrapoda. Air bladders and gills appear to be the standard ar- 

 rangement for most fishes. It is probable, therefore, that the function of 

 external respiration rests mainly upon the branchiae or gills in all fishes other 

 than the Dipnoi, while the function of buoyancy is the responsibility of the 

 air bladder. In some fishes {Dipnoi and ganoids), the functions of buoyancy 

 and respiration converge into one structure, the air bladder or lung, as they 

 do in many Tetrapoda. 



In development, air bladders, like the lungs of all Tetrapoda, arise as di- 

 verticula of the posterior pharyngeal area. In most cases, the air bladder arises 

 as a dorsal diverticulum (fig. 304A, B), while, in other instances, its origin 

 appears to be from the lateral wall (fig, 304C). In Salmonidae , Siluridae, etc., 

 for example, it arises from the right wall, while in Cyprinidae, C haracinidae , 

 etc., it takes its origin from the left wall. The air bladder generally is a 

 single structure (fig. 304A, C, D), but in some cases it is double or bilobed 

 (fig. 304E). 



Generally speaking, the air bladder receives blood from the dorsal aorta 

 or its immediate branches (fig. 304G), but in Dipnoi and Polypterus, the 

 blood supply to the air bladder comes from the pulmonary arteries as it does 

 in Tetrapoda. 



4. Lunglessness 



Many urodele amphibia have reduced or lost their lungs entirely. In many 

 cases the reduced condition of the lungs or absence of lungs is compensated 

 for by the development of buccopharyngeal respiration. The latter type of 

 respiration depends upon an extreme vascularization of the pharyngeal and 

 caudal mouth epithelium and rapid throat movements which suck the air in 

 and then expel it. In Aneides (Autodax) lugubnis, a land form, these throat 

 movements may reach 120 to 180 movements per minute (Ritter and Miller, 

 1899). Lungless aquatic salamanders also practice buccopharyngeal respira- 

 tion, although, in Pseudotriton ruber, cutaneous respiration evidently is re- 

 sorted to (Noble, '25). 



Bibliography 



Clements, L. P. 1938. Embryonic develop- Eycleshymer, A. C. 1906. The growth and 

 ment of the respiratory portion of the regeneration of the gills in the young 



pig's lung. Anat. Rec. 70:575. Necturus. Biol. Bull. X: 171. 



