644 RESPIRATORY AND BUOYANCY SYSTEMS 



be regarded as a lung which secondarily is associated with the function of 

 buoyancy. The latter condition is found also in the Dipnoi (lungfishes). 



The lung of the mud puppy, Necturus maculosus, is capable of considerable 

 extension, particularly in the antero-posterior direction, is devoid of air cells 

 within, and, hence, probably serves the buoyancy function as much or more 

 than that of respiration. The lungs of sea turtles are capable of great distension 

 and aid the animal in maintaining a position near the surface of the water. 

 In the bird group, air sacs are united directly to the lungs, as sac-like exten- 

 sions of the latter. 



Thus, the formation of structures which assume the responsibility for the 

 functions of buoyancy and respiration is a characteristic feature of pharyngeal 

 development in most vertebrate species. 



2. Development of Lungs 



a. Development of Lungs in the Frog and Other Amphibia 

 In the 5- to 6-mm. embryo of Rana pipiens, the lungs arise as a solid evagi- 

 nation of the midventral area of the pharynx at the level of the fifth branchial 

 pouches and over the developing heart. At the 7-mm. stage from this evagina- 

 tion, two lung rudiments begin to extend caudally below the developing 

 esophagus (fig. 305). In the 10-mm. embryo, the lungs extend backward 

 from a common tracheal area above the heart and liver area (fig. 258D). 

 At this time, the entodermal lung buds are surrounded by a mass of mesen- 

 chyme and coelomic epitheHum. The entodermal lining eventually becomes 

 folded to form larger and smaller air chambers. 



In Necturus, the development of lungs is similar to that of the frog, but the 

 inner surface of the lungs remains quite smooth. The tracheal area of the 

 frog and Necturus shows little differentiation and represents a comparatively 

 short chamber from the lungs to the glottis. In some urodeles, the trachea is well 

 differentiated, possessing cartilaginous, supporting structures (e.g., Amphiuma, 

 Siren ) . 



NEURAL TUBE 



NOT OChORD 

 E SOPMAGUS 



Fig. 305. Lung rudiment of 7-mm. of frog tadpole. (Cf. fig. 258.) 



