642 



RESPIRATORY AND BUOYANCY SYSTEMS 



the mouth, through the gill slit, over the gill filament, and, from thence, 

 through the opercular opening to the exterior. Both types of gill filaments, 

 external and internal, fundamentally are similar. 



4) Resorption and Obliteration of Gills. The resorption of gills is a phe- 

 nomenon associated with metamorphosis in dipnoan fishes and in Amphibia, 

 although certain species of Amphibia, as indicated on p. 639, retain certain 

 larval characteristics in the adult condition. Most species metamorphose into 

 an adult form which necessitates many changes in body structure (Noble, '31, 

 p. 102). This transformation has been related to the thyroid hormone (Chap. 

 21 ). In frogs, toads, and salamanders, the thyroid hormone produces degenera- 

 tion and resorption of gills, the branchial clefts fuse, and the larval branchial 

 skeleton is changed into the adult form (fig. 317). 



An interesting feature of gill resorption in the anuran tadpole is that the 

 degenerating gills produce a cytolytic substance which brings about the for- 

 mation of the hole in the operculum through which the foreleg protrudes 

 during metamorphosis (Hellf, '24; Noble, '31, p. 103). 



C. Development of Lungs and Buoyancy Structures 



1. General Relationship Between Lungs and Air Bladders 



The functions of buoyancy and external respiration are related closely. 

 Lungs and air bladders (sacs) constitute a series of pharyngeal diverticula 

 associated with these functions (fig. 304A-F). (For an historical approach 

 to the work on developing lungs, see Flint, '06; for studies on air bladders, 

 consult Goodrich, '30.) Air bladders (sacs) are a characteristic feature of 



GILL RUDIMENTS 



INTERNAL GILL FILA 



EXTERNAL GILL FILAMENTS 



Fig. 303. Gill development in the tadpole of Rana pipiens. (All drawings are original.) 

 (A) Five- to six-mm. tadpole. (B) Frontal section of 7-mm. tadpole. (C) External, 

 ventral view of 10-mm. tadpole, showing opercular fold covering gill area. (D) Gill 

 bar, internal and external gill filaments of 10- to ll-mm. stage. 



