DEVELOPMENT OF THE LEOPARD FROG 415 



grows posteriorly on either side is obviously homologous with the 

 operculum, or lateral covering of the gills, in teleost fishes. The 

 external gills disappear as this operculum develops and the internal 

 gills become functional. In its subsequent development the 

 posterior margin of the operculum fuses with the body surface, 

 leaving a small tubular opening on either side. Later, the opening 

 on the right side closes, leaving the one on the left to function 

 for both sides, since the right and left opercular cavities are in 

 communication ventrally, until the gills disappear at the time of 

 metamorphosis. The tadpole "breathes" Hke a fish, by taking 

 water into the mouth, passing it through the gill slits, and dis- 

 charging it through the opercular opening; but with the develop- 

 ment of lungs in later stages (Fig. 11 G, p. 23) the animal comes 

 to the surface to take in and discharge air by the mouth, and thus 

 the branchial respiration becomes less important. 



After reaching the condition described in the first two or three 

 weeks of larval life, the tadpole develops more slowly until the 

 time of metamorphosis, and increases considerably in size. The 

 legs begin to appear at about the end of the first month. The 

 fore legs develop first, although they are not seen externally because 

 covered by the operculum until the time of metamorphosis; the 

 hind legs develop a little later on either side of the anal opening at 

 the posterior end of the globular body. In the leopard frog the 

 tadpole stage lasts for a period of two or three months, depending 

 upon food and temperature, and may be continued through the 

 following winter under exceptional conditions. In the bullfrog, 

 Rana catesbiana, which lays its eggs later in the season than the 

 leopard frog, the first winter is normally passed in the tadpole 

 stages. 



Since all the more important organs of the body are present, 

 at least as rudiments, in the later tadpole stages, this general 

 description and discussion of the developmental processes may be 

 concluded before describing the metamorphosis by which the tad- 

 pole becomes a miniature frog. To continue the account of 

 internal development from the stage shown in Fig. 215 E, the 

 central nervous system is fomied from the neural tube by a 

 thickening of the walls, and a consequent reduction in the relative 

 size of the internal cavity until it remains only as a microscopic 

 canal in the cord of the adult (cf. Fig. Gl A, p. 109). The brain 

 region of the neural tube becomes differentiated first into three 



