Metazoa Rana. 



333 



contractions of which movements of the body as a whole 

 are effected. There are at first no limbs. The mouth is 

 round and sucker-like, and quite unlike the wide gape so 

 characteristic of the adult. From the sides of the neck 

 project three pairs of external gills (branchiae), prolonga- 

 tions, in short, of the vascular membrane covering the 

 branchial arches of Amphioxus. By them respiration is 

 effected just as in that type, for the lungs have not yet 

 been developed. Subsequently the gills become covered 

 over by a fold of skin, though a communication with the 

 exterior is still maintained late in embryonic life. Respira- 

 tion is carried on by means of the gills, which are now 



FIG. 190. METAMORPHOSIS OF THE FROG. (Owen.) 



much reduced in size and hidden beneath the fold in a sort 

 of branchial chamber comparable with the atrial cavity of 

 Amphioxus. The lungs are then developed as buds from 

 the alimentary canal, and the blood-stream is carried to 

 their walls by means of the pulmonary artery. The hind- 

 limbs are the first to appear, springing from the origin of 

 the tail. Concomitantly with the development of the hind- 

 limbs the tail atrophies (i.e. becomes absorbed) ; the fore- 

 limbs make their appearance from beneath the membrane 

 covering the branchiae. The branchial slits close, and the 

 gills atrophy also, the entire respiration being now under- 

 taken by the lungs. Finally the frog becomes a truly 



