THE DEVELOPMENT OF AMPHIBIA. 



191 



brancliial aperture, and, in the Batrachia, extends over and 

 eventually covers tte gills, a rounded aperture persisting 

 for a certain time only on tlie left side. The anterior pair 

 of limbs is developed before the posterior, but in the Frog 

 they are not so soon visible, being hidden by the opercular 

 membrane. 



The lungs make their appeai-ance as diverticula of the 

 ventral wall of theoesophagus. The nasal sacs are at firstmere 



Fig. 60. — A. B., Tadpoles with external branchiae: n, nasal sacs; a, 

 eye; o, ear; kb, branchicg ; m, mouth; z, horny jaws; s, suckers; 

 d, opercular fold. 



C. a more advanced Frog's larva : ;/, the rudiment of the hind limb ; k s, 

 the single branchial aperture. The figure has not been reversed, so 

 that this aperture appears to lie on the right side instead of the left. 



caecal involutions of the integument, but nasal passages com- 

 municating with the mouth are soon formed, and both 

 aerial and aquatic respiration are completely established. 



In the Batrachia, as development proceeds, the external 

 branchiae disappear, and are succeeded, functionally, by 

 short branchial filaments developed upon the whole length 

 of each of the branchial arches, of which there are four. 



