THE LATER DEVELOPMENT OF THE FROG 161 



pouch is never perforated; shortly before the opening of the 

 second and third clefts it loses its connection with the ectoderm 

 and gradually disappears. The formation, from the dorsal wall 

 of the hyomandibular pouch, of the rudiment of the tubo- 

 tympanic cavity of the ear, has already been described (Fig. 57). 



The chief structures associated internally with the visceral 

 arches are the aortic arches and the visceral skeleton, the devel- 

 opment of which will be described in later sections. From the 

 external surface of certain visceral arches are developed the 

 gills, both external and internal. Reference has previously 

 been made to the external gills: these appear just before hatch- 

 ing, as small outgrowths from the outer surfaces of the dorsal 

 ends of the first and second branchial (second and third visceral) 

 arches (Fig. 22, G.) Later a small external gill appears on the 

 third branchial arch. The two anterior pairs grow very rapidly 

 and about the time the mouth opens -they form large branched 

 or lobed processes, extending out from the sides of the pharynx 

 (Figs. 64, 65). They become extremely vascular and are the 

 earliest respiratory organs of the tadpole. The posterior pair 

 remains small and comparatively simple. 



After a few days the external gills become covered by the 

 operculum, and then they are gradually absorbed and finally 

 disappear completely. The operculum makes its first appear- 

 ance before the perforation of the mouth, as a pair of out- 

 growths from the posterior borders of the hyomandibular 

 arches. These folds grow backward rapidly, and just as the 

 external gills reach their maximum development, the operculum 

 extends to and outside of them, enclosing them in an opercular 

 cavity. This cavity finally becomes entirely enclosed on the 

 right side by the fusion of the posterior border of the right 

 opercular fold with the surface of the body; this fusion 

 extends across the ventral side also, and thus puts the right 

 cavity in connection with the left (Fig. 58, B). The left fold 

 remains partly free and the margins of the opening are drawn 

 out forming a short opercular tube or "spiracle." 



The internal gills appear just after the gill clefts are perfo- 

 rated, about the time the mouth opens, as a series of small 



