THE LATER DEVELOPMENT OF THE FROG 165 



sent the vestiges of a sixth pair of visceral pouches, although 

 they never extend out as far as the surface ectoderm. They 

 soon separate from the pharyngeal wall and acquire internal 

 cavities, remaining in the floor of the pharynx, in a supraperi- 

 cardial position (Figs. 54, 55, 56). 



The remaining structures of the pharyngeal region have no 

 genetic relationship with branchial structures. The thyroid 

 body appears just before hatching, as a median evagination, 

 narrow and elongated, from the floor of the pharynx (Fig. 57). 

 This slowly pinches off from the pharyngeal epithelium, 

 forming a solid rod of cells, and a few days after the opening of 

 the mouth it divides into a pair of bodies, which then enlarge 

 rapidly and become very vascular. 



The lungs appear just before hatching as a pair of solid 

 proliferations from the ventral wall of the posterior part of the 

 fore-gut, between the yolk-mass and the heart. These rudi- 

 ments slowly extend posteriorly along the sides of the fore-gut, 

 and early acquire cavities proximally. Some time after the 

 opening of the mouth, the wall of the fore-gut between and 

 around the openings of these diverticula, becomes depressed 

 as a transverse groove, the laryngeal chamber (Fig. 58, B), 

 which is then partly constricted off from the alimentary tract; 

 the opening that remains is the glottis. The lungs now rapidly 

 elongate, pushing out into the body cavity and becoming very 

 vascular (Fig. 72); the mesodermal constituents surrounding 

 the endodermal lining of the lungs, are derived from the 

 splanchnic mesoderm. 



The tongue appears very late, just before metamorphosis. 

 It is first indicated as an elevation in the floor of the anterior 

 part of the pharynx, just back of the region from which the 

 thyroid body was derived. In front of this elevation, between 

 it and the lower jaw, the floor of the pharynx is depressed and 

 glandular. During metamorphosis the rapid anterior extension 

 of the tongue carries this glandular area upward so that it 

 lies on the free anterior tip of the tongue. 



The liver appears very early, even before the embryo begins 

 to elongate, as a postero-ventral extension of the cavity of the 



