244 



COMPARATIVE MORPHOLOGY OF VERTEBRATES. 



those belonging to the cleft become the functional organs, the water 

 taken in through the mouth passing over them in its way to the exterior 

 via the extrabranchial chamber. Then, with the completion of the 

 metamorphosis, the lungs become functional, the gill clefts are closed 

 and the gills absorbed, the legs are developed and the anterior pair 

 released from the extrabranchial chamber, the tail is absorbed, and the 

 tadpole (larva) becomes the adult. 



FIG. 252. Cast of oropharyngeal region of pig embryo, 17 mm. long, after Fox. alf, 

 alveo-lingual fold; ctm, cervical cord of thymus; dp 1 , dp 2 , dorsal apex of first and second 

 pharyngeal pouches; dptm, dorsal plate of thymus;/, filiform appendix of second pouch; 

 tlr, lateral thyreoid; stt, sulcus tubo-tympanicus; tm, thymus; vf, vestibular fold of mouth. 



Little is known of the gills in the stegocephals, but the presence of well developed 

 branchial arches in the larvae of some species (p. 83) would imply the existence 

 of functional gills. 



For some time it was thought that the fish gills were of entodermal origin, and 

 those of the amphibia were derived from the ectoderm. Hence the conclusion was 

 that the two had no genetic connexion, the gills of the amphibia being a new 

 acquisition, developed within the group or arising from the external gills of some 

 form like Polypterus. Lately the doubts thrown upon the entodermal origin of the 

 gills of fishes (p. 237) render it possible that all vertebrate gills are homologous. 



Gills are never developed in the amniotes, but in the embryos the 

 paired visceral pouches are formed (figs. 208, 252) five in the saurop- 

 sida, four in mammals in the same way as in the fish-like forms. 

 Few, if any, of them break through to the exterior, although their 

 position is indicated by grooves on the outside of the neck. The proc- 

 ess of obliteration of these external grooves is interesting. The ante- 

 rior arches enlarge and slide back over the posterior, so that at least 

 the external branchial grooves lie in the wall of a pocket, the cervical 

 sinus, on either side of the neck (fig. 253). Later a process of the 

 anterior (hyoid) arch extends over and closes the sinus, a process re- 



