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. 
Fic. 252.—Cast of oropharyngeal region of pig embryo, 17 mm. long, after Fox. al/; 
alveo-lingual fold; ctm, cervical cord of thymus; dp, dp”, dorsal apex of first and second 
pharyngeal pouches; dptm, dorsal plate of thymus; /, filiform appendix of second pouch; 
tlr, lateral thyreoid; stf, sulcus tubo-tympanicus; ¢m, 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 larve 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- 
