AMPHIBIA. 225 



of the throat six thickened bars, the visceral arches, passing from 

 above downward, are developed. These are termed mandibular, 

 hyoid, 1st, 2nd, 3rd, and 4th branchial arches respectively. From 

 branchial arches 1 and 2 a branched tuft-like external gill grows 

 out on either side, and after hatching, which occurs about this 

 period, another proceeds from the third branchial arch. The tail 

 now assumes great size and importance as a locomotor organ. In 

 front of each branchial arch a visceral cleft, leading into the 

 throat, makes its appearance, and a fold of skin, the operculum, 

 grows back from the hyoid arches over the external gills. By 



Fig. 67. DIAGRAMMATIC LONGITUDINAL SECTION THROUGH A FROG EM- 

 BRYO (after Gotte). Ectoderm unshaded, endoderm shaded, mesoderm 

 (ra) darkly shaded. nc, Neural canal contained within neural tube, 

 which on the left is dilated into the rudimentary brain already 

 exhibiting flexure; al, alimentary canal, with mass of yolk (yk) be- 

 neath ; x, placed at blastopore, the reference line runs to posterior end 

 of neural canal, which is here continuous with alimentary. 



the union of this fold with the body-wall a branchial chamber is 

 formed on each side. The right one soon closes, but remains 

 connected by a cross-passage under the throat with the left, 

 which opens to the exterior by a small rounded opening. The 

 external gills now shrink and disappear, being succeeded by inter- 

 nal gills, which are vascular folds on the walls of the clefts. As 

 the lungs develop, these too disappear, and the visceral clefts 

 close up. 



The limbs have been arising meanwhile as bud-like out- 

 growths, the anterior pair being hidden at first by the operculum. 

 They appear at the surface after a shedding of skin which follows 



15 



