1 6 THE COMMON FROG. [ch\p. 



br^ and br"^). These openings are termed ''visceral 

 clefts," which lead from the exterior into the throat, 

 as already described. The solid pillars (or intervals) 

 between the clefts, i.e., the " visceral arches," become 

 furnished with gills,i or braiicJiicE, and are therefore 



41 rm 



Fig. 2.— View of left side of Embryo Tadpole (after Parker), br^ and br^, first and 

 second external branchiae ; cl'^—cl^, the six visceral clefts ; cp the left " holder " ; 

 d, the olfactory organ ; e, the eye : //, the left lip ; m, the aperture of the mouth ; 

 op, the hinder margin of the rudimentary operculum. 



called ".branchial arches." The eggs are hatched 

 towards the end of April, and the tadpole emerges 

 in the stage represented at Fig. 3, 1. It has a rela- 

 tively large head, a rounded body, and a long tail, by 

 lateral undulations of which the little creature swims 

 about. From behind the head, on each side, jut forth 

 external branchiae as a small plume-like structure, but 

 no limbs are visible. 



As the tadpole grows, the external plumose gills 

 at first greatly enlarge (Fig. 3, 2 and 2«), but after- 

 wards become gradually absorbed, and are succeeded 

 by short gill filaments, which are developed along 

 each of the branchial arches. These latter filaments 



^ Gills (or branchiae) are delicate processes of skin richly supplied 

 mth minute blood-vessels, wherein the blood becomes exposed to the 

 purifying action of the air dissolved in the water. 



