272 OUTLINES OF E VOLUTION AKY BIOLOGY 



Embryos which, having hatched at a comparatively early stage 

 of their development, lead independent, self-supporting lives, are 

 commonly spoken of as larvae, and amongst such larvae we again 

 find abundant instances of special larval organs developed in 

 relation to their special requirements. 



The tadpole of the frog (Fig. 121, 411) affords an excellent 

 example of a free-living larva. It has certain special organs, 

 such as its horny jaws and the suckers on the under surface of 



FIG. 121. Stages in the Life History of the Frog. (From Brehm's " Thier- 

 leben.") 



1, freshly laid eggs; 2, eggs slightly magnified, adhering together by their gelatinous 

 envelopes to form " spawn " ; 3, young tadpole still enclosed in the jelly ; 4, newly 

 hatched tadpoles adhering by their suckers to a blade of grass ; 5, 6, tadpoles with 

 external gills ; 7, 8, tadpoles with the gills concealed by the operculum ; 9, 10, tad- 

 poles with conspicuous hind legs but with the front legs still concealed beneath the 

 operculum ; 11, tadpole during metamorphosis, with front legs protruded ; 12, young 

 frog with tail not completely absorbed. 



(For microscopic details of the early stages see Fig. 119.) 



the head, which are developed in relation to its larval life, but 

 there can be no question about the phylogenetic significance 

 of the tadpole stage as a. whole. In every essential feature of its 

 organization the tadpole, before its legs develop, is a fish. It 

 swims like a fish by means of its well developed tail, it breathes 

 like a fish by means of its gills and gill-slits, and its internal 

 structure is that of a fish. There can be no doubt that it 

 represents a fish-like stage in the evolution of the frog. 



In the tadpole, owing to its free-living, aquatic mode of life, 



