DEVELOPMENT 271 



rial from the surface of aquatic plants. Moreover, increased res- 

 piration is necessary, and this is met by the appearance of branched 

 external gills on the sides of the head, which are the first of a 

 series of three different respiratory organs that succeed one another 

 during the life history. Indeed, these external gills are soon cov- 

 ered over by a fold of the skin, the operculum, which finally 

 leaves but a single opening, the spiracle, to the exterior. And 

 no sooner is the operculum fully developed, than the external 

 gills are resorbed and a new set of fish-like internal gills take their 

 place in the gill slits. Then hind legs slowly make their appear- 

 ance, followed by the fore legs, already developed under the 

 operculum. 



Now the larva is ready for metamorphosis — its transforma- 

 tion from a gill-breathing tadpole to a lung-breathing juvenile 

 Frog. During metamorphosis one stage melts rapidly into the 

 next: the tail is resorbed, the legs increase in size, the long coiled 

 intestine becomes shorter and specialized to digest animal food, 

 the internal gills are resorbed, and lungs are developed which 

 make it necessary for the tadpole to come to the surface of the 

 water for air. Finally, as a juvenile Frog, the animal transfers 

 its abode largely to land, and grows. 



The process of metamorphosis varies in length from a few 

 weeks to several years in different species of Frogs. In all cases it is 

 some time after metamorphosis before sexual maturity arises. Pre- 

 ceding the first breeding season, the gonads develop rapidly, their 

 ducts become fully differentiated, and adult male and female 

 individuals are established. (Figs. 132, 163.) 



C. Embryonic Membranes of the Higher Vertebrates 



The embryological development of the higher Vertebrates de- 

 parts rather widely in certain ways from that of the Earthworm 

 and the Frog. Thus the eggs of Reptiles and Birds contain much 

 more yolk than the egg of the Frog, with a consequent greater 

 obscuring — though not obliteration — of the characteristic blas- 

 tula, gastrula, etc. (Fig. 169.) 



Furthermore, in the Frog the whole egg becomes converted 

 into the body of the tadpole, whereas in Reptiles, Birds, and 

 Mammals a part of the egg forms a hood-like membrane, the 

 amnion, about the embryo. This is cast off at birth and with it 

 another membrane, the allantois, which extends into the am- 



