DEVELOPMENT OF THE ORGANISM 



an embryo before it bursts the egg-shell and begins as a 

 larva to earn its own living. Furthermore, an animal is not 

 hatched in exactly the form of the adult; it attains this form 

 only after a period of growth, and during this period it may 

 properly be termed a larva. 



The salamanders show us clearly that the embryonic phase 

 is a secondary modification of the larval phase — in a word, 

 that the embryo is a larva which is provided with shelter and 

 food. 



There are two species of European salamander, the black 

 and yellow salamander (^Salamandra maculosa), commonly 

 known as the fire salamander, which inhabits the plains and 

 lower reaches of the valleys, and the black salamander 

 (Salamandra atra) , which is found in Alpine pastures. Both 

 species bring forth the young alive, but the young of the 

 fire salamander are provided with long, feathery gills and 

 gill slits and pass the first six months of their life in water, 

 whereas the young of the black salamander are devoid of 

 gills and gill slits at birth and are ready to take up the 

 parental habit of life on land. 



If now we open the womb of a pregnant black sala- 

 mander we shall find in it a number of embryos with long, 

 feathery gills, and if these embryos are thrown into water 

 some of them will survive and develop like the young of the 

 fire salamander. The gills are an adaptation to life in water, 

 and when we find them in an embryo we may be sure that 

 the embryo was once a larva and that the larval phase is 

 therefore the primary one. 



Similar examples could be adduced from almost every 

 group of animals. We have already alluded to the well- 

 known tadpole larvae of frogs and toads. There is, to men- 

 tion one more animal, a small West Indian tree frog 

 {Hylodes) which produces a few large eggs from which 



[55] 



