420 DEVELOPMENT OF FROG AND OTHER VERTEBRATA 



Development of Other Vertebrates 



The development of other vertebrate animals may now be exam- 

 ined in its general features. The Amphibia lay their eggs in 

 water, with but few exceptions, and develop through tadpole 

 stages to salamanders, or, by a more pronounced metamorphosis, 

 attain to the tailless condition of frogs and toads. The develop- 

 ment of the frog has been described in detail as an example of the 

 fundamental features of vertebrate embryology. Although the 

 fishes represent a lower type of vertebrate organization, their 

 development is more specialized because of the presence of a 

 greater amount of yolk in their eggs. For this reason the brief 

 account of the protochordate type, Amphioxus, was inserted by 

 way of introduction. The eggs of reptiles and birds are even more 

 highly modified than those of fishes by the presence of yolk, but 

 their mode of development holds additional interest because of its 

 adaptation to terrestrial life. Fishes and amphibians are both 

 aquatic animals insofar as their developmental stages are con- 

 cerned. Reptiles and birds are completely adapted for terrestrial 

 life, since their developmental stages, as well as their adult lives, 

 can be passed on land surrounded by the atmosphere. It is true, 

 however, that the embryo within the egg of a bird or reptile 

 develops in a fluid medium which is kept from evaporating by the 

 enclosing egg shell. Thus, the leathery shell of a snake's egg and 

 the calcareous shell of a bird's egg are devices that make possible a 

 thorough-going terrestrial existence such as is impossible for an 

 amphibian. The development of mammals in turn presents inter- 

 esting points for comparison with that of birds and reptiles. In 

 all the classes of vertebrates the formation of the principal parts 

 of the embryo is substantially the same in the early stages, being 

 modified only by the conditions of development and the nature of 

 the adult animal. The comparative study that is undertaken in 

 the following sections shows not only the mode of fife of the embryo 

 in each of the classes, but also the changes in the mode of devel- 

 opment that have probably occurred in the evolution of verte- 

 brates from an aquatic habitat to the present terrestrial hfe of 

 reptiles, birds, and mammals. 



Development of Fishes. — Although the majority of fishes 

 laj' their eggs in the open water, where fertiUzation and develop- 

 ment occur, there are many exceptions (c/. Fig. 219). The skates 



