CHAPTER 17 



DEVELOPMENT OF THE FROG AND OTHER VERTEBRATA 



The distinction was made in earlier chapters between repro- 

 duction, or the origin of a new individual, and development, or 

 the process by which a fertilized egg undergoes cell division and 

 differentiation to form a many-celled animal. Examples of 

 development have been seen in the representative animal types 

 that have been described. We shall now undertake a more inten- 

 sive study of this important biological process, as specifically illus- 

 trated by vertebrates, and of some of the more general problems 

 involved. The frog is selected for special study because we have 

 become familiar with its adult organization and because its devel- 

 opmental stages have long been favorite objects of study for the 

 zoologist. Moreover, the development of the frog is representa- 

 tive of what occurs in vertebrates, since the early stages are less 

 modified by the presence of yolk than are the eggs of fishes, and 

 the later stages are not complicated by the special conditions that 

 exist in reptiles, birds, and mammals. It might be supposed that 

 fishes would represent the simplest mode of development in the 

 backboned animals; but the egg of the fish has a relatively large 

 amount of yolk and is quite specialized, although it develops into 

 an adult which is the simplest type of vertebrate animal. To 

 find a mode of development that is simpler than the frog's, it is 

 necessary to disregard the fishes and examine the protochordate 

 type, Aviphioxus. As this animal presents interesting points of 

 comparison, its development will be briefly described before the 

 account of the amphibian is begun. Briefer accounts of develop- 

 ment in the fishes and in reptiles, birds, and mammals are included, 

 and a discussion of general problems in development is undertaken 

 in the chapter that follows. Throughout all this discussion the 

 importance of the cell will be apparent, since development is a 

 process of cell division and differentiation. 



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