Embryology and Growth of Fishes 133 



and fully grown fish. Thus the development of most fishes is 

 almost wholly embryonic development— that is, development 

 within the egg or in the body of the mother— while the develop- 

 ment of some of them is to a considerable degree post-embry- 

 onic or larval development. There is no important difference 

 between embryonic and post-embryonic development. The de- 

 velopment is continuous from egg-cell to mature animal and, 

 whether inside or outside of an egg, it goes on with a degree of 

 regularity. While certain fishes are subject to a sort of meta- 

 morphosis, the nature of this change is in no way to be com- 

 pared with the change in insects which undergo a complete 

 metamorphosis. In the insects all the organs of the body are 

 broken down and rebuilt in the process of change. In all fishes 

 a structure once formed maintains a more nearly continuous 

 integrity although often considerably altered in form. 



General Laws of Development. — The general law of develop- 

 ment may be briefly stated as follows : All many-celled animals 

 begin life as a single cell, the fertilized egg-cell ; each animal 

 goes through a certain orderly series of developmental changes 

 which, accompanied by growth, leads the animal to change from 

 single-cell to many-celled, complex form characteristic of the 

 species to which the animal belongs; this development is from 

 simple to complex structural condition ; the development is the 

 same for all individuals of one species. While all animals begin 

 development similarly, the course of development in the dif- 

 ferent groups soon diverges, the divergence being of the nature 

 of a branching, like that shown in the growth of a tree. In the 

 free tips of the smallest branches we have represented the 

 various species of animals in their fully developed condition, all 

 standing clearly apart from each other. But in tracing back 

 the development of any kind of animal we soon come to a 

 point where it very much resembles or becomes apparently 

 identical with some other kind of animal, and going farther back 

 we find it resembling other animals in their young condition, 

 and so on until we come to that first stage of development, that 

 trunk stage where all animals are structurally alike. Any ani- 

 mal at any stage in its existence differs absolutely from any 

 other kind of animal, in this respect: it can develop into only 

 its own kind. There is something inherent in each develop- 



