DEVELOPMENT OF THE INDIVIDUAL 



563 



ment, it is often difficult to recognise the 

 adult characteristics. An adult frog, for ex- 

 ample, only vaguely resembles the tadpole 

 from which it was derived; similarly, the 

 tadpole shows few of the anatomical fea- 



until finally the adult characters emerge. 

 It is remarkable how all animals follow 

 similar paths in their earlier stages, becom- 

 ing more unlike as they approach maturity. 

 It is difficult, for example, to distinguish 



Fig. 23-3. The early embryology of the starfish, in this photograph the fertilized egg, the two- 

 celled stage, the four-celled stage, a blastula, and the young free-swimming larva are shown. 



tures of earlier stages, and surely there is 

 no resemblance between any of them and 

 the fertilized egg. It is apparent that in 

 development animals go through a long 

 series of changes. At first they are very sim- 

 ple, with few parts, but as development 

 proceeds complexity is added to complexity 



between early embryos of a fish, bird, cat, 

 or man ( Fig. 25-9 ) . We shall speak of this 

 again in a later chapter on evolution. 



Let us examine briefly the stages in the 

 early development of an animal. It might 

 seem more meaningful to select human de- 

 velopment for our study, but because of the 



