278 ELEMENTARY BIOLOGY 



the organism may be compared to animals related to the 

 hydra, which get to this stage of development but never get 

 much farther. Then the two-layered cup becomes longer, 

 suggesting certain kinds of worms. 



When we compare the embryos of animals that are closely 

 related, such as several kinds of backboned animals, or several 

 kinds of insects, we find still more remarkable facts. Thus, 

 the fish, the bird, the salamander, and the rabbit are very much 

 alike early in their development, not only when each consists 

 of a single cell, but later, when it is possible to distinguish 

 head and trun and limbs (see Fig. 114). In a somewhat 

 later stage each has developed a little farther, and it is not 

 difficult to distinguish the bird from the fish or the tortoise. 

 But at this stage we can see certain resemblances between 

 the bird and the reptiles. Moreover, if we compare the 

 embryos of several mammals (such as the rabbit, the pig, the 

 sheep, and man) at this stage, we shall find them strikingly 

 similar. As they become older they become more and more 

 different. 



328. Recapitulation. Now if we imagine a series of ani- 

 mals of different degrees of complexity, beginning with the 

 one-celled ameba and ending with man, we shall have before 

 us a picture resembling in many ways the series of stages 

 through which each individual human being passes, from his 

 one-celled stage to his maturity (see Fig. 114). This paral- 

 lelism between the stages in individual development and in 

 the whole animal series was observed long ago, and is known 

 as Von Baer's Law of Recapitulation. Some biologists have 

 gone so far as to say that each individual passes through stages 

 representing all the types of his ancestors. In a general 

 way this is true only as a restatement of Von Baer's law. 

 But, strictly speaking, it is not true, for example, that you once 

 passed through a hydra stage or a fish stage. All we can say 

 is that we have passed through stages that are similar to 

 corresponding stages in many classes of animals. 



