THE ONTOGENETIC SERIES 399 



mental processes the first identification would be possible after 

 gastrulation, because at this point the coelenterate development 

 ceases. The other embryos continue their development by the 

 formation of the third layer, the mesoderm. The body becomes 

 elongated and the mesoderm assumes the form of a series of 

 mesoderm] c somites by which the body is divided into a homono- 

 mous series of metameric segments. Here the annelid larva 

 has attained the form of the adult worm, and hence the end of 

 its development. The other larvae develop gill slits, and at this 

 stage the development of the lancelet is virtually completed. 

 The further addition of large cerebral vesicles and paired eyes 

 would indicate vertebrate embryos, the fish, the bird and the 

 mammal. But only the bird and the mammal would develop 

 lungs and pentadactyl appendages. In the bird the aortic 

 arch would lie on the right side of the body, while in the mammal 

 it would be on the left. 



8ii. In this comparison of the development of animals 

 details are, of course, omitted. The purpose of the comparison 

 is to show that the difference in the result of development of the 

 lower and higher forms is due not to a difference in direction of 

 development so much as to its extent. All forms pursue the 

 same course, but the higher forms continue their development 

 farther. Stating the same thing in another way, we may say 

 that the higher forms, in their development, pass through 

 stages which are the permanent adult condition of lower forms. 

 Why? If it is granted that the baleen whale is descended 

 from a terrestrial quadruped with the dentition characteristic 

 of Mammalia, then the rudimentary teeth of the young whale 

 are a relic of the former condition, and in its development the 

 whale passes beyond the tooth-bearing stage to a stage of rudi- 

 mentary teeth. This argument applies to all rudimentary or 

 vestigeal organs. The same line of reasoning may be applied 

 to another type of development in which an organ, instead of 

 remaining rudimentary, passes beyond the normal type. 



