54 EVOLUTION 



along the dorsal median line of the embry- 

 onic gut. In a few old-fashioned types, such 

 as lancelet and lamprey, there is no backbone, 

 but the notochord persists throughout life. 

 From fishes upwards it is gradually replaced 

 in development by the backbone. The noto- 

 chord does not become the backbone, but is 

 replaced by it. The two are quite different 

 embryologically, the notochord arising from 

 the inner germinal layer or endoderm, the 

 backbone arising, like the rest of the internal 

 skeleton, from the middle germinal layer or 

 mesoderm. In point of fact, the backbone 

 develops from a mesodermic sheath around 

 the notochord, a permanent structure around 

 a temporary structure, as a tall tower might 

 be built around an internal scaffolding of 

 wood. Now, what is the relation between the 

 more primitive axis or notochord and its 

 more effective substitute the backbone, seeing 

 that the former does not become the latter .f^ 

 In his interesting theory of "the substitution 

 of organs," Xleinenberg suggested that the 

 notochord supplies the stimulus, the neces- 

 sary developmental condition, for the forma- 

 tion of the backbone when suitable materials 

 are forthcoming. Of course we require to 

 know more precisely how the old-fashioned 

 structure prepares the way for and stimulates 

 the growth of its future substitute, but the 



