The Theory of Evolution 65 



the digestive tract. In the cartilaginous fishes the notochord 

 also appears at a very early stage, and also from the dorsal 

 wall of the digestive tract. In later embryonic stages it 

 becomes surrounded by a cartilaginous sheath, or tube, 

 which then segments into blocks, the vertebrae. The noto- 

 chord becomes partially obliterated as the centra of the 

 vertebras are formed, but traces of it are present even in 

 adult stages. In the lower amphibians the notochord arises 

 also at an early stage over and perhaps, in part, from the 

 dorsal wall of the digestive tract. It is later almost entirely 

 obliterated by the development of the vertebrae. These 

 vertebrae first appear as a membraneous tube which breaks 

 up into cartilaginous blocks, and these are the structures 

 around and in which the bone develops to form the per- 

 manent vertebrae. 



In higher forms, reptiles, birds, and mammals, the noto- 

 chord also appears at the very beginning of the develop- 

 ment, but it is not certain that we can call the material out 

 of which it forms the dorsal wall of the archenteron (the 

 amphibians giving, perhaps, intermediate stages). It be- 

 comes surrounded by continuous tissue which breaks up into 

 blocks, and these become the bases of the vertebrae. The 

 notochord becomes so nearly obliterated in later stages that 

 only the barest traces of it are left either in the spaces 

 between, or in, the vertebrae. 



In this series we see the higher forms passing through 

 stages similar at first to those through which the lower forms 

 pass ; and it is especially worthy of note that the embryo 

 mammal begins to produce its notochord at the very begin- 

 ning of its development, at a stage, in fact, so far as compari- 

 son is possible, as early as that at which the notochord of 

 amphioxus develops. 



The development of the skull gives a somewhat similar 

 case. The skulls of sharks and skates are entirely cartilagi- 

 nous and imperfectly enclose the brain. The ganoids 



