346 THE EVOLUTION OF MAN. 



In Fig. 107 there are seven, in Fig. 108 there are 

 eight, and in Fig. 109 ten pairs of primitive vertebrae. Their 

 number afterwards increases considerably, amounting in 

 Man to upwards of thirty. As we shall presently see, out 

 of each pair of these primitive vertebral segments an indi- 

 vidual section of the trunk, a metameron, develops. Each 

 pair of primitive vertebrae is not, as the name seems to 

 indicate, merely the rudiment of a future vertebra, but, in 

 addition to the vertebra, the appropriate muscles also 

 develop from it, as does a pair of nerve-roots, etc. It is 

 only the innermost portion of the primitive vertebra, the 

 part lying next to the notochord, that gives rise to the 

 rudiment of the articulated vertebral column, extending 

 from the cranium to the tail, and composed of a number of 

 bony vertebral rings.^^ 



The breaking up of the vertebral cord into a double 

 chain of primitive vertebral segments, or, briefly, the 

 forming of the metamera, is of the greatest importance, 

 because it is in this process that the body of the Vertebrate 

 passes from its originally inarticulate to its permanent 

 articulate conditions. The developed Vertebrate is composed 

 of a chain of homogeneous parts, lying one behind another 

 precisely as are the Articulated Animals {Artliropoda). 

 In the latter class, in Crabs, Spiders, Millipedes, and insects, 

 this articulation is very clearly marked externally, the 

 skin between each two members (metamera) having a ring- 

 shaped contraction or dent round the circumference of the 

 body. In Vertebrates the articulation of the body is equally 

 complete, but it does not appear externally, though internally 

 it is fundamental. Every Vertebrate, in its perfect state, is 

 an articulated person. Its personality forms a chain of 



