DEVELOPMENT OF THE VERTEBRAL COLUMN. 287 



This perfectly simple, inarticulate, primary axial 

 skeleton is soon replaced by an articulated, secondary 

 axial skeleton, called the " vertebral column." On each side 

 of the notochord the primitive vertebral bands or primitive 

 vertebral plates (vol. i. p. 306, Fig. 92, uiv) differentiate from 

 the inner portion of the skin-fibrous layer. The inner part 

 of these primitive vertebral bands, which immediately sur- 

 rounds the notochord, is the skeleton-plate, or skeleton 

 stratum (i.e., the cell-layer forming the skeleton), which 

 furnishes the tissue for the rudiments of the permanent 

 vertebral column and of the skull. In the anterior half 

 of the body the primitive vertebral plate remains a simple, 

 continuous, unbroken layer of tissue, and soon expands into 

 a thin- walled vesicle, which surrounds the brain ; this is the 

 primordial skull. In the posterior half, on the contrary, 

 the primitive vertebral plate breaks up into a number of 

 homologous cube-shaped pieces, lying one behind the other, 

 these are the several primitive vertebrae. The number 

 of these is at first very small, but soon increases, as the 

 germ grows in the posterior direction (Figs. 258-260, uw). 

 The first and earliest primitive vertebras are the foremost 

 neck-vertebras ; the posterior neck-vertebras then originate ; 

 then the anterior breast-vertebrae, etc. The lowest of the 

 tail-vertebrse arise last. This successive ontogenetic growth 

 of the vertebral column in a direction from front to rear 

 may be explained phylogenetically by regarding the many- 

 membered vertebrate body as a secondary product, which 

 has originated from an originally inarticulate parent-form 

 by progressive metameric development, or articulation. 

 Just as the many-membered Worms (Earth-worm, Leech) 

 and the closely allied Arthropods (Crabs, Insects) originally 



