NATUHE OF THE VEIITEBE.E. 285 



which are enclosed the large blood-vessels on the under 

 surface of the vertebral column — the aorta and the tail vein. 

 In hip"her Vertebrates most of these inferior vertebral arches 



o 



are lost or become merely rudimentary. But in the breast 

 section of the vertebral column they develop into strong, 

 independent bony arches, the ribs (costce). The ribs are, in 

 fact, merely large vertebral arches which have become 

 independent, and have broken their original connection 

 with the vertebral bodies. The gill arches, of which we 

 have spoken so often, are of similar origin ; they are actual 

 head-ribs in the strictest sense — processes which have 

 actually originated from the lower arches of the skull- 

 vertebrae, and which correspond with the ribs. Even the 

 mode of connection of the right and left halves of the arches 

 on the ventral side is the same in both instances. The 

 chest is closed in front by the intervention, between the 

 upper ribs, of the breast-bone (sternum) — a single bone 

 originating from two corresponding side-halves. The gill- 

 body is also closed in front by the intervention of a single 

 piece of bone — the copula lingualis. 



In now turning from this anatomical examination of the 

 constitution of the vertebral column to the question of its 

 development, I may, as regards the first and most important 

 features in the evolution, refer the reader to the explanation 

 already given of the germ-history of the vertebral column 

 (Chap. XII., vol. i. pp. 369-378). In the first place, it is ne- 

 cessary to recollect the important fact that in Man, as in all 

 other Vertebrates, a simple, unarticulated cartilaginous rod 

 at first occupies the place of the articulated vertebral column. 

 This firm but fiexible and elastic cartilaginous rod is the 

 well-known notochord (cJiorda dorsalis). In the lowest Ver- 



