THE ENDOSKELETON 123 



ments of the vertebrae are derived. The notochord, which 

 is endodermic and arises from the primitive alimentary canal 

 in the manner related above, is the only portion of the endo- 

 skeleton not formed from the mesenchyme, and is hence not a 



FIG. 32. Diagram of vertebrate, showing relation of skeleton to soft 

 parts. 



AXIAL SKELETON: tr, trabecula; p, parachordal; d, dermal bones of skull; nt, 

 notochord; np, neuropophyses ; hp, haemapophyses: 



VISCERAL SKELETON: m, mandible; br, branchial arches. 



APPENDICULAR SKELETON: ga, anterior; d, its dermal element; gp, posterior 

 girdle; x, anterior free limb; y, posterior free limb. 



SOFT PARTS: nv, nervous system; hy, hypophysis; a, aorta; v, sub-intestinal vein 

 (an embryonic organ) ; int, intestine. 



connective tissue; but this becomes gradually replaced by 

 skeletal elements formed from true mesenchymatous tissue, so 

 that in the higher forms the adult skeleton is wholly from this 

 latter source. The notochord seems to have been a very 

 ancient form of endoskeleton, antedating that formed of con- 

 nective tissue and functioning in the immediate ancestors of 

 the present-day vertebrates. It is present in what may be 

 nearly its original condition in Amphioxus, where it appears 

 as a cylindrical rod, extending through the longitudinal axis 

 of the body from end to end. This rod furnishes a certain 

 degree of rigidity and allows the animal to maintain a fixed 

 length, while permitting a considerable amount of flexibility 

 through its elasticity. In about the same condition it appears 

 as a constant organ during the early embryonic life of every 

 vertebrate ; and, although it is usually replaced in great measure 

 by mesenchymatous elements, yet in some fishes, even in those 



