VERTEBRAL COLUMN. 481 



formed. Further, in spite of the developmental differences, the mem- 

 brane and cartilage bones become closely united to one another, or 

 even fused, and there is thus formed "a firm, closed, bony receptacle 

 of mixed origin," as exemplified by the skull of any of the higher 

 Vertebrates. 



We may thus say that in the evolution of the skull there 

 is first a cartilaginous capsule, that this becomes invested 

 to a greater or less extent by dermal ossifications, and that 

 finally the dermal bones lose their superficial position, and, 

 fusing with the ossified remainder of the cartilaginous 

 cranium, form a complete bony capsule. In Cyclostomes 

 and Elasmobranchs the brain-box is wholly cartilaginous ; 

 above Elasmobranchs the cartilage is more or less thoroughly 

 replaced or covered by bones. In the individual develop- 

 ment there is a parallel progress. 



The segmentation of the head, in contradistinction to the unseg- 

 mented skull, is expressed, although indistinctly, by the muscle seg- 

 ments and by the nerves supplying these, perhaps also by the lateral 

 sense organs, the ganglia, and the arches. 



There are three pro-otic head-segments (pre-mandibular, mandibular, 

 and hyoid), which correspond to the orbital region, their walls forming 

 the six eye-muscles. Behind the auditory capsule there are ten or 

 eleven r\ead-segments. 



Vertebral column. A dorsal skeletal axis is character- 

 istic of Vertebrata, and its usefulness is evident. It gives 

 coherent strength to the body ; it is usually associated very 

 closely with a skull, with limb girdles, and with ribs; it 

 affords stable insertion to muscles ; its dorsal parts usually 

 form a protective arch around the spinal cord. 



To understand this skeletal axis, we must distinguish 

 clearly between the notochord and the backbone. 



The notochord is the first skeletal structure to appear in 

 the embryo. It arises as an axial differentiation of endo- 

 derm along the dorsal wall of the embryonic gut or 

 archenteron beneath the nerve-cord. The backbone, which 

 in most Vertebrates replaces the notochord, has a meso- 

 blastic origin. It develops as the substitute of the noto- 

 chord, and not from it, but from a skeletogenous sheath 

 surrounding it. 



According to Kleinenberg, the notochord supplies the 

 necessary growth stimulus for the rise of its substitute, the 

 backbone. 



