428 COMPARATIVE ANATOMY. 



or less homogeneous vertebral column, which extends to the caudal 

 end of the body. The upper arches remain in close connection with 

 the centra. Movable girder-like pieces are, however, separated off 

 from the lower arches in the region which encloses the ccelom ; these 

 are the ribs. 



Lastly, there are the skeletal portions of the appendages 

 which are connected by special organs — the pectoral and pelvic 

 girdles — with the skeleton of the trunk. 



The cartilaginous stage of the primitive skeleton is found 

 in all of the higher divisions, but in them it has no function 

 after a short time, for it is gradually replaced by osseous tissue, 

 whereby the skeletal parts come to have a greater physiological 

 importance. In correlation with this we note a greater differen- 

 tiation in morphological points. Even in the osseous skeleton, 

 however, the cartilage is of great importance. A modified form of 

 cartilage, which is characterised by the deposit of calcareous matter 

 in it, is also of importance. This form is not only antecedent to the 

 ossification of the parts of the skeleton, which are laid down in 

 cartilage, but — as is seen in that superficial calcification of the car- 

 tilaginous skeleton of the lower Griiathostomata — is also sometimes 

 a permanent arrangement. 



Vertebral Column. 



§ 328. 



The separation of the rachis into skull and vertebral column is 

 not completely effected in Amphioxus ; the whole axial skeleton is 

 represented by the notochord. In the Crauiota they begin to be 

 separated. The lowest characters of the spinal column obtain in 

 the Cyclostomata, where the more highly-developed notochord, with 

 its sheath, forms the chief portion of it. Around the sheath there is 

 cartilaginous tissue, which is continued into lateral ridges as well as 

 into the wall of the dorsal canal. This tissue has its origin in the 

 continuous differentiation of the skeletogenous layer, and must not 

 be confounded with the cartilages, which ordinarily form the 

 vertebral segments. Speaking exactly, therefore, the spinal column 

 is not here separated into distinct vertebras ; of which, indeed, there 

 are indications only in Petromyzon, where cartilaginous pieces, which 

 correspond to the superior arches, are enclosed in the wall of the 

 more anterior division of the dorsal canal. We meet also with 

 indications of inferior arches. 



The notochord also retains its primitive characters in Chimera 

 and the Dipnoi. In the Chimajras circular calcifications of the large 

 chordal sheath point to a segmentation of the notochordal tube, but 

 they arc more numerous than the primitive vertebra), which are 

 merely represented by the arches on the chordal sheath. In the 

 most anterior region they grow round the chord and give rise by 

 fusion to a larger undivided piece. In the Dipnoi a strong tube, 



