36 COMPARATIVE ANATOMY. 



articulation at its base, and thus give rise to a rib (cp. p. 48). 

 The relations of these parts in Elasmobranchs and Teleosteans is 

 similar to that above described. For the further strengthening of the 

 vertebral column so-called "intercalary pieces" (Figs. 21, 22, 

 7c) appear between the upper and lower arches in Cartilaginous 

 Ganoids and Elasmobranchs. The vertebral column of Bony 

 Ganoids reaches a much higher stage of development. Paired 

 dorsal and ventral cartilages arise above and below the notochord, 



FIG. 24. PORTION OF THE VERTEBRAL COLUMN OF Polypicrus. 

 WK, centra ; BF, basal processes ; Ob, upper arches ; Ps, spiuous processes. 



but in the course of development so extend at the base as to com- 

 pletely surround it. From the dorsal cartilages the upper arches 

 take their origin, and from the ventral the lower, while the car- 

 tilage surrounding the notochord gives rise to the vertebral 

 bodies or centra. The whole vertebral column also becomes 

 strongly ossified (Fig. 24). The notochord is now no longer equal in 

 diameter throughout, but becomes constricted or actually divided 



1VK 



FIG. 25. DIAGRAM SHOWING THE INTERVERTEBRAL REMAINS OF THE 

 NOTOCHORD. 



C, C 1 , expanded and constricted portions of notochord ; WK, centra ; Li, inter- 

 vertebral ligaments. 



in the centrum of each vertebra (that is, vertebrally), while 

 intervertebrally it remains expanded, and so serves as a sort of 

 connecting- or packing- substance between two contiguous vertebrae 

 (Figs. 25 and 27, C, C 1 ). The same thing takes place in other 

 Fishes Elasmobranchii and Teleostei and thus deeply 

 biconcave (amphiccelous) centra are formed. 



One of the Bony Ganoids, Lepidosteus, forms a marked exception to other 

 Fishes as regards its vertebral column, inasmuch as definite articulations are 

 formed between the vertebra. A concavity is formed at the hinder end of 



