36 COMPARATIVE AN ATOM V. 



articulat : on at its base, and thus give rise to a rib (cp. p. 4S). 

 Tin' relations of these parts in Elasmobranehs and Teleostcans is 

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

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

 Ic) appear between the upper and luwer arches in Cartilaginous 

 Ganoids and Elasmobranehs. The vertebral column of Bony 

 Ganoids reaches a much higher stage of development. Paired 

 dorsal and ventral cartilages arise above and below the notochord, 



M" 



FIG. 24. PORTION OF THE VERTKBRAL COLUMN OF 

 //"A", centra ; HF, basal processes ; Oft, upper a relies ; Ps, spinous processes. 



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

 pletely surround it. From the dorsal cartilages the upper arcln-- 

 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 



Zi f" 



Y 

 . 



//A \< 



I'l... -jr.. In M.I: \M MiowiM: mi [NTEBVEUTEBRAL I;I:M\IV> a}- mr. 



NOTOCHORD. 



(7, C* 1 , expanded and constricted portions of notocliord ; //'A', centra; /./, intcr- 



in the <eiitn.ni nf each vertebra 'that is, vertebrally), while 

 i n t'Tvert ebrally it remains rxpamled, and so serves as a sort of 

 r.iiiiMM tiii-4- or packing- substance l>et\veen two contiguous vertebraB 

 (Figs. -~> and '11, (', C 1 ). The same thing takes place in other 

 Fi.-lies ElasmobrancWi and Teleostei and thus deeply 

 biconcave (amphicoelous) lentra are formed. 



< >in' nf I li.' l',..n\ ( !an nil 1<, Lrji id 1 1- ! r n >, forms a m.ii \,< <\ .\ci-pl i. in to other 



I'i-lH :\ regard rtebral ix-lumn, inasmuch as definite articulations are 



liet \vee|| tile \ i ! t c ] ,] ie. A eolna\il\ i< I'oniieil at tlie llill']']' i)i(l of 



