VERTEBRAL COLUMN 



51 



with that of the centra, or the number of the latter with that of 

 the myntomes. 



Articular processes (zygapophyses) are usually present on the 

 neural arches of Bony Fishes. 



In Plagiostomes, the cartilage which has invaded the sheath of 

 the notochord is segmented into definite vertebral centra, which 

 become partially calcified in various ways. The calcification (Fig. 

 40) may in each centrum take the form of a double cone, con- 

 stricted in the middle, as in Scymnus and Acanthias (cyclo- 

 spondylic form) ; concentric layers may be added to this, as in the 

 Rays (tectospondylic) ; or longitudinal plates may be formed radiating 

 outwards from the double cone, as in Scyllium (aster ospondylic). 

 The dorsal and ventral arches usually extend round the centrum 



ha 



Fie. 40. DIAGRAMMATIC TRANSVERSE SECTION THROUGH THE MIDDLE OF A 



C \CLOSPONDYLIC (A), A TKCTOSPONDYLIC (B), AND AX ASTEROSPONDYLIC 



VERTEBRA (C). (From Zittel, after Hasse. ) 



d, middle portion of the calcified double cone ; d e , additional concentric calcified 

 layers ; d", double cone with radiating calcified layers ; ex.m, external elastic 

 membrane ; h.a, ha?mal arch ; n.a, neural arch ; n.c, uotochordal cavity. 



so as to enclose it, and in the tail there may be two or more sets of 

 vertebral elements to each body segment. 1 



In Bony Ganoids and Teleosts, there is a tendency towards a 

 reduction of the cartilage ; that which forms the centra is entirely 

 outside the notochordal sheaths, and the vertebrae become more or 

 less densely ossified. 



In the course of development of the centra in all cases, the 

 notochord becomes constricted by the growth of the cartilage at 

 regular intervals while the latter undergoes segmentation into 

 centra. Each point of constriction corresponds to the middle of a 

 centrum, i.e., it is intra vertebral in position, and the notochord 

 may here disappear entirely ; inter vert ebrally it remains expanded 



1 In Rays and Chimaroids the anterior vertebral elements become fused 

 into a single mass, on which a definite condyle is formed for articulation with the 

 skull ; amongst Sharks and in Dipnoans also, a concrescence of the anterior vertebral 

 elements with one another and with the skull may occur. 



E '2 



