THE ENDOSKELETON: VERTEBRAL COLUMN AND RIBS 



63 



C. SOME PRIMITIVE VERTEBRAL COLUMNS 



It has already been stated that in some living and in many extinct forms the 

 vertebrae are embryonic in structure, consisting of separate arcualia and without 

 a centrum. In other cases diplospondyly is evident. We may glance at a few 

 of these. 



1. The vertebral column of the sturgeon. — In the sturgeon the vertebral col- 

 umn is very primitive — acentrous, and consisting of separate arcualia. Examine 



A 



Fig. 21.— Some primitive vertebral columns. A, cyclostomes; the vertebral column consists of a 

 row of arches, two arches to each segment, resting on the dorsal surface of the notochord. B, sturgeon; 

 the vertebral column consists of separate arcualia partially surrounding the notochord. C, tail region 

 of Amia, showing the two centra, k and I, to each segment, illustrating diplospondyly. D, vertebrae 

 of an extinct amphibian belonging to the Stegocephala, showing the arcualia around the notochord. 

 a, segmental blood vessels; b, nerve; c, neural tube; d, notochord; e, basidorsal or neural arch; /, basi- 

 ventral; g, interdorsal; h, interventral; i, neural spine or supradorsal; j, rib; k, posterior centrum; 

 I, anterior centrum, for each segment; m, haemal arch; n, haemal spine; 0, blood vessels. {A and B 

 after Goodrich in Part IX of Lankester's Treatise on Zoology, courtesy of the Macmillan Company, 

 C from Zittell, D from Kingsley's Comparative Anatomy of Vertebrates, after Zittell, copyright by 

 P. Blakiston's Son and Company.) 



the demonstration specimens and compare with Figure 21B. The large noto- 

 chord traverses the center of the vertebral column and is covered dorsally and 



