6o 



LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



basidorsal 



notochord 



basidorsal 



basiventral 



neural arch 



neural canal 



notochord 



haemal canal 



basiventral 



interventral 



FIG. 17. Diagrams to show the arcualia from which the vertebrae are formed. A, side view, 

 showing the four arcualia of one side to each segment. B, cross-section through the basiventrals and 

 basidorsals, showing their relation to the notochord. C, later stage of the same cross-section as in B, 

 showing union of the basidorsals to form the neural arch and of basiventrals to form the haemal arch. 



g 



E F 



FIG. 18. Diagrammatic sections through developing vertebrae to show formation of the arches 

 and the centra. A, early stage, showing the skeletogenous regions b around the notochord. B-D, 

 formation of the chordal type of centrum: B, appearance of the arcualia/ and the sheath of the noto- 

 chord e; C, invasion of skeleton-forming cells from the arcualia into the sheath of the notochord, as at.;'; 

 D, completion of the chordal centrum in the sheath of the notochord, completion of the arches by fusion 

 of the arcualia, and union of centrum and arches. E and F, formation of the perichordal or arch type 

 of centrum: E, appearance of the arcualia/ and the sheath of the notochord e; F, formation of the 

 centrum and the arches by the fusion of the arcualia around and above and below the notochord. 

 a, notochord; b, skeletogenous regions; c, neural canal; d, haemal canal; e, sheath of the notochord; 

 /, arcualia; g, neural arch; h, centrum; ", haemal arch. (From Wiedersheim's Comparative Anatomy 

 of Vertebrates, courtesy of the Macmillan Company ) 



