197 



another kind of connective tissue, namely, cells which form 

 true bone. 



The principal parts of the skeleton are : 



{(a) Notochord of embryo. The noto- 

 chord sheath becomes cartila- 

 ginous in adult Cyclostomata, 

 Elasmobranchii, Holocephali and 

 Dipnoi ; and in Elasmobranchii 

 is further transformed into a 



segmented vertebral column. 



1. Axial portion 



(b) Vertebral Column (backbone), with 



associated Ribs, becomes ossified 

 in certain fishes (e.g., Teleosteans), 

 in Amphibians, Reptiles, Birds, 

 and Mammals. 



(c) Skull, with associated Visceral 



Arches. 



2. Appendicular portion "1 



(absent or nidi- I ( d ) Fore Limbs > and Pectoral Girdle. 



mentary in certain { ( e ) Hind Limbs, and Pelvic Girdle. 



Craniata. J 



Name the Chief Parts of a typical Vertebra or Segment of 

 the Vertebral Column. 



A vertebra consists of a body or centrum, a dorsal neural 

 arch (around the spinal cord) culminating in a neural spine, 

 and transverse processes which project at the sides of the 

 centrum, and which are either without or with connected 

 ribs. In the tail region, instead of transverse processes 

 there is a ventral haemal arch (around the blood-vessels). 

 There are articular processes and facets on the centrum. 

 The centra may be concave at both ends, i.e., amphicoelous 

 (e.g., Skate, Teleostean Fishes), or concave in front and 

 convex behind, i.e., procoelous (e.g., Frog), or convex in 

 front and concave behind, i.e., opisthocoelous (e.g., certain 

 Fishes and Amphibians, Horse, Ox). The cervical verte- 

 brae of Birds are heterocoelous, i.e., the front end of the 

 centrum is convex from above downwards and concave 

 from side to side, and the hind end has the curvatures 

 reversed. 



How are the Vertebrae formed ? 



The cartilage-forming (mesenchyme) tissue around the 

 notochord constitutes the " skeletogenous layer." Cells 



