198 



ESSENTIALS OF ZOOLOGY 



Bone expands by growth of the periosteal layer, successive 

 layers being formed. The skeleton thus gains in hardness and 

 in rigidity, but it loses in elasticity. 



The vertebral column is formed around the notochord of 

 the larva. Cartilaginous arches are developed in the meso- 

 derm around the notochord and spinal cord, those on the 

 dorsal side being conspicuously large. The latter expand 

 upwards to form the neural arches. Along the notochord a 

 series of chondrifications of the sheath of the notochord form 

 a series of intersegmental rings. A split appears in each ring 

 separating the centra ; and in the middle of each centrum 



cartilage is formed in the 

 notochord. A ring of bone 

 appears early in each cen- 

 trum, and the cartilage is 

 gradually replaced by bone. 

 The haemal arches become 

 more conspicuous posteriorly. 

 In the frog there are nine 

 FIG. 98. Development of vertebra, vertebrae and the urostyle. 



Cartilaginous neural and haemal 

 arches fusing to form neural arch 

 and centrum. 



All the figures in this chapter 

 refer to Rana temporaria. 



Rib 

 Spinal cord 



Neural process 



Notochord 

 Haemal process 



The second to the seventh 

 vertebrae are formed in each 

 case of a flattened centum 

 bearing a neural arch and 

 produced laterally into the prominent transverse processes. The 

 arch extends upwards into a small spine, and anteriorly and 

 posteriorly bears articular processes, or zygapophyses. The 

 articulation is, as usual, the posterior processes overlapping the 

 anterior of the succeeding vertebrae. Each of these vertebrae 

 is procoelous ; that is to say, is hollow in front and rounded 

 behind. The first vertebra differs in the absence of transverse 

 processes and anterior articular processes. It is attached to 

 the occipital condyles of the skull by two diverging facets on 

 its anterior surface. The eighth is amphicoelous hollow at 

 both ends of the centrum. The ninth forms the sacrum, and is 

 at once distinguished by the stronger upwardly directed trans- 

 verse processes, which are articulated to the ilia of the pelvis. 

 The centrum is convex in front and presents two projecting 

 processes behind for the articulation of the urostyle. The 



