THE VERTEBRAL COLUMN. 199 



delicate skeletal tube, at first soft, but soon becoming cartilagi- 

 nous, is formed from the mesoblast surrounding the notochord 

 (Fig. 84, CH). This grows upwards at the sides of the spinal 

 cord as a pair of longitudinal ridges, with which a series of 

 cartilaginous arches, which appeared at the sides of the spinal 

 cord at a slightly earlier stage, soon become continuous (Figs. 

 87, 88). 



By the appearance of transverse lines of demarcation, this 

 axial skeleton becomes cut up into a series of nine vertebrae, 

 followed by a posterior unsegmented portion, which later on 

 gives rise to the urostyle. The vertebras are cut off in order, 

 from before backwards, and the division at first involves the 

 cartilaginous tube alone, the notochord remaining as a continuous 

 structure until the complete absorption of the tail, at the end of 

 the metamorphosis (Fig. 89). 



Shortly before the metamorphosis, thin rings of bone, slightly 

 constricted in their middles so as to be hour-glass shaped in 

 section, are developed in the membrane investing the cartila- 

 ginous sheath of the notochord ; these from their first appear- 

 ance correspond with the nine vertebrae, to the bony centra of 

 which they give rise. Like the cartilaginous vertebrae, they 

 develop in order from before backwards. 



In the intervertebral regions, between the successive bony 

 rings, annular thickenings of the cartilaginous sheath occur, 

 which grow inwards so as to constrict, and ultimately obliterate 

 the notochord. Each of these intervertebral rings, after the 

 metamorphosis, becomes divided into anterior and posterior 

 portions, which fuse with the bony centra of the adjacent 

 vertebras, and ossify to form their articular ends. 



From the circumference, and from the articular ends of each 

 vertebra, ossification gradually spreads inwards ; but a small 

 portion of the notochord persists in the middle of each centrum 

 for a long time, or even throughout life. 



The vertebras do not lie opposite the muscle segments or 

 myotomes, but alternate with these ; so that each vertebra is 

 acted on by two myotomes on each side, one pulling it forwards, 

 and the other backwards. 



The transverse processes arise independently of the corre- 

 sponding vertebras, bat very early fuse with these. They extend 

 along the connective-tissue septa between the successive 



