THE VERTEBRAL COLUMN. 199 



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

 nous, is formed from tlie mesoblast surrounding the notochord 

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

 cord as a pair of l ongitudinal ridge s, 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 skele ton be comes cut up into a series of nine vertebree , 

 follnwe d^ by a, pos terior unsegmented portion, which later on 

 giv sH rise to the iirnst vle. The vertebree 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, t hin rings of bone, slight lv 

 cons tricted in their middle s so as to be hour-glass shaped in 

 section, ar e developed in the membrane investing the carti la- 

 gi nous sheath of the no tochord ; these from their first appear- 

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

 which they give rise. Like the cartilaginous vertebrae, they 

 develop in order from before backwards. 



In the intervertebral regions, be tween the successive b ony 

 rings, a nnular thicken ings of the cartilagin ous sheath occur , 

 w hich gr ow inwards so as to constrict, and u ltimately 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 

 vertebra, and ossify to form their articular ends. 



Fro m the circmnfereTic.p,^ a.n d from th e articular ends of ea ch 

 vertebra , os sification 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 tra nsverse processe s ar ise independe ntly ofthe_corre- 

 sponding vertebrae, but s ^ry early fuse with the se. 'I hey extend 

 alon^ tne connective-tissue septa between the successive 



