THE SKELETON. 145 



tudinal ridges with which a series of cartilaginous arches, which 

 appeared at the sides of the spinal cord at a slightly earliei 

 stage, very soon become continuous. 



By the appearance of transverse lines of demarcation, the 

 cartilaginous sheath of the notochord becomes cut up into a 

 series of nine vertebrae, followed by a posterior unsegmented 

 portion, which later becomes the urostyle. This transverse 

 division does not affect the notochord, which remains as a 

 continuous structure until the complete absorption of the tail 

 at the end of the metamorphosis. 



Shortly after the metamorphosis thin rings of bone, slightly 

 constricted in their centres, so as to be hour-glass-shaped in 

 section, are developed in the membrane investing the cartila- 

 ginous sheath of the notochord : these correspond with the nine 

 vertebrae already present, and form the first rudiments of the 

 vertebral centra. 



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 becomes, 

 after the metamorphosis, divided into an anterior and a posterior 

 portion, which fuse with the bony centra of adjacent vertebrae, 

 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 notochord persists in the middle of each centrum 

 for a long time, or even throughout life. 



The vertebrae are not placed opposite the 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 are at first independent of the 

 corresponding vertebrae, but very early fuse with them. They 

 extend into the septa between the myotomes, and probably 

 correspond to the ribs of other vertebrates. 



The urostyle is the part of the axial skeleton behind the 

 vertebrae ; it is not divided into vertebrae at any stage in 

 development. 



The anterior end of the notochord, imbedded in the base of 

 the skull, is gradually encroached on by the cartilage and bone 

 around it, and ultimately completely absorbed. 

 K 



