32C THE CHICK. 



the notochord. Opposite the places of attachment of the neural 

 arches, the matrix becomes more abundant, and the cartilage 

 cells fewer; while between the successive neural arches the 

 matrix remains comparatively scanty, and the cartilage cells 

 more numerous. In this way the cartilaginous tube round the 

 notochord, while still remaining a continuous unsegmented 

 structure, becomes marked into alternate vertebral and inter- 

 vertebral rings, the vertebral rings being the parts to which the 

 neural arches are attached, and in which the cartilage is of a 

 more mature type ; and the intervertebral rings being the parts 

 between successive neural arches, in which the cartilage remains 

 of a more embryonic character. 



Each intervertebral ring, about the end of the fifth day, 

 divides into two portions, anterior and posterior, which attach 

 themselves to the vertebral rings in front of, and behind, them 

 respectively. By this division the originally continuous carti- 

 laginous sheath of the notochord becomes cut up into a series of 

 segments ; each segment consisting of a vertebral ring, fused 

 with the posterior and anterior halves of successive inter- 

 vertebral rings, and fused also with a cartilaginous neural arch. 

 The segments so formed become the adult vertebrae (Fig. 110). 



The original vertebral rings, and the neural arches, lie oppo- 

 site the intervals between successive pairs of muscle-plates ; 

 while the intervertebral rings lie opposite the muscle-plates 

 themselves. As the division into vertebrae takes place across 

 the centres of the intervertebral rings, it follows that the 

 planes of division between the vertebraa do not coincide with the 

 planes of division between the muscle-plates, i.e. with the 

 original protovertebral planes of division, but are midway 

 between these. Hence the division, or segmentation, of the 

 vertebral column has been spoken of as secondary or permanent 

 segmentation, in contrast to the primary or protovertebral 

 segmentation which is retained by the muscle-plates or 

 myotomes. 



It must be borne in mind, however, that the permanent 

 segmentation is the only one ever shown by the skeletal elements 

 themselves. The primary segmentation is essentially a divi- 

 sion into myotomes or muscle segments, and occurs at a time 

 when the notochord is the only skeletal structure present. The 

 cartilaginous skeleton is at its first appearance unsegmented, and 



