Development of the Fourth Day 229 



of a vertebral ring, with its attached neural 

 arch, and the anterior and posterior halves, 

 respectively, of the succeeding and preceding 

 intervertebral rings. The segments so formed 

 become the vertebra of the adult. The 

 planes of this secondary or permanent segmen- 

 tation, as it is sometimes called, do not corre- 

 spond with the planes of segmentation of the 

 mesoblastic somites, or the primary segmen- 

 tation. The secondary segmentation takes 

 place in such a way that the lines of separation 

 between the newly formed vertebrae lie oppo- 

 site the centres of the muscle plates. By this 

 alternate arrangement of the muscle segments 

 and the vertebral segments, each vertebra is 

 acted upon, on each side, by two muscles, the 

 preceding muscle being attached to the an- 

 terior half of the vertebra, the succeeding 

 muscle to the posterior half. The advantage 

 of this arrangement in producing motion or 

 bending of the spinal column is too evident to 

 need explanation. 



Although the segmentation of the cartilagi- 

 nous tube that surrounds the notochord has 

 been called " secondary," it is really not sec- 

 ondary in the strictest sense of the word. 

 This segmentation is concerned only with the 



