DEVELOPMENT OF THE ENDOSKELETON 683 



(fig. 321 J). The less dense mesenchymal mass represents the rudiment of 

 the interdorsal vertebral element, while the posterior dense mass of mesen- 

 chyme is the basidorsal element. As development proceeds, the basidorsal 

 mass of cells from one segment and the interdorsal mass of the next posterior 

 segment on either side of the notochord move toward each other and align 

 themselves in the intersegmental area as shown in figure 32 IK, L. The basi- 

 dorsal element thus comes to lie along the anterior portion of the interseg- 

 mental area, and the interdorsal rudiment occupies the posterior part of 

 this area. The four vertebral elements, two on either side of the notochord 

 in the intersegmental area, form the basic vertebral rudiments, although rudi- 

 mentary basiventral and interventral elements possibly are present. The inter- 

 segmental artery eventually comes to lie laterally to the forming vertebra. 



Once these basic rudiments of the vertebra are established, the vertebra 

 begins to form. In doing so, there is an increase in the number of mesen- 

 chymal cells present, and the sclerotomic masses move toward and around 

 the notochord in the intersegmental position. The two dense basidorsal ele- 

 ments from either side expand dorsally around the neural tube as the two 

 interdorsal rudiments coalesce to form the body of the centrum (fig. 321M). 

 Laterally, the rudiment of the rib arises as a condensation of mesenchyme con- 

 tinuous with the forming neural arch and centrum. The rib element continues 

 to grow ventro-laterally, particularly in the thoracic area (fig. 321N). In the 

 lateral growth of the rib rudiment, surrounding mesenchyme is organized and 

 incorporated into the growing structure of the rudiment. 



Once the vertebral rudiment is established as a dense mass of mesenchyme, 

 the pre -cartilage stage of cartilage development occurs (fig. 313A). The pic- 

 cartilage stage is followed soon by cartilage (fig. 313B, C). Later, centers 

 of ossification arise as indicated in figure 3210, and the cartilaginous con- 

 dition becomes converted into a bony condition. Secondary centers of ossifi- 

 cation, forming bony epiphyses, ultimately arise after birth at the anterior 

 and posterior ends of each centrum. When the ultimate size of the vertebra 

 is attained, the epiphyseal cartilages between the epiphyses and the centrum 

 of each vertebra become ossified, and the epiphyses thus unite with the 

 centrum. The intervertebral discs of fibrocartilage form in the segmental 

 position between the vertebrae. 



It is to be observed that the intersegmental arrangement of the vertebrae 

 permits direct passage of the spinal nerves to the developing musculature 

 within each segment and also permits the musculature of each segment to 

 attach itself to two successive vertebrae. The latter feature is particularly 

 advantageous in lateral bending movements, so prominent in the swimming 

 movements of water-dweUing forms. 



See legend, fig. 321, for vertebral development in various vertebrates. 



c) Divisions of the Vertebral Column. In fishes, two main divisions 

 of the vertebral column are recognizable, the caudal region where the ver- 



