DEVELOPMENT OF FOURTH DAY 517 



around the notochord throughout its entire length, while at each side of 

 the spinal cord paired bars of cartilage form, which will shortly fuse 

 with the cartilaginous sheath of the notochord to form the beginnings of 

 the neural arches. 



Toward the end of the fifth day the points opposite the attachment 

 of the neural arches become thickened and more mature, but the por- 

 tions between the neural arches retain their embryonic character. This 

 causes what has been called a secondary segmentation of the cartilag- 

 inous tube. Later this segmentation becomes still greater until the en- 

 tire cartilaginous tube is made up of a series of vertebral rings or seg- 

 ments, each segment consisting of a vertebral ring with its attached 

 neural arch, and the anterior-posterior halves, respectively, of the suc- 

 ceeding and preceding intervertebral rings. Each of these segments 

 becomes one of the vertebrae which constitute the spinal column. 



-Med. ossif. center 

 Fig. 306. 



Thoracic vertebra and ribs of human embryo of 55 mm. 



(Middle of 3rd month) to show ossification centers. Cartilage is 



indicated by stippled areas, and ossification centers by irregular 

 black lines. (After Kollmann.) 



It must be understood, however, that these so-called secondary seg- 

 ments do not correspond with the somites from which they were formed. 

 The secondary lines of segmentation lie at about the center of the mus- 

 cle plates (Fig. 305), so that each of these secondary segments obtains 

 approximately one-half of the muscle action from the immediately an- 

 terior muscle plate, and one-half from the immediately posterior muscle 

 plate, thus making it possible for each one of the vertebrae to have the 

 muscles from two regions act upon it. 



The spinal column develops around the notochord. 



Ossification of the vertebrae begins about the twelfth day in the 

 centrum of the second or third cervical vertebra, gradually extending 

 caudad. The neural arches ossify still later, there being two centers of 

 ossification for these. (Fig. 306.) 



On about the seventh day, the centrum of the first cervical vertebra, 

 or atlas, separates from the rest of the bony ring and becomes attached 

 to the axis to form the odontoid process. 



On the seventh day there are present about forty-five vertebrae. 



