82 



OSTEOLOGY 



This stage is succeeded by that of the cartilaginous vertebral column. In the 

 fourth week two cartilaginous centers make their appearance, one on either side of 

 the notochord; these extend around the notochord and form the body of the cartil- 

 aginous vertebra. A second pair of cartilaginous foci appear in the lateral parts of 

 the vertebral bow, and grow backward on either side of the neural tube to form 

 the cartilaginous vertebral arch, and a separate cartilaginous center appears for 

 each costal process. By the eighth week the cartilaginous arch has fused with the 

 body, and in the fourth month the two halves of the arch are joined on the dorsal 

 aspect of the neural tube. The spinous process is developed from the junction of 

 the two halves of the vertebral arch. The transverse process grows out from the 

 vertebral arch behind the costal process. 



In the upper cervical vertebrae a band of mesodermal tissue connects the ends of 

 the vertebral arches across the ventral surfaces of the intervertebral fibrocartilages. 

 This is termed the hypochordal bar or brace; in all except the first it is transitory 

 and disappears by fusing with the fibrocartilages. In the atlas, however, the entire 

 bow persists and undergoes chondrification; it develops into the anterior arch of the 

 bone, while the cartilage representing the body of the atlas forms the dens or 

 odontoid process which fuses with the body of the second cervical vertebra. 



Anterior 



longitudinal 



ligament 



Posterior longitudinal 



ligament 

 Cartilaginous end 



of vertebral body 



Nucleus pulposus 



Intervertebral fibro- 

 cartilage 



Slight enlargement 

 of notochord in 

 the cartilaginous 

 vertebral body 



FIG. 66. Sagittal section through an intervertebral fibrocartilage and adjacent parts of two vertebrae of an advanced 



sheep's embryo. (Kolliker.) 



The portions of the notochord which are surrounded by the bodies of the verte- 

 brae atrophy, and ultimately disappear, while those which lie in the centers of the 

 intervertebral fibrocartilages undergo enlargement, and persist throughout life as 

 the central nucleus pulposus of the fibrocartilages (Fig. 66). 



The Ribs. The ribs are formed from the ventral or costal processes of the 

 primitive vertebral bows, the processes extending between the muscle-plates. In 

 the thoracic region of the vertebral column the costal processes grow lateralward to 

 form a series of arches, the primitive costal arches. As already described, the 

 transverse process grows out behind the vertebral end of each arch. It is at first 

 connected to the costal process by continuous mesoderm, but this becomes differ- 

 entiated later to form the costotransverse ligament; between the costal process 

 and the tip of the transverse process the costotransverse joint is formed by 

 absorption. The costal process becomes separated from the vertebral bow by the 

 development of the costocentral joint. In the cervical vertebrae (Fig. 67) the trans- 

 verse process forms the posterior boundary of the foramen transversarium, while 

 the costal process corresponding to the head and neck of the rib fuses with the 



