THE CONNECTIVE TISSUES AND THE SKELETAL SYSTEM. 



185 



part and gradually enlarges and involves the denser cephalic part. It is to be 

 noted that the denser tissue of the cephalic part of a vertebral body corresponds 

 to the caudal part of a sclerotome. Two chondrification centers appear, one on 



Costal process 



Aorta 



Arch of 

 vertebra 



Notochord 



Body of 

 vertebra 



Mesonephros 



FIG. 162. Transverse section (dorsal part) of pig embryo of 14 mm. Photograph. 



each side of the medial line, but the two soon fuse around the notochord to 

 form a single center. In addition to the center in the body of the vertebra, one 

 also appears in each half of the vertebral arch, and one in each costal process 



Arch of 



vertebra Interdorsal membrane 



Notochord 



Costal process 

 FIG. 163. Models of three vertebrae in the blastemal stage; from an embryo of n mm. Bardeen. 



(Fig. 164). All these centers then enlarge and unite to form a single mass of 

 cartilage which corresponds in shape quite accurately to the future bony 

 vertebra. Processes then grow out from the vertebral arch. These represent 



