186 



TEXT-BOOK OF EMBRYOLOGY. 



the transverse and articular processes (Fig. 165). Each half of a vertebral 

 arch meets its fellow of the opposite side dorsal to the spinal cord, and from 

 the point of meeting the spinous process grows out. The costal processes do 



Costal process 

 (rib) 



Body of 



vertebra 



Costal process 

 (rib) 



Aorta 



Pleural cavity Liver CEsophagus Lung 



Fig. 164. — Transverse section (dorsal part) of pig embryo of 35 mm. Photograph. 



not retain their connection with the body of the vertebra, but break away 

 and become the rib cartilages, as will be noted again in connection with the 

 development of the ribs. 



Following the cartilaginous stage is the stage of ossification in which the 



Arch of vertebra 

 Post, articular 

 process 



Transverse process 



Anterior articular process 



Fig. 165.— Models of the 6th, 7th and 8th thoracic vertebra; of an embryo of 33 mm. 



(dorsal view). Bardeen. 



On the right the cartilage is shown, on the left the surrounding fibrous tissue. 



vertebrae become ossified and acquire the adult condition. Ossification begins 

 during the third month of foetal life and extends over a long period, even up to 

 the age of twenty-five years. A single center of ossification appears in the body 



