188 



TEXT-BOOK OF EMBRYOLOGY. 



five years. About the twenty-fifth year the sacral vertebrae unite to form a 

 single mass of bone, and a similar union also takes place between the more or 

 less rudimentary coccygeal vertebrae. 



While the general plan of development is practically the same in all the 

 vertebras, there are a few noteworthy modifications. The greatest modification 

 is in the atlas and epistropheus (axis). The entire atlas is formed from the 

 denser caudal part of a sclerotome. The lateral mass and the posterior (dorsal) 

 arch represent the vertebral arch. The anterior (ventral) arch represents the 

 hypochordal bar, a plate of cartilage which develops in all vertebrae ventral to 

 the notochord but disappears in all except the atlas. A body also develops 

 but instead of forming part of the atlas it unites with the body of the epistro- 

 pheus to form the dens (odontoid process) of the latter. 



Clavicle 



Suprasternal cartilage 



Sternal bar 



7th rib 



FIG. 168. Ventral view of developing sternum of human embryo of 30 mm. 

 (beginning of 3rd month). Rugc, Kollmann's Atlas. 



The various ligaments of the vertebral column are derived from the embry- 

 onic connective tissue surrounding the vertebrae. The embryonic connective 

 tissue in the clefts separating the developing vertebrae is transformed into the 

 intervertebral fibrocartilages. 



The Ribs. It has been stated in a previous paragraph that the costal proc- 

 esses arise as outgrowths from the denser caudal parts of the sclerotomes; that 

 they grow in a ventro-lateral direction and consequently are at first connected 

 with and are parts of the bodies of the vertebrae (Figs. 162 and 165). These 

 costal processes are the anlagen of the ribs, and they continue to grow ventrally 

 until they practically encircle the body, the ventral ends of a number of them 

 fusing in the medial line to form the sternum. The primary junctions between 

 the costal processes and vertebrae are dissolved, and the embryonic connective 

 tissue in this region gives rise to the costo-vertebral ligaments. The dissolu- 

 tion of the junctions leaves the ribs simply articulating with the vertebrae. 



