230 Vertebrate Embryology 



vertebral column, not with the musculature, 

 and before this segmentation takes place, the 

 vertebral column is represented by the unseg- 

 mented notochord. 



Until about the sixth or the seventh day, the 

 notochord is undiminished in size and of nearly 

 uniform diameter throughout ; but after that 

 time it is constricted and encroached on, at regu- 

 lar intervals, by the growth inwards of the cen- 

 tra of the vertebrae, and it finally disappears, 

 though a trace of it long remains, in the inter- 

 vertebral regions, as the ligamenta suspensoria. 



Ossification begins about the twelfth day, in 

 the centrum of the second or the third cervical 

 vertebra, and gradually extends backwards. 

 The neural arches ossify a little later, and 

 independently of the centra, each having two 

 centres of ossification. 



About the seventh day, the centrum of the 

 first cervical vertebra, or atlas, separates from 

 the rest of the bony ring, and becomes at- 

 tached to the axis, of which it forms the odon- 

 toid process. 



In the embryo of seven days there are 

 present forty-five vertebrae, of which the hind- 

 ermost five or six fuse, at a later period, to 

 form the pygostyle. 



