The centres from which the bone is formed are those of its constituent vertebrae plus costal and epiphysial centres in the lateral mass, but the number of these last is variously estimated by different observers : possibly this bone, like others, is not absolutely constant in the number of its small secondary centres.

Each segment is laid down in cartilage, including the body and the neural half-arches with their transverse and articular processes. Costal chondrification forms the costal bars, which quickly fuse with the other cartilaginous parts proximally, and with each other distally to make the basis of the outer part of the lateral mass carrying the articular surface. Thus the bone as a whole is preformed in cartilage. Primary centres appear in the cartilage at very variable times, those for the proximal segments generally much earlier than the distal centres. The centres for the bodies appear between the third and seventh or eighth month, those for the neural arches after the mid-fcetal period, and the costal centres shortly after the neural centres : within these limits there seems to be much individual variation.

The centres extend very slowly, so that at birth the sacrum consists mainly ol cartilage. Secondary centres begin to show about puberty and continue to appear until after twenty-one : they include epiphysial plates for the bodies, epiphysial plates for the lateral masses, and a variable number of centres for the different bony prominences.

The centres mav be classed as follows :-

I. Primary.

1 for each body .......

5

1 for each half neural arch .....

10

1 for upper three costal bars .....

6

21

(Costal centres may be 2 or 4 on each side).

II. Secondary.

(a) (Constant).

Upper and lower epiphysis for each body .

10

Epiphysial plate for auricular surface

2

Epiphysial plate for margin below this

2

14

(b) (Variable).

Epiphysial tips for spinous, transverse, and mammillary processes. Additional centres for the margins.

The fusion of all these component parts commences in the lateral region, where the costal elements join the neural arches before the sixth year. A httle later the arches and costal bars join the centre, the more distal ones fusing first. As soon as this has occurred, the laminae commence to unite from before backward, completing the process at puberty, and then the costal processes unite with each other distally.

After this the epiphysial plates of the bodies commence their junction, from below upwards, the process not being completed until twenty-five.

Lastly, about this time, the lateral epiphysial plates and the remaining secondary centres consolidate with the bone.