The atlas is usually ossified from three centers.

Of these, one appears in each lateral mass about the seventh week of fetal life, and extends backward; at birth, these portions of bone are separated from one another behind by a narrow interval filled with cartilage.

Between the third and fourth years they unite either directly or through the medium of a separate center developed in the cartilage.

At birth, the anterior arch consists of cartilage; in this a separate center appears about the end of the first year after birth, and joins the lateral masses from the sixth to the eighth year.

The lines of union extend across the anterior portions of the superior articular facets.

Occasionally there is no separate center, the anterior arch being formed by the forward extension and ultimate junction of the two lateral masses; sometimes this arch is ossified from two centers, one on either side of the middle line.
The atlas is usually ossified from three centers.
Of these, one appears in each lateral mass about the seventh week of fetal life, and extends backward; at birth, these portions of bone are separated from one another behind by a narrow interval filled with cartilage.
Between the third and fourth years they unite either directly or through the medium of a separate center developed in the cartilage.
At birth, the anterior arch consists of cartilage; in this a separate center appears about the end of the first year after birth, and joins the lateral masses from the sixth to the eighth year.
The lines of union extend across the anterior portions of the superior articular facets.
Occasionally there is no separate center, the anterior arch being formed by the forward extension and ultimate junction of the two lateral masses; sometimes this arch is ossified from two centers, one on either side of the middle line.
Atlas (anatomy)