head-half

Updated: August 21, 2026

But this unsegmented primary axial skeleton is soon replaced by the segmented secondary axial skeleton, which we know as the vertebral column. In the head-half of the embryo the skeletal plate remains a continuous, simple, undivided layer of tissue, and presently enlarges into a thin-walled capsule enclosing the brain, the primordial skull.

Ernst Heinrich Philipp August Haeckel - The Evolution of Man — Volume 2