The Spinal Column and Cranium
Now appear the primitive cartilaginous and membranous elements from which a bony wall is to be built around the central nerve axis, primitive vertebrae, the upper known as cranial and numbering four, and the lower, or spinal, numbering usually thirty-three. These bone structures develop around the brain and spinal cord. Later the cephalic vertebrae fuse into a solid vault, the cranium, completely enclosed except for various foramina for the passage of spinal cord, nerves, and blood-vessels. The succeeding twenty-four vertebrae remain separate and movable upon each other and leave between them the openings for the emergence of the spinal nerves. The last nine segments fuse eventually into two immovable or false vertebrae called Sacrum and Coccyx. These latter also contain foramina from which nerves issue.