A. The Vertebral Column and Ribs.
The vertebral column consists of a series of some hundred and thirty vertebrae, each of which is united with its predecessor and successor in such a way as to allow a large amount of flexibility.
These vertebrae are developed round an unsegmented rod, the notochord, which forms the axial support of the embryo. The notochord remains continuous throughout the whole vertebral column, but is greatly constricted opposite the middle of each vertebra, and thus rendered moniliform. The vertebrae are divided into two groups, an anterior group of trunk vertebrae, and a posterior group of caudal or tail vertebrae.
A typical vertebra consists of a middle portion, the centrum, a dorsal portion, the dorsal or neural arch, which surrounds the spinal cord, and a ventral portion, the ventral or haemal arch, which similarly encloses a space.
The tail vertebrae of the dogfish have this typical arrangement, the trunk vertebrae have the haemal arches modified.
Each centrum is a short cylinder of cartilage surrounding an hourglass-shaped cavity occupied by the notochord. The neural arches are composed of three separate elements, the vertebral neural plates (basidorsalia), intervertebral neural plates (interdorsalia), and neural spines (supradorsalia).
The vertebral neural plates are in the adult fused with their respective centra, and are notched behind for the exit of the ventral (motor) roots of the spinal nerves. The intervertebral neural plates are polygonal pieces alternating with the vertebral neural plates; they are notched behind, but at a more dorsal level than are the vertebral neural plates, for the exit of the dorsal or sensory roots of the spinal nerves.
The neural spines are small patches of cartilage filling up the gaps between the dorsal ends of the neural plates.
The haemal arches (basiventralia) differ much in the trunk and tail portions of the vertebral column. In the trunk portion the centra are flattened below, and the two halves of the haemal arch diverge from one another as blunt ventri-lateral processes to which short cartilaginous rods, the ribs, are attached. Further back at about vertebra 37, the two halves of the haemal arch project downwards and meet forming a complete arch. Further back still, towards the hind end of the tail, the haemal arches bear median haemal spines (ventrispinalia).
B. The Skull.