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. The skull of the dogfish remains cartilaginous throughout the life of the animal, and has consequently a far more simple structure than have the skulls of higher animals, in which complication has been produced by the development of bone. The skull consists of the following parts:-- (1) a dorsal portion, the =cranium=, which lodges the brain, and to the sides of which the capsules of the auditory and olfactory sense organs are united. The cranium may be compared to an unsegmented continuation of the vertebral column; (2) a number of ventral structures, disconnected or only loosely connected with the cranium. These together constitute the =visceral skeleton= forming the jaws and supporting the gills. (1) THE CRANIUM.