In =imperfect joints=, such as the intervertebral joints of mammals, the two contiguous surfaces are united by a mass of fibrous tissue which allows only a limited amount of motion. In =perfect joints= the contiguous articular surfaces are covered with cartilage, and between them lies a synovial membrane which secretes a viscid lubricating fluid. The amount of motion possible varies according to the nature of the articular surfaces; these include-- _a._ =ball and socket joints=, like the hip and shoulder, in which the end of one bone works in a cup provided by another, and movements can take place in a variety of planes. _b._ =hinge joints=, like the elbow and knee, in which as in ball-and-socket joints one bone works in a cup provided by another, but movements can take place in one plane only. THE ENDOSKELETON. The endoskeleton is divisible into =axial= and =appendicular= parts; and the =axial= skeleton into-- 1. the spinal column, 2. the skull {_a._ the cranium, {_b._ the jaws and visceral skeleton, 3. the ribs and sternum[7]. I. THE AXIAL SKELETON. 1. THE SPINAL COLUMN. The spinal column in the simplest cases consists of an unsegmented rod, the notochord, surrounded by the =skeletogenous layer=, a sheath of mesoblastic origin, which also envelops the nerve cord. Several intermediate stages connect this simple spinal column with the vertebral column characteristic of higher vertebrates. A typical vertebral column may be said to consist of (1) a series of cartilaginous or bony blocks, the vertebral =centra=, which arise in the sheath surrounding the notochord. They cause the notochord to become constricted and to atrophy to a varying extent, though a remnant of it persists, either permanently or for a long period, within each centrum or between successive centra. (2) From the dorsal surface of each centrum arise a pair of processes which grow round the spinal cord and unite above it, forming a =dorsal= or =neural arch=. (3) A similar pair of processes arising from the ventral surface of the centrum form the =ventral= or =haemal arch=. To the ventral arch the ribs strictly belong, and it tends to surround the ventral blood-vessels and the body cavity with the alimentary canal and other viscera. A =neural spine= or spinous process commonly projects upwards from the dorsal surface of the neural arch, and a pair of =transverse processes= project outwards from its sides. When, as is commonly the case, the two halves of the haemal arch do not meet, the ventral surface of the centrum often bears a downwardly-projecting =hypapophysis=.