1888 - 89 .] Mr D. Hepburn on Diarthrodial Joints. 
261 
(b) It may differentiate into fibrous tissue and form a 
syndesmosis or synarthrosis. 
(c) It may partially cleave and form a joint cavity. 
3. The joint cavity appears within the articular disc at a period 
when the process of chondrification is at some distance from the 
cavity. 
4. If the cavity remain of small size, and the surrounding articular 
disc develop into fibrous tissue, an amphiarthrosis is formed, e.g ., 
the joint between vertebral bodies. (This is specially well seen 
in some Cetacea, and probably the epiphyseal plates on the bodies 
of vertebrae are also derived from the articular disc.) 
5. The cavity may enlarge to form a diarthrosis. 
6. When the joint cavity is single we have a simple diarthrosis ; 
when there ^re two cavities we have a diarthrosis with an interposed 
meniscus ; when the two clefts unite in the centre we have an 
incomplete meniscus ; when the cleft is single, but does not extend 
across the axis of the disc, an interarticular ligament is formed. 
7. The proximal and distal segments of the articular disc 
develop into the articular cartilages of the joint, and probably form 
part, if not all, of the epiphyseal ends of the bones. 
8. The circumference of the articular disc develops into the 
capsule of the joint. 
9. Interarticular fibrocartilages and ligaments are derived from 
the articular disc as the result of modifications of the joint cavit3 r . 
10. The cells lining the joint cavity have a double fate : — 
(a) Those applied to the ligamentous structures are 
specialised as synovial membrane. 
(b) Those upon the articular cartilage persist until the 
period of hatching in the chick, but undergo 
degeneration in the mammal ; in both cases dis- 
appearing by friction as the result of movement. 
