260 Proceedings of Royal Society of Edinburgh. [sess. 
gone differentiation into fibrous tissue, and in it may be found the 
rows of cells and wavy fibres characteristic of that structure. This 
fibrous capsule also continues to be lined by a row of flattened cells 
continuous with those on the surface of the articular cartilages. A 
similar series of changes may be traced in connection with inter- 
articular fibro-cartilages and ligaments. 
In mammals the formation of the articular disc and the appear- 
ance of the joint cavity, as seen in embryo mice and rabbits, are 
practically identical with those just described. There is therefore 
no reason to doubt that the subsequent changes in the component 
parts of the disc are also similar. 
There is, however, one striking difference in connection with the 
layer of flattened cells which line the cavity, and this was observed 
on the articular cartilages taken from the phalangeal joints of a 
human foetus approaching the full period of uterogestation. Here 
the usual flat cells were found lining the interior of the capsular 
ligament, but on tracing them towards the articular cartilages they 
were seen to be replaced by a narrow band, staining somewhat more 
freely than the hyaline cartilage, but presenting no cell structure. 
The free surface of this band was slightly ragged, and it appeared 
to be undergoing degeneration. Thus it would appear that the flat 
cells lining the primitive joint cavity have a double fate. Those in 
relation to the ligamentous structures, and thus within reach of a 
direct blood supply, become specialised into a synovial membrane; 
whereas those in relation to the articular cartilages, although present 
in the chick at the period of hatching, probably disappear as the 
result of friction ; while, in the case of the mammal, they undergo 
degenerative changes, which lead to their early disappearance from 
the same cause. 
Summary of Conclusions. 
1. The bone matrices and the articular disc possess a tissue con- 
tinuity, and are derivative of a common blastema of which the 
articular disc is at first the undifferentiated form. 
2. The articular disc may conduct itself as follows : — 
(a) It may develop into a plate of cartilage and form a 
synchondrosis, e.g., the articulation between basi- 
occipital and basi-sphenoid bones. 
