952 THE ARTICULATIONS OR JOINTS. 
manner of their appearance forecasts their ultimate destination as immovable or mov- 
able arrangements. 
All joints arise in mesodermic tissue which has undergone more or less differentia- 
tion. 
When this differentiation has produced a continuous membranous layer, in which 
ossific centres representing separate skeletal segments make their appearance, we get the 
primitive form of suture. The plane of the articulation merely indicates the limit of 
the ossifie process extending from different directions. If, again, the differentiation of 
the mesoblast has resulted in the formation of a continuous cartilaginous layer, in which 
ossification commences at separate centres, the plane of the articulation is marked 
out by the unossified cartilage—in other words, the articulation is a synchondrosis. Ulti- 
mately this disappears through the extension of the process of ossification. 
To some extent sutures also disappear, although their complete obliteration is not 
usual even in aged people. Developmentally, therefore, synarthroses or immovable 
joints do not present any special structural element, and, speaking generally, they have 
only a temporary existence. 
The development of all movable joints is in marked contrast to that of synar- 
throses. Not only are they permanent arrangements so far as concerns normal conditions, 
but they never arise merely as planes which indicate the temporary phase of an ossific 
process. From the outset they present distinct skeletal units, from which the special 
structures of the joint are derived. 
The primitive movable joint is first recognised as a mass of undifferentiated 
mesodermic cells situated between two masses, which have differentiated into primitive 
cartilage. 
The cell-mass which constitutes the joint-unit presents the appearance of a thick 
cellular disc, the proximal and distal surfaces of which are in accurate apposition with the 
primitive cartilages, while its circumference is defined from the surrounding mesoderm by 
a somewhat closer aggregation of the cells of which the disc is composed. From this 
cellular dise or joint-unit, all the structures characteristic of amphiartbrodial and diar- 
throdial joints are ultimately developed. 
Thus by the transformation of the circumferential cells into fibrous tissue, the invest- 
ing ligaments are produced. Within the substance of the disc itself, a transverse cleft, 
more or less well-defined and complete, makes its appearance. In this manner the disc is 
divided into proximal and distal segments, separated from each other by an interval 
which is the primitive joint-cavity. This cleft, however, never extends so far as to inter- 
rupt the continuity of the circumferential part of the disc which develops into the fibrous 
tissue of the investing ligaments. From the proximal and distal segments of the 
articular disc, the various structures, distinctive of movable joints, are developed. 
Thus, in amphiarthrodial joints the cellular articular disc or primitive joint-unit gives 
origin to the following structures :—From its circumference, investing ligaments ; from 
its interior, the fibro-cartilaginous plate or disc in which an imperfect joint-cavity with 
corresponding imperfect synovial may be found. 
In the case of a diarthrodial joint the changes take place on a more extended scale. 
The joint-cavity becomes a prominent feature, in relation to which, the surrounding 
fibrous structures form an investing capsule, lined by a synovial membrane. 
When a single cleft arises, but does not extend completely across the longitudinal axis 
of the articular disc, the undivided portion develops into fibrous interarticular ligaments. 
On the other hand, when two transverse clefts are formed, that portion of the cellular 
dise which remains between them becomes transformed into a fibro-cartilaginous inter- 
articular disc or meniscus, which in its turn may either be complete or incomplete, and 
thus we may obtain two distinct synovial joint cavities belonging to one articulation.' 
In considering the development of the synovial membrane, and the surfaces on which | 
it is found in the interior of a joint, it is necessary to keep clearly in mind that a synovial 
membrane is a highly specialised structure, whose function it is to secrete a lubricating 
fluid or synovia, and that, therefore, its position is determined by the essential necessity 
of proximity to a direct blood supply. In other words this condition of secretion is pro- 
vided by all parts of the interior of a joint-cavity except the articular encrusting carti- 
lage. Consequently synovial membrane is only absent from the free surface of articular 
cartilage. 
1 From a series of observations upon the development of diarthrodial joints, the writer considers that 
there is evidence to show that the ‘‘cellular articular disc” is directly responsible for the production of 
the epiphyses which adjoin the completed joint cavity, and that, among such amphiarthroses as exist 
between the bodies of vertebre, not only the intervertebral disc, but the proximal and distal epiphyses 
which ultimately unite with the vertebral bodies have a common origin in the joint-unit. 
