Bone fusion by connective or (later) cartilaginous tissue was the ele­mentary form of bone joining (in lower aquatic vertebrates). Such compact joining of bones, however, limits the volume of movements. With the forma­tion of bony levers of movement, slits and cavities began appearing in the interstitial tissue between the bones because of its resorption. As a result a new type of bone joining, interrupted joining, articulation, developed. The bones not only became joined to each other, but they articulated so that joints formed which allowed the bone levers to perform wide movements especially necessary for terrestrial animals. Thus, two types of bone joining developed in the process of phylogenesis: the elementary, compact joining with a limited range of movements and a later, interrupted type allowing wide movements. In reflection of this phylogenetic process of the adaptation of animals to the environment by movement in the joints, the development of bone articulations in the embryogenesis of man also goes through these two stages. At first the skeletal germs are connected to each other by layers of mesenchyme. The mesenchyme transforms to connective tissue from which the bone-joining apparatus develops. If the areas of connective tissue locat­ed between the bones are compact, then compact contiguous joining of the bones, called fusion or synarthrosis occurs. If a cavity forms between these areas as the result of resorption of the connective tissue, then another type of joining, cavitary or interrupted, forms; it is called diarthrosis. Thus, according to development, structure, and function, all bone arti­culations (articulationes ossium) can be divided into two large groups. Contiguous articulations, synarthroses (BNA), earliest in develop­ment, fixed in function or allowing slight movement. Interrupted (synovial) articulations, diarthroses (BNA), latest in development and permitting more movements functionally. Between these forms there is a transitional form, from contiguous to interrupted or vice versa. It is characterized by the presence of a small gap which does not have the structure of a true articular cavity, and because of this the articulation is called a half-joint, or hemiarthrosis.