THE ORGANS OF THE INTERMEDIATE LAYER OR MESENCHYJIE. 645 



In the development of the joints the following general phenomena 

 occur : 



Young cartilaginous fundaments, as, e.g., those of the thigh and 

 leg, are in early stages separated at the place where the articular 

 cavity is subsequently formed by a very cellular intermediate tissue 

 (the intermediate disc of HENKE UND KEYHER). This subsequently 

 diminishes in extent, because the ends of the cartilages grow at its 

 expense. In many cases it disappears entirely, so that the terminal 

 surfaces of the skeletal parts concerned are for some distance in 

 immediate contact. 



The specific curvature of the articular surfaces is by this time 

 more or less well established. This is accomplished at a time when 

 there is as yet no articular cavity, and when, moreover, movements 

 of the skeletal parts cannot be executed, because the muscles are not 

 capable of functioning. 



From this it follows that during embryonic life the articular 

 surfaces cannot acquire their specific form under the influence of 

 muscular activity, and that they are not formed, as it were, by 

 attrition and adaptation to each other in consequence of definite 

 recurrent movements in a simply mechanical way, as has been 

 assumed by many. The early appearing typical form of the joint 

 seems therefore to be inherited (BERNAYS). Muscular activity can be 

 effective only for alterations at later stages; it is, however, not 

 without influence in the further development and formation of the 

 articular surfaces. 



When, after the disappearance of the intermediate tissue, the 

 surfaces at the ends of the developing cartilages come into immediate 

 contact, there arises between them a narrow fissure as the first 

 fundament of the articular cavity. This is bounded directly by the 

 hyaline articular cartilage, which does not here possess any peri- 

 chondriuin. Then a sharper delimitation of the articular cavity 

 from the surrounding connective tissue gradually takes place, inas- 

 much as a firmer connective-tissue layer, which becomes the capsular 

 ligament, is developed from one cartilage to the other, and addi- 

 tional fibrous tracts are converted into separate tense articular 

 ligaments. 



The process of development takes a somewhat different course 

 when the articular surfaces do not fit into each other. In these 

 cases the ends of the cartilages cannot come into immediate contact 

 in the manner previously described ; they now remain separated by 

 ui ore or less considerable remnants of the richly cellular intermediate 



