646 EMBRYOLOGY. 



tissue, which then assumes more and more the condition of compact 

 fibrous tissue. 



When the intermediate tissue is preserved in its whole extent, 

 there arises a nbro-cartilaginous interarticular disc (intermediate 

 or interpolated cartilage), which is inserted as an elastic cushion 

 between the skeletal pieces. There is formed an articular fissure 

 between the ligamentous disc and the terminal surfaces of each of 

 the articular cartilages, or, in other words, there is developed an 

 articular cavity, which is divided into two by means of an interpolated 

 disc. 



Finally, a special modification of the joint occurs when the carti- 

 lages are partly in contact and partly remain separated by inter- 

 mediate tissue. In this case there appears at the place of contact 

 a single articular cavity ; laterally, however, this is enlarged by 

 the incongruent parts of the cartilaginous surfaces becoming split oft 

 from the intermediate tissue separating them. Thus there arises an 

 articular cavity which, it is true, is single, but into which are thrust 

 from the articular capsule the metamorphosed products of the inter- 

 mediate tissue, which constitute the so-called semi-lunar fibro-carti- 

 lages or the menisci, as in the case of the knee-joint. 



As was previously described in treating of the development of the 

 bones of the extremities, there is preserved, even after the termination 

 of the process of ossification, an exceedingly small remnant of the 

 cartilaginous fundament, which forms on the articular surfaces a 

 cartilaginous covering only a few millimetres thick. The articular 

 ends of all bones that are developed out of a cartilaginous fundament 

 possess such a covering. 



It is different when bones that have been produced directly in 

 connective tissue (the covering bones) are united to each other by 

 a veritable joint. Such a case occurs in the articulation of the 

 lower jaw in Mammals. The glenoid process of the lower jaw, as 

 well as the glenoid fossa of the squamous portion of the temporal 

 bone, is in this case covered with a thin layer of unossified tissue. It 

 looks like cartilage, and usually is described as such. But microscopic 

 examination shows that it is composed exclusively of layers of con- 

 nective : tissue fibres. 



As there are bones which are preformed in cartilage and others which 

 are preformed in connective tissue, so a distinction must be made 

 between joints with a covering of hyaline cartilage and joints with 

 a covering of fibrous connective substance. 



