THE CONNECTIVE TISSUES AND THE SKELETAL SYSTEM. 



211 



DIARTHROSIS. Where a great degree of mobility is necessary, the arrange- 

 ment of the joint is different. The cells in the central part of the embryonic 

 connective tissue between the ends of adjacent bones (or cartilages) (Fig. 190) 

 liquefy so that a relatively large cavity, the joint cavity, is formed (Fig. 191). 

 The liquefaction of the connective tissue cells may also extend for a short dis- 

 tance along the sides of the bones so that the joint cavity surrounds the ends 

 of the bones (Figs. 192 and 193). The origin of the synovial fluid is not known 



Humerus 



ii 







Radius 



FIG. 190. Section through axilla and arm of a human embryo of 26 mm. (2 months). Photograph. 

 Note the mesenchymal tissue between the humerus and the radius the site of the elbow joint. 



with certainty, but it is probably in part the product of liquefaction of the con- 

 nective tissue cells. The more peripheral part of the connective tissue which 

 encloses the joint cavity is transformed into a dense fibrous tissue, the joint 

 capsule. The cells lining the cavity become differentiated into oval or irregular 

 cells, among which is a considerable amount of intercellular substance. By 

 some it is held that these cells form a continuous single layer like endothelium, 

 but the most recent researches tend to disprove this. The cells lining the 



