DYNAMICS OF HISTOGENESIS 105 



femur in length is then due to the proliferation of the cells con- 

 stituting its extremities. This terminal growth of a skeletal 

 segment opposes the active growth of the cells of a contiguous 

 skeletal segment. 



The first objective evidence in joint formation is significant. 

 There is seen (fig. 4) a linear flattening of the blastemal cells 

 along the line representing the zone of juncture between opposed 

 zones of accelerated growth of neighboring segments. The 

 contour of this linear condensation of cells, which outlines the 

 subsequent position of the incipient or primordial joint cleft, is 

 dependent upon the intensity of the mutual forces of growth 

 opposed in action. That particular skeletal segment presenting 

 the greater intensity of growth force per square millimeter of 

 cross-section will bore into and possess the convex component, 

 constituting a diarthroidal joint, whereas the component pre- 

 senting the lesser force of growth per square millimeter of cross- 

 section will possess the concave component of a ball-and-socket 

 joint. 



The formation of a joint, therefore, is a resultant along the 

 lines of juncture between the zones of accelerated growth of 

 neighboring segments opposed in action. The segment present- 

 ing the greater force of growth possesses the convex element, 

 whereas, the segment presenting the lesser force possesses the 

 concave element entering into a movable joint. If the opposed 

 forces are equally distributed between the surfaces of the two 

 segments, a joint with more or less opposed plane surfaces, as the 

 intervertebral joints, is the result. 



According to this evidence, muscles, bones, and joints are not 

 hereditary nor do they spontaneously crystallize out of embryonic 

 tissue by some unknown, non-biological method of self-differ- 

 entiation, but they are the mechanical resultants of an appar- 

 ently continuous blastemal skeleton, possessing segmental centers 

 of accelerated, longitudinal growth opposed in action. This 

 segmental skeleton is in an environment with which it is syncyti- 

 ally continuous. The effect of longitudinal skeletal growth is 

 traction of the surrounding mesenchyme along the lines of 

 optimum tension evidenced by muscle origin. The perfected 



THE AMERICAN JOURNAL OF ANATOMY, VOL. 29, NO. 1 



