THEFUNCTIONAL PERIOD OF DEVELOPMENT 



433 



of the more rapid multiplication of the cells subjected to the stress, 

 these experiments have completely confirmed the epoch-making 

 essay of W. Roux (1881), by whom the principles of functional 

 differentiation were first clearly stated. From these and other lines 

 of evidence, it appears highly probable that the size and fibre- 

 direction of all the tendons of the body have no direct hereditary 



■^'/T^ .- y 







i4\ ^--v. 





■■■* 



Fig. 211 

 Portion of a tissue-culture of chick fibroblasts exposed to regional tension (by 

 cultivation as a film in a quadrangular frame). In the region under tension (left) 

 the cells are arranged in fibres parallel to the directions of the tensile force, and 

 are more numerous than in the remainder, where they are scattered and of 

 irregular form. (From Weiss, Arch. Entwmech. cxvi, 1929.) 



basis, but are determined epigenetically de novo in each individual 

 by the stresses and strains to which they are exposed during develop- 

 ment. The fact that fibroblasts arrange themselves along lines of 

 mechanical stress, and multiply faster when exposed to tension, 

 automatically accounts for the production of a mechanically 

 adaptive structure. 



HEE 28 



