longitudinal growth of the femyr the progressive appearance of more and more 

 definite traction lines is seen in the surrounding mesenchyme retarded in longi- 

 tudinal growth. Along these lines the fasciculi of the developing muscles form, 

 due to the tension of differential longitudinal growth. The determination of 

 accelerated and retarded growth is based on the number of mitotic figures and the 

 compactness of the nuclei per square millimeter of cross-section in a field of dif- 

 ferential growth. 



3. The force of longitudinal growth of the femur is from ten to fifteen times 

 greater than that of the acetabulum. Compare along the line labeled 'limb axis.' 

 Note, at the same time, the inevitable mechanical effects of muscular pull. These 

 factors, together with the strengthening influence of femoral ossification, deter- 

 mine the location of the convex ball of the hip, ball-and-socket joint, on the femur 

 and not on the region of the acetabulum. 



4. The tensile perichondria! strain fibrosis is clearly detected as a limiting 

 membrane, first appearing on the convex aspect of the bent femur (figs. 3, 4, 5, 

 6, 7, no. 3). Figure 4 represents the limb of a living embryo injected with india 

 ink. Note that the injected capillaries are outside of the perichondrium. This 

 membrane strangles the cartilage cells at the center of the bent femur and later 

 becomes modified into the periosteum from which the osteoblasts proliferate. 

 The osteoblasts, by appositional growth, replace the degenerating cartilage scaf- 

 fold with a more rigid bony base. These cellular reactions are elicited by the 

 progressive intensity of the strain to which the femur is subjected by the resist- 

 ances to advancing femoral extension. Longitudinal femoral accelerated growth 

 and extrinsic resistances to this growth are interactions that must be intensively 

 studied in order to appreciate the competition and the resultant products of dif- 

 ferential growth. 



Chart A Curve of relative decrease of femoral volume with increased resist- 

 ance due to hind-limb weight. 



112 



