DYNAMICS OF HISTOGENESIS 103 



These resistances may be enumerated as follows : first, the weight 

 of the hind limb ; second, the reacting muscular force of elasticity 

 of traction of the surrounding mesenchyme retarded in growth; 

 third, the active muscular contractility manifested during the 

 rotation of the hind limb, and, fourth, the restriction to longi- 

 tudinal, interstitial growth of the femur at the proximal acetabu- 

 lar accelerated growth center and at the distal tibial accelerated 

 growth center. 



These resistances to femoral growth are active and formative 

 during development. They are just as efficient in causing 

 femoral differentiation as the intrinsic accelerated growth of the 

 femoral center itself. Intrinsic growth of the femur and extrinsic 

 resistance are factors pari passu in the genesis, growth, and 

 perfected maturity of the femur. Growth and resistance are 

 inseparable; one is just as important as the other. The forma- 

 tive influence of this resistance has been hitherto entirely over- 

 looked. 



In embryos 10 to 14 mm. in length, the skeletal condensation 

 of the central core of mesodermal nuclei is purely a mechanical 

 function of position (fig. 2). The syncytial nuclei, located at 

 the center of the limb, have less volume to expand in and a greater 

 resistance to overcome than those located more peripherad. 

 The less volume and the greater resistance are the two factors 

 determining the compactness of the skeletal nuclei. 



Once the limb begins to grow rapidly in length, the appar- 

 ently continuous, compact skeletal core presents two centers 

 of accelerated growth. These centers, dense with nuclei, are 

 separated by transverse zones lighter in texture. The lighter 

 zones are the indefinite lines of demarcation between the centers 

 of accelerated growth of the primordial and segmental blastemal 

 skeleton. 



The ultimate external form and internal structure of the vari- 

 ous components of the mature skeleton are dependent upon the 

 following factors: First, the varying continuation of the differ- 

 ential intensity of the force of each accelerated growth center of 

 the segmental blastemal skeleton, and, second, the varying 

 resistances encountered by this force. Starting with these two 



