DYNAMICS OF HISTOGENESIS 107 



5. CONCLUSIONS 



1. In an embryo 10 mm. in length the hind limb bud is filled 

 with a uniform mass of unmodified mesenchjonal cells. 



2. The central condensation of the nuclei forming the blastemal 

 skeleton is due to two factors: first, the less volume centrally 

 in which these cells have to expand; second, the resistance of 

 the surrounding mesenchyma. 



3. The blastemal skeleton at first grows relatively more rapid 

 in width than in length in embryos 10 to 14 mm. long. 



4. With the formation of the periblastemal membrane the 

 subsequent growth of the femur is relatively and absolutely 

 greater in length than in width in embryos 14 to 25 mm. long. 

 This is due to the shifting of the planes of mitosis on account 

 of the compression of the periblastemal membrane. A central 

 skeletal core of accelerated longitudinal growth is now clearly 

 demarked from the peripheral mesenchyme retarded in longi- 

 tudinal growth. 



5. The interaction of these differential growing zones results 

 in a tension of the peripheral mesenchyme. The nuclei of the 

 latter are drawn out in traction, thus resulting in the first step 

 of myogenesis. The direction of the elongated cytoplasmic 

 myofibrils is arranged, therefore, in the line of longitudinal 

 skeletal growth. 



6. The formation of skeletal muscle is a dependent differ- 

 entiation, relying on the accelerated longitudinal growth of the 

 skeleton for its genesis, growth, and continued differentiation. 

 The tension of differential growth is the efl&cient interacting 

 stimulus to myogenesis in the limb. 



7. The following law of direction of the skeletal muscular 

 fasciculi may be formulated from the evidence presented : 



The elongation of the developing muscular fasciculi of the limb 

 is in the direction of the accelerated longitudinal growth of the related 

 dominant skeletal segment. As regards developing muscles in 

 general, the elongation of muscular fasciculi is in the direction of a 

 dominant force extrinsic to the zone of myogenesis. 



