106 EBEN J. CAREY 



limb represents a continuance of this interaction between skeletal 

 segments and developing musculature until equilibrium is estab- 

 lished. This is reached at maturity when the forces of growth 

 are counterbalanced by the resistances to growth. This active 

 interaction of a dominant growth force and its concomitant 

 resistances, playing a dynamic role in histogenesis and morpho- 

 genesis, has never been heretofore considered in ontogenetic 

 development of the limb. 



4. GENERAL CONCLUSIONS 



From the facts presented in this paper, the generalization is 

 clearly apparent that the volume of a skeletal segment decreases 

 relatively to the increase of the intensity of the external pressure 

 or resistance. The less the extrinsic resistance to growth early 

 in development, the greater the relative volume of the growing 

 skeletal segment is found to be. 



With the decrease in the relative volume of the skeletal seg- 

 ment as development advances, another fact is self-evident, 

 namely, there is an increase in density. The greater the resist- 

 ances overcome by the growing skeletal segment, the greater 

 the density becomes; conversely, the less the resistance encoun- 

 tered in growth, the less the density, the more gelatinous the 

 consistency. These facts may be summarized in the following 

 law^s : 



The law of density of a growing tissue: The density of a 

 growing tissue is directly proportional to the resistances (pressure) 

 encountered during growth. 



The law of relative volume of growing tissue: The relative 

 volume of a given quantity of growing tissue is inversely as the 

 resistances (pressure) which it bears. 



From the evidence leading to these laws it is conclusive that 

 the various stages of the developing skeleton are resultants of 

 the mechanical resistances to growth which, interpreted, means 

 that cartilage and bone are not self-differentiated, nor are they 

 spontaneously self-crystallized products, but they are the im- 

 mediate cellular responses to the varying intensity of the stresses 

 and strains produced by resistances (pressure) counteracting 

 the growth of the blastemal skeleton. 



