484 SCIENCE PROGRESS 



experimental facts. In the first place those facts upon which 

 it was based, the storage of oxygen within the cell in chemical 

 combination alongside of oxidisable material to which it is 

 finally handed over. In the second place such instances as 

 those in which minute traces of specific chemical materials 

 modify the life and activities of the cell. In this latter case the 

 concept has justified its existence by indicating fruitful lines of 

 experiment. Its influence has also been clearly felt in further 

 elucidating the importance of oxygen not only to life, but also 

 to function. 



We must ask, however, whether no position of importance 

 should be accorded to states and changes of a purely physical 

 kind. Surely these also play their part within the cell. Their 

 influence upon extra-cellular material is often paramount, and 

 it is perhaps best by a consideration of this influence that their 

 case may be introduced. Let us for example consider a rod 

 of cartilage. Here cells are situated in a mass of tough 

 material which they have manufactured. Unlike their epithelial 

 cousins situated on the surface of the body, they do not pass 

 their manufactured products on to a surface from which in the 

 course of events these are swept away. The cells are therefore 

 separated by a greater amount of material, which first oozing 

 from them as a viscid mass no more rigid than its source, then 

 sets into the tough elastic substance to which the cartilage owes 

 its physical characteristics and its sole use. Now who can 

 deny the probability that this process of " setting " is due to 

 stimulation provided by the stresses and strains to which the 

 exuded material is at once subjected ? We thus get a picture 

 of this extracellular material undergoing a change of physical 

 state of functional importance, and that as the result of excitation. 

 Again, as the cartilage plays a frequently repeated part in the 

 stresses and strains to which the limb is subjected by muscular 

 action and external pressure, certain points within it are selected 

 as the foci through which resultant forces pass. These most 

 probably are the " centres of ossification." The forces in action 

 arrange the cells and matrix in columns radiating from this 

 centre. The stimulated matrix continues with its change of 

 state, the colloid material passes from the condition of a 

 homogeneous jelly into one in which there is a precipitation 

 of denser " calcified " material from a more simple solution 

 readily removed ; condensation and liquefaction. Is this a 



