THE PHYSICAL BASIS OF LIFE 



17 



they may be — variations in its environment and variations in 

 its activities being reflected directly or indirectly in its ap- 

 pearance. (Fig. 6.) 



Under the highest magnifications, not only does the finer struc- 

 ture of protoplasm differ in various specimens, but also in the same 

 cell under slightly different physiological conditions. At one time 

 it presents the appearance of a fairly definite net-like structure, or 





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* . * 



Fig. 9. — Protoplasm of the living Starfish egg (highly magnified), exhibit- 

 ing alveolar, or foam-like, structure. This arises by the appearance, growth, 

 and crowding together of minute bodies in a homogeneous ground substance. 

 (From Wilson.) 



reticulum, the meshes of which enclose a more fluid substance; at 

 another, a frothy, or alveolar, appearance due to a more liquid 

 substance scattered or emulsified as spherical bodies in a less 

 liquid medium. Again, at other times, the denser portion seems 

 to take the form of minute threads, or fibers, or of tiny granules 

 distributed in a somewhat fluid matrix. (Fig. 9.) 



These appearances have given rise to various theories which 

 emphasize one or another as the universal formula for the physical 

 structure of protoplasm, from which the other appearances are 

 merely secondarily derived. But the trend of recent work has 

 been to indicate that although the general similarity of proto- 

 plasmic activity, wherever we find it, might lead us to expect to 

 find also a visible fundamental structural basis, such does not exist 

 within the range of magnifications at our command. Reticular, 

 alveolar, and other structures which our microscopes reveal are, 

 as it were, merely surface ripples from underlying physico-chemical 

 changes in the colloidal system which, thus far, are unfathomable. 



