26 ANDREWS. 



multiplication of pellicles, even to the end of microscopic 



vision 



[22] Pellicles differ one from the other, or at different 

 moments of time, from their own former states; in point of 

 thickness, of viscidity, of finer structure, and of stability in the 

 component protoplasm, as well as in activities of this. The 

 same is true of the interalveolar substance throughout its 

 formations. 



The varying viscosity of pellicles is shown by optical increase 

 of density, which also brings a characteristic change of hue ; 

 and by change of refractive value; with increased resistance to 

 pressure and increased elasticity. At such times, owing to 

 these optical qualities, structures so minute, or expanses of 

 substance so tenuous, as to be in more fluid states wholly 

 invisible, become readily detected.''^ 



It has been urged that to be capable of contraction, proto- 

 plasm must needs be a solid substance ; it has also been 

 affirmed that if a fluid it cannot be elastic as contractile sub- 

 stances are. 



That both of these conditions are fulfilled by the substance 

 and that it yet does not for these renounce its typical fluidity 

 can be proven by hundreds of instances. A single instance 

 will suffice to make this clear. 



The pellicle of the contractile vacuole of amoeba, which is 

 at times so fluid as to mix freely with the surrounding proto- 

 plasm, becomes at other moments, notably while in that very 

 viscid refractive state which rhythmically precedes the collapse 

 of the organ; so very firm and so truly elastic that the vacuole 

 suffers constant change of shape under pressure of the endo- 

 sarc, of the included substances also, and the peri-neuclear area, 

 if present. At such times it looks most like a transparent 

 rubber bag filled with water. 



Conversely, the instant return of a most elastic and highly 

 organized area to a truly fluid state is beautifully shown, when, 

 by breaking or tearing tension, the stalk of a retractile vorti- 

 cellid is torn apart. Then the contractile muscular fibre, which 



1 See Activities — filose ; also Substance as Such, description of Choano- 

 Flagellata. 



2 See Striation. 



