CELLS AND TISSUES. 29 



tion of a liquid state of aggregation for protoplasm meets 

 in many cases with still greater difficulties than does the 

 assumption of a solid state. While, for example, all the 

 different portions of the cell body of an amoeba show 

 the same behavior, in that any element within the pro- 

 toplasmic mass may become a surface element, and con- 

 versely ; in other words, every particle is equally capable 

 of the functions of assimilation, stimulation, and move- 

 ment, there exist peculiarities in many of the more highly 

 developed unicellular organisms, or the individual cells 

 of higher animals, which 'can scarcely be interpreted 

 otherwise than as expressions of polarity. Under this 

 heading belong, for example, the fact that absorption and 

 secretion take place predominantly in certain directions, 

 the dependence of muscular stimulation upon the angle 

 of the current and the direction of the muscular fibrils, 

 and the polarity of phenomena of regeneration in plants 

 and animals. These phenomena, which indicate a per- 

 sistent inner differentiation, can scarcely be explained 

 without the assumption of a solid orientation of the par- 

 ticles of living matter. 



A way out of this dilemma, of which the details con- 

 stitute a literature that cannot be entered into in this 

 paper, is rendered possible through a study of the colloidal 

 state. 



II. 



Those gels which were said above to be swollen or 



solidified (for example, ordinary gelatine or agar-agar) 

 show the properties of both solids and liquids united in 

 one, in much the same way as protoplasm. They are 

 capable of existing in all states of aggregation, varying 



