INTRODUCTION PEOTOPLASM CELL 



TRiCHOtYSTS 



T CANAL9 



an aqueous solution, holding in suspension colloidal substances of great 

 complexity. Crystalloids are divisible into two groups: electrolytes and 



non-electrolytes. The one (salts, 

 acids, bases) in solution conduct 

 the electric current, the others 

 (urea, sugar) do not. Colloids ex- 

 ist in two states, a liquid or sol 

 state, and a semi-solid or gel state. 

 There exists no sharp line of di- 

 vision between colloids and crystal- 

 loids ; these terms designate phases 

 or states rather than substances; 

 between them lie all kinds of inter- 

 mediate grades. Protoplasm is a 

 sol ; and since its fluidity is due to 

 water, it is commonly classed as a 

 hydrosol. It passes readily into a 

 gel condition, thus becoming a hy- 

 drogel. In living protoplasm this 

 metamorphosis is a reversible proc- 

 ess. Agents which effect an irre- 

 versible gelation of protoplasm 

 tend to bring life to a standstill. 

 Fixation, or killing, of tissue for 

 microscopic study consists in a sep- 

 aration of the more solid part of 

 colloidal protoplasm from a more 

 liquid part. Death is histologically 

 such a process of coagulation. Liv- 

 ing protoplasm may be studied to 

 good advantage in the one-cell ani- 



CONTRACTILE 



FIG. 3. PARAMECIUM CAUDATUM. 



Note the peripheral cilia and the gran- 

 ulo-alveolar character of the protoplasm. 

 (From Calkins' "Biology," H. Holt 

 &Co.) 



mal forms, Ameba (Fig. 2) or 

 Paramecium (Fig. 3). These and 

 other equally favorable protozoan 

 forms are readily available from hay infusion cultures, and can be profit- 

 ably employed for the demonstration also of the simpler modes of proto- 

 plasmic activity, and of the changes suffered by protoplasm in passing 

 from the living to the dead condition. Since protoplasm is commonly 

 organized into cells, the next step demands a knowledge of a typical or 

 generalized cell. 



