62 PROTOPLASM AND THE AMCEBA. PROTOCOCCUS 



going on in the living protoplasm is called metabolism,' 

 and these larger granules and droplets are metabolites, 

 i.e. products of metabolism. They must be carefully 

 distinguished from the ultramicroscopic disperse particles 

 which we cannot see, but whose existence we confidently 

 infer from different lines of evidence, and which form 

 the permanent structure of protoplasm. 



In the clear protoplasm of the pseudopodium of the 

 amoeba (see Fig. i), in which there are no coarse granules, 

 very numerous fine particles can be seen in Brownian 

 movement. This movement is suddenly brought to 

 an end by electrical stimulation. When the stimula- 

 tion ceases the Brownian movement recommences. The 

 mobile sol has been momentarily converted into a 

 reversible gel. When protoplasm is killed by certain 

 " poisons " (what are called in biology " fixing reagents," 

 such as acetic acid, chromic acid or dilute iodine solu- 

 tion, etc.) the sol is converted into an irreversible gel. 

 If protoplasm is heated to 60° C, however, chemical 

 changes occur in the proteins, and the whole colloid 

 structure is destroyed. The proteins are then said to 

 be " coagulated." 



The protein particles forming the disperse phase of 

 the protoplasmic sol very readily run together to form 

 a membrane (gel) in any plane on the two sides of which 

 they are subjected to different physico-chemical action. 

 This occurs for instance on the outer surface of a 

 detached unit of protoplasm such as a cell, and this is 

 accordingly covered with a semi-permeable protoplasmic 

 membrane, the ectoplasm, which is relatively fixed and 

 sharply distinguished from the rest of the cell proto- 

 plasm (endoplasm). Similar gel membranes are formed 

 round the vacuoles (segregated drops of liquid enclosed 

 ? Greek /lera^o^, a changing over. 



