276 PROTOPLASM 



interior of the masses that they subtend, because they are sub- 

 jected to different influences on their two sides. Conductivity 

 measurements indicate that the surface of protoplasm differs 

 from the interior. There is also anatomical evidence that a 

 definite morphological membrane surrounds cells. 



One important fact in regard to the plasma membrane must be 

 borne in mind: It may possess an outer layer of fat which is 

 essentially a nonliving film, but the membrane as a whole is of 

 protoplasm, and while differing in constitution from the inner 

 material, it is still living matter, as irritable and capable of change 

 and adjustment as is protoplasm generally. 



Physical Properties. — The protoplasmic membrane may be of 

 any degree of substantialness, from that of a delicate, impercepti- 

 ble film to a tough, anatomical pellicle. The former type of 

 membrane is at the surface of most animal cells and of so-called 

 "naked" masses of plant protoplasm such as the Plasmodium of 

 slime molds. The substantial type of membrane forms the 

 covering of unicellular organisms such as the protozoan Euplotes 

 (Fig. 28). No one doubts the presence of a pellicle on Euplotes. 

 Less certain in the minds of some is the existence of a membrane 

 on animal tissue cells and naked plant protoplasm. It has long 

 been debated whether the red blood cell is a bit of firm jelly 

 devoid of a covering, or a sac with hemoglobin in it. The presence 

 of "ghost" cells in blood suggests the second alternative. The 

 question has apparently been settled by microdissection, which 

 has shown that blood cells possess a resistant and highly elastic 

 membrane (Fig. 47). Whether delicate or substantial, and no 

 matter on what kind of cell, the protoplasmic membrane is never 

 permanently of the same quality. It is of living matter and 

 therefore constantly undergoing changes — usually imperceptible 

 but often very conspicuous ones. Amoeba and slime molds move 

 about by the process known as amoeboid movement, which 

 involves the formation of finger-like protrusions or pseudopodia. 

 When these are formed, a change occurs in consistency from the 

 firm, highly viscous state of the quiescent membrane to the fluid 

 condition of the moving membrane. That such a change in the 

 physical state of the surface layer of Amoeba occurs can be 

 readily demonstrated by microdissection. The membrane when 

 quiet offers considerable resistance to, and is torn by, a micro- 

 needle; but when a pseudopodium is advancing, the surface layer 



