PROTOPLASM 39 



reactions, such as rapid oxidation, ordinarily occurring only at high tem- 

 peratures, may take place at relatively low temperatures in the organism. 



Finally, in the film-partitioned structure of protoplasm there is a 

 basis for further electrical phenomena, a subject fully discussed by Lillie. 

 Electric currents have been shown to accompany a variety of vital 

 processes and to have a fundamentally important bearing upon problems 

 pertaining to the reception and conduction of stimuli, automatic or reflex 

 activity, muscular contraction, the movement of plant parts, correlation, 

 polarity, growth, and other biological phenomena. "They .appear, in 

 fact, to be as essential a feature of protoplasmic action as the consumption 

 of oxygen or the evolution of CO2. " 



Ectoplasm and the Plasma Membrane. — It has long been known 

 that there is at the normal surface of any mass of 

 protoplasm a layer whose physical properties differ 

 somewhat from those of the substance within the 

 mass. Sometimes such a differentiation can be 

 easily seen: in amoebse and myxomycetes a layer of 

 hyaline ectoplasm surrounds the granular endoplasm 

 within (Fig. 18) . Frequently the boundary between 

 these two is not at all sharp, and it seems evident 

 that they may be readily converted one into the 

 other. Among Protista, as will be shown farther 

 on, the edoplast is often elaborately differentiated 

 and may be accompanied by additional envelopes. pj^ is.— An amceba, 



There is a large body of evidence for the view showingectopiasm.endo- 



,•■ , 1- 1 xi • 7 1 plasm, and contractile 



that an exceedingly thin plasma membrane, or vacuole. 

 plasmalemma, is always present at the surface of 



the protoplast, whether any special ectoplasmic layer can be directly 

 observed or not. In case such a layer is present, the plasma membrane 

 represents its external surface film. Much has been learned about the 

 plasma membranes of amoebae and other cells by observing the behavior 

 of particles adhering to it (Schaeffer, 1920) and by the use of the micro- 

 manipulator. -^ In myxomycetes Seifriz finds the outer membrane to be 

 distinctly more elastic and tenacious than the hyaline substance imme- 

 diately beneath. Although it is very thin, it is a definite morphological 

 structure which may be removed after death with dissecting needles. 

 Moreover, it is capable of constant repair. Such a capacity to form a 

 surface membrane immiscible with water, provided the injury has not 

 been too sudden and extensive, seems to be generally present in healthy 

 protoplasm. The development of the membrane is in many respects like 

 that of the formation of superficial films by other colloidal systems, 

 although in the case of protoplasm certain metabolic syntheses seem to be 



"Kite (1913), Chambers (1917 et seq.), Seifriz (1918, 1921), Chambers and 

 Hofler (1931), Plowe (1931a). 



