44 INTRODUCTION TO CYTOLOGY 



interpret the ectoplast as an emulsion. According to Czapek (1910, 

 1911, 1915) the ectoplast is an emulsion of lipoids, proteins, and other 

 substances, the lipoids forming a suspended phase. "Protoplasm is a 

 colloidal emulsion of lipoids in hydrocolloidal media, the latter containing 

 proteins and mineral salts." Lepeschkin (1910, 1911) advanced the 

 contrary view that the lipoids form the medium of dispersion. In at- 

 tempting to account for changes in permeability Clowes (1916) points 

 out that inversion of phases probably plays an important role, while 

 Spaeth (1916) ascribes changes in permeability to alterations in the 

 degree of dispersion of the colloids, with resulting changes in the vis- 

 cosity of the membrane. A more definitely stated hypothesis of the 

 latter type is that tentatively suggested by Lloyd (1915) and Free (1918). 

 Colloids are known that "have two liquid phases which differ in composi- 

 tion only in the relative proportion of water and of the substance of the 

 colloid" (Free). It is accordingly possible that 

 alterations in permeability may be due to changes 

 in the distribution of water between two such 

 phases present in the plasma membrane. When 

 water passes from the internal (suspended) to the 

 external (continuous) phase, the droplets of the 

 former would become very small; when the move- 

 ment is in the opposite direction they would be- 

 come very large and closely packed. As a result 

 N- l^.( there would be such changes in the physical 

 ec t . ^Sj nature of the membrane as would aid in interpret- 

 ,, _ ing the behavior of the latter toward substances 



FIG. 9. Amoeba, show- . 



ing ectoplasm, endo- entering or leaving the cell. It is held that such a 

 ntractile hypothesis accounts more readily for the gradual 

 changes in permeability observed than does the 

 inversion theory of Clowes, according to which the change might be ex- 

 pected to occur suddenly. It is pointed out, however, that both 

 processes are probably involved. 



Whatever the degree of correspondence between the above inter- 

 pretations and reality may be, it is scarcely open to doubt, especially 

 since the work of Bancroft (1913) and Clowes (1916) on colloids, that in 

 such theories we have our best prospect of reaching an adequate knowl- 

 edge of the plasma membrane, which, because of its great importance in 

 the life of the cell, is to be regarded as a definite "osmotic organ." 



Protozoa -It is in the Protozoa that the ectoplast shows its most 

 elaborate structural differentiations. (See Minchin, 1912, Chapter V.) 

 Here the ectoplast clearly has several functions: protective, motor, excre- 

 tory, and sensory. In most forms other than the Sarcodina there is a 

 resistant envelope of some sort. This may represent (a) the entire ecto- 

 plast modified (the "periplast" of Flagellata); (6) a superficial modified 



