66 PROTOPLASM OF PROTOZOA 



found to be approximately 0.026 cubic micra per minute through each 

 square micron of surface. These authors concluded that the permeability 

 to water was regulated by the plasmalemma. 



It is well known that many marine Protozoa, especially Sarcodina, do 

 not possess contractile vacuoles, but, when transferred to fresh water, 

 contractile vacuoles may appear (Schaeffer, 1926). The contractile vacu- 

 oles of fresh-water forms either work very slowly or disappear entirely 

 upon the organism being transferred to various concentrations of sea 

 water. Furthermore, some Protozoa of the same species, for example 

 Actinophrys, may be found in both fresh and salt water. The cytoplasm 

 of the fresh-water form is greatly vacuolated and possesses a contractile 

 vacuole, while the cytoplasm of the salt-water form is relatively free of 

 vacuoles, including the contractile vacuole, and the general appearance 

 of the cytoplasm has changed to a granular condition. Thus a gradual 

 acclimatization of fresh-water Protozoa to salt water seems to reduce the 

 difference between the external and internal osmotic pressures by a loss 

 of water from the cell. This is probably accompanied by changes in the 

 physical state of the protoplasm, particularly its consistency and specific 

 gravity. It is of interest to recall here the extraordinary case of Noctiluca, 

 in which the specific gravity of the organism is less than that of the sur- 

 rounding sea water, owing to lower concentration of salts. Here the 

 water, instead of entering the organism, tends to diffuse out of it, so 

 that osmotic work must be done to retain its constant state. 



Experiments designed to test the selective permeability of the surface 

 membrane of the Protozoa have been carried out by utilizing various 

 dyestuffs (Chambers, 1922; Ball, 1927). For example, according to 

 Chambers (1922) an aqueous solution of eosin does not stain Amoeba 

 from the exterior. However, if injected into the interior of the cell, it 

 readily diffuses throughout the protoplasm. 



Attempts to visualize the mechanism of permeability control in sur- 

 face membranes have been made by assuming a solubility of the permeat- 

 ing substance in the membrane inself or by assuming the membrane to 

 have a sieve-like structure similar to a filter, but much smaller. In any 

 case, whatever may prove to be the final answer to this problem, it will 

 undoubtedly involve a change in molecular aggregation, organization, 

 and polarity of the elements of the physiological membrane. 



2. Nuclear memhrane. — The interphase nucleus, like the cytoplasm of 



