426 THE CONTRACTILE VACUOLE 



through the cell surface. The amount of water expelled by the vacuole 

 corresponds within reasonable limits to that necessary for the excretion 

 of carbon dioxide, if it is assumed that this water is saturated with the 

 gas. From this Ludwig concludes that the vacuole is of special significance 

 not only in the regulation of osmotic pressure within the cell, but also 

 in excretion of carbon dioxide. 



Evidence bearing directly on the excretory nature of the vacuole func- 

 tion is exceedingly scant, and for the most part negative. The reason 

 for the relatively few observations is immediately apparent to all who 

 have attempted experiments of this nature. Perhaps a more thorough 

 investigation of the nature of nitrogenous waste products in other Protozoa 

 will suggest more effective methods for answering the question, by reveal- 

 ing other chemicals which may be detected more readily. 



Hartog (1888), Degen (1905), Zuelzer (1910), Doflein (1911), 

 and others maintain that the contractile vacuole is concerned primarily 

 in the regulation of hydrostatic pressure within the cell, or the preven- 

 tion of overdilution of the cell contents by water taken into the cell 

 in feeding as well as through the cell membrane by osmosis. Harvey 

 (1917) found that Noctiluca, which normally lives near the surface of 

 sea water, sinks when transferred to diluted sea water, but ultimately 

 rises to the surface again. Meanwhile, expansion takes place, owing 

 to the taking up of water by the organism. This passage of water from 

 exterior to interior is from a region of higher concentration to one of 

 lower concentration, and therefore contrary to the laws of osmosis in 

 simple systems. When organisms are transferred from diluted sea water 

 to pure sea water, they shrink, vacuoles are formed, and these appear to 

 discharge to the exterior in somewhat the same manner as contractile 

 vacuoles. This appears to aid in reestablishing the normal salt concentra- 

 tion within the organism. Hance (1917) made extensive observations 

 on a race of Paramecium possessing extra contractile vacuoles. He found 

 that these animals cannot withstand immediate immersion in water 

 containing 0.5 percent sea salt, but can be acclimated gradually to this 

 'concentration. The number of vacuoles is not reduced by this treatment, 

 but the pulsation frequency is reduced. This response may be taken 

 to indicate a decreased rate of entry of water into the cell, presumably 

 because of the higher external osmotic pressure; but Hance observed 

 also an increased viscosity and toughness of the pellicle, which may indi- 



