68 PROTOPLASM OF PROTOZOA 



King ( 1935 ) , and Kitching ( 1938) . It is well known that the contractile 

 vacuole often arises from the coalescence of smaller vacuoles. 



C. V. Taylor (1923) studied the structure of the contractile vacuole 

 of Euplotes and found that it appeared to be composed of a definite 

 "wall" of measurable thickness. However, he later concluded that the 

 apparent "wall" was an optical illusion and that careful study revealed 

 only the internal surface of the "wall" to be sharply delimited; externally 

 it merges not abruptly, but gradually, into the surrounding medium. By 

 moving the needle point against and about the contractile vacuole, he 

 found it could be displaced and that its boundary was tolerably durable 

 and its viscosity distinctly higher than the surrounding endoplasm. With 

 completion of contraction, the vacuole wholly disappears. 



In a similar study upon the contractile vacuole of A. verrucosa and P. 

 caudatum, Rowland (1924b) and Howland and Pollack (1927) have 

 been able to dissect the contractile vacuole out of the organisms and ob- 

 serve it floating freely in the water. Here it may be stained by alizarin 

 blue and manipulated by microneedles. Upon puncture, the surface of 

 the membrane was observed to wrinkle. In other experiments, when the 

 contractile vacuole was forced into contact with the plasmalemma, a 

 fusion took place and, because of this fact, Howland and Pollack were 

 led to suggest that the surface membranes of both the cell and the con- 

 tractile vacuole must possess similar physical properties. These authors 

 further found the contractile vacuole to lie in a region of gelated endo- 

 plasm, a condition they think necessary for its functioning. 



Mast (1938) has described the membrane at the surface of the con- 

 tractile vacuole in A. proteus as a well-differentiated structure about 

 0.5 micron in thickness. 



King (1935) has described the permanent components of the contrac- 

 tile vacuole system of P. mtdthnhronucleata as including the pore with 

 its discharging tubule, and the feeding canals, each made up of a distal 

 excretory portion, an ampulla, and an injection tubule. The membrane 

 of the contractile vacuole itself is considered a temporary structure, dis- 

 appearing at systole and closing the pore, which ruptures at the next 

 systole. The new membrane of the contractile vacuole appears by the 

 coalescence of the membranes of vesicles which lie just under the pore, 

 and becomes continuous with that of the pore. In centrifuged P. cauda- 

 tum, King and Beams (1937) observed that in some cases the vacuole 



