CHAPTER VII 



THE CONTRACTILE VACUOLE 

 J. H. Weatherby 



Introduction 



Since the first description of the protozoan contractile vacuole, prob- 

 ably made by Spallanzani in 1776, few structures in these organisms have 

 received such intensive investigation. Unfortunately, solutions of many 

 of the perplexing questions which have arisen as a result of these studies 

 are not yet at hand. Indeed, much of the more recent work has given rise 

 to entirely new questions which are no less insistent in their demands 

 for answers than were the earlier ones. In the literature claims and 

 counterclaims are abundant; important discoveries have been made only 

 to be discarded because of lack of confirmation, or, in some instances, 

 because of direct contradiction. In view of the somewhat confused state 

 of the evidence concerning contractile vacuoles a re-survey of some of 

 the more important questions seems to be in order. 



Probably the first question asked by the first investigator to see a con- 

 tractile vacuole was "What is its function?" Needless to say, this first 

 investigator did not learn the answer, and, in the opinions of many, the 

 most recent investigation probably does not supply the complete answer. 

 Following this question there have been others hardly less interesting. 

 Is it essential to life? Is it a permanent structure, or does it arise anew 

 at the beginning of each new cycle? Does it always occupy the same 

 position in the organism with respect to other structures? Is the vacuole 

 surrounded by a permanent membrane? Is its discharge to the exterior 

 through a permanent excretory pore? If there is no pre-formed excretory 

 pore, how may one explain the formation even of a temporary pore, and 

 once formed how is it closed again? What natural forces operate to expel 

 the contents of the vacuole? There are many other equally interesting 

 questions, but only a few can be considered at this time. 



The most promising order for discussing these problems seems to be 



