414 THE CONTRACTILE VACUOLE 



1920, one must admit that only limited importance can be attached to 

 their opinions. Unquestionably most of these investigators were careful 

 observers, and expressed opinions only after due consideration of all the 

 factors which they were able to recognize. But the microscope of today 

 is a far different instrument from that of a hundred years ago, or even 

 fifty years ago; and chemical procedures, particularly those dealing with 

 colloids, have undergone extensive development. However highly one 

 may regard this or that early investigator, the fact remains that none 

 could have been better than the tools with which he worked, and ad- 

 mittedly the tools were poor. Consequently, the author maintains that 

 it is neither unkind nor unappreciative to propose that these various early 

 opinions be considered mainly as of historical interest, and of little 

 worth in settling the question as to the presence or absence of permanent 

 membranes, or of any kind of membrane for that matter, around con- 

 tractile vacuoles. The employment of the best of modern instruments 

 and techniques leaves the question in an unsatisfactory state. 



Before presenting the more recent evidence concerning this question 

 of membranes, perhaps it will not be unwise to present briefly the more 

 fundamental question of what constitutes a membrane. Most textbooks 

 either avoid the issue more or less completely or describe the structure 

 and properties of the artificial membranes so often used in the laboratory 

 for experimental purposes. Although reliable information concerning 

 living membranes is scant, there is sufficient evidence to justify the divi- 

 sion of membranes into two types: morphological membranes and physi- 

 ological membranes. Morphological membranes are permanent struc- 

 tures which are frequently visible in living material viewed through the 

 microscope, and usually may be demonstrated more or less clearly by 

 suitable staining techniques. Apparently they consist mainly of a reticu- 

 lum, or framework, which is described by some authors as being com- 

 posed largely of protein. Such membranes are usually thought to possess 

 an appreciable amount of rigidity, and to serve primarily as supporting 

 structures. Free permeability in both directions is usually assigned to 

 them. 



Physiological membranes are entirely different from morphological 

 membranes in many important respects. Usually they are considered to 

 be so thin as to be invisible even with the highest magnification. They 

 may possess a certain degree of rigidity, but probably much less than 



