INTRODUCTION 



across certain natural protistan relationships (such as that between 

 the more advanced multicellular algae and their motile, unicellular, 

 "protozoan" relatives, and, with even less logic, that between 

 certain groups of unicellular forms — diatoms, desmids, and 

 other non-motile chlorophytes — and related forms here treated 

 as "protozoa"). The justification for this is the purely pragmatic 

 one of having had to establish limits in order to get the job of 

 writing this book done, combined with the author's own interest 

 in animal-like forms and their ancestors. Of course it often will 

 be necessary to draw upon comparative data about non-protozoan 

 protists, as well as metaphytes and metazoa, but such data will 

 be cited only to clarify or amplify available information on 

 protozoa. 



Techniques and Interpretations in "Electron Microscopy 



It is not necessary here to reiterate the potentialities and 

 problems of electron microscopy. This has been done repeatedly. 

 Recently, Schmitt (1960) has presented a judicious evaluation of 

 the position of the electron microscope in the spectrum of 

 powerful modern biophysical and biochemical tools and has 

 reviewed some of the methods by which data accruing from these 

 may be cross-checked. As familiarity with the literature in 

 electron microscopy increases, the reader who asks the question 

 "How far can I trust what I see — or what the author says I 

 see?" needs a set of criteria neither different in principle nor more 

 mysterious than those he would apply to evidence from any other 

 source. These are essentially the same criteria applicable to light 

 microscopy, but magnified. The most important single qualifica- 

 tion is that the electron micrograph illustrates a structure 

 artificially fixed in a moment of time. It can yield information 

 on cell dynamics only if carefully controlled ancillary techniques 

 are used. 



Almost all of the information to* be discussed is drawn from 

 protozoan cells fixed in buffered solutions of osmium tetroxide, 

 dehydrated, embedded in plastic or resin, and thin-sectioned. 

 These techniques were first employed little more than ten years 

 ago; since then they have improved so rapidly that, in general, 

 more recent reports in this short interval can be assumed to be 

 more accurate than earlier ones (although of course improvement 



