Z ELECTRON-MICROSCOPIC STRUCTURE OF PROTOZOA 



against the uncomfortable boundaries of light-microscope resolu- 

 tion, this same quality now has the advantage of encompassing 

 a multitude of structures and functions within a single package 

 that the electron microscope can open. Because each cell is also 

 a relatively independent organism, groups of individuals can be 

 studied in parallel as fully and naturally functioning units and as 

 assemblages of structural parts. 



From ancestors whose identity is obscured by time, the earliest 

 protozoa inherited the ability to segregate many vital activities 

 within membrane-bounded intracellular compartments, and they 

 were ingenious experimenters with this equipment. They 

 explored countless possibilities and achieved enough successful 

 combinations to give rise to the metaphytan and metazoan lines 

 as well as to the myriad forms that have survived admirably 

 without ever finding it necessary to conscript battalions of 

 interdependent units. During the course of this experimentation 

 they hit upon most of the special functions that are differentially 

 assigned to less versatile cells, tissues, and organs in the metazoa 

 and metaphytes. In addition to the every-day miracles that each 

 living cell must perform, protozoa perfected such remarkable 

 skills as contraction, skeleton formation, co-ordinated locomo- 

 tion, sexual reproduction, photosynthesis, symbiosis both benign 

 and pathogenic, and the morphogenesis of complex form and 

 pattern. 



As innovators, specialists, models, and extremists, the protozoa 

 have been exploited for only a tiny fraction of the information 

 that is ours for the seeking. While we are beginning to feel rather 

 comfortably at home within the minute dimensions of some of 

 the more domesticated vertebrate cells, there is not yet a single 

 protozoan type whose ultra-structure and ultra-function have 

 been so thoroughly mapped. Too many of the clarifying micro- 

 graphs are still wanting, even for the most familiar species, and 

 the combined application of electron microscopy with other, 

 analytic techniques has rarely been attempted. The generaliza- 

 tions that we can begin to make about protozoa are based on 

 comparisons with "higher" organisms, and it ought to be the 

 other way around. 



The fraction of information on protozoan ultrastructure that 

 we have obtained, and the tentative generalizations emerging 



