14 ELECTRON-MICROSCOPIC STRUCTURE OF PROTOZOA 



phase and a membrane-enclosed phase, and that the membrane- 

 limited spaces are at least potentially all in communication with 

 each other and with the outside environment of the cell. 



Cytoplasmic membranes always surround and enclose some- 

 thing, even though the enclosed space may become nearly 

 eclipsed by flattening. They are, as would be expected, phase 

 boundaries and their formation and function must be intimately 

 related to the existence of the two phases. At least some of them 

 are exceedingly labile structures, yet their molecular framework 

 cannot be haphazard and (if the unit membrane concept is valid) 

 is always basically the same. The cytoplasmic matrix must 

 contain in considerable abundance molecules capable of being 

 rapidly incorporated into existing membranes or aligned to form 

 new ones ; it would seem economically sound if already formed 

 membrane skeletons could, where available, serve as templates 

 in this process. The specific nature of the membrane surface 

 facing the cytoplasmic continuum should be roughly the same in 

 adjacent regions of the cytoplasm; the specific nature of the 

 surface facing the enclosed phase should vary with the source and 

 composition of that phase. Temporary or permanent fusion of 

 neighboring membranous compartments could under these cir- 

 cumstances be expected at least accidentally and might subserve 

 important functions of transport. Where very precisely pre- 

 determined enzyme assemblages are required to carry out specific 

 localized reactions, membrane packages of a high degree of order, 

 such as mitochondria, chloroplasts, etc., are formed, and these 

 are generally more stable. 



What all this means is that for most cells, and particularly for 

 most protozoa, the significance of miscellaneous cytoplasmic 

 membranes is not known. When we use the term "endoplasmic 

 reticulum" for such membranes we use it descriptively, recogniz- 

 ing that the structures may be evanescent, and without implying 

 any certitude about their source, fate, or function. 



In a majority of cells, including protozoan, at least some of the 

 cytoplasmic membranes are studded on their outer (facing the 

 cytoplasmic matrix) surfaces with Palade particles (Fig. 1, PL I). 

 The outer surface of the nuclear envelope is typically granular and 

 often is continuous with granular endoplasmic reticulum. In 

 several types of metazoan cells that are actively engaged in 



