are commonly found in a variety of epitheliums. The points along the 

 cell surface where the membrane has invaginated to divide the cytoplasm 

 into compartments are considered to be regions of active transport. 



^- Pf 



cm / 



^f 



16 



Figure 3-32. Electron Micrograph Showing Five Body Cilia of Par- 

 amecium multimicronucleatum in cross section. The individual cilium con- 

 sists of nine peripheral filaments (pf) evenly spaced in a circle around a 

 central pair of filaments (cf), and surrounded by the cell membrane (cm). 

 Each of the nine peripheral filaments is, itself, double with each member of 

 the pair measuring 150 to 200 A in diameter, which is equivalent in size 

 to one of the central filaments. (From Sedar, A. W. and Porter, K. R., 1955. 

 'The Fine Structure of Cortical Components of Paramecium Multimicro- 

 nucleatum." /. Biopliys. Biochem. CyloL, 1, Fig. 6, Plate 153.) 



VACUOLES 



It is a debatable matter whether a vacuole should be treated as a cell 

 organelle. It consists essentially of an aggregation of solid or fluid sub- 

 stances. The question of whether or not it is delimited by a membrane or 

 a protoplasmic interface is open, as are the questions of its origin and 

 function. Vacuoles vary in size from virtually submicroscopic to almost 



66 / CHAPTER 3 



