Fig. 1. Section of R. ruhrum growing exponentially at a light intens- 

 ity of 50 foot-candles. The membrane bounded vesicles (v) extend deep 

 into the cytoplasm, but are most abundent at the periphery of the cell. 

 Main fixation 18 hours, no post staining. X 84,000. 

 Fig. 2. Section of a dividing cell of R. rubriim growing exponentially 

 at a light intensity of 6000 foot-candles. Membrane-bounded vesicles 

 are sparsely and irregularly distributed around the periphery of the 

 cell. At the point indicated by an arrow, a portion of the vesicular 

 membrane is continuous with the cytoplasmic membrane and the light 

 central area of the vesicle opens into the space that lies between the 

 cell wall (w) and the cell membrane. Nucleoplasm (n) and ribosomes 

 (r) are particularly well preserved in this section. Main fixation 2 

 hours, post staining with lead hydroxide. X 120,000. 



