ELEMENTS OF MICROBIAL CYTOLOGY 23 



has long been known, on the other hand, that the starch and paramylum 

 are always formed in the chloroplastids. 



MEMBRANE. The cell is usually enveloped in a more or less heavy 

 membrane, secreted by the cytoplasm, which acts as a protective 

 organ for the cell. 



The presence of the membrane is not, however, indispensable; 

 many protozoa do not have it, and are consequently naked cells. 

 Motility in many microorganisms is closely associated with the mem- 

 brane, for the movement of cytoplasm and the flexibility of the mem- 



FIG. 13. A, Metachromatic corpuscles (cm) in Sarcosporidia, Sarcocystis tenella. 

 (After Erdmann.} B, Fat globules (g) in Trypanosoma rotatorium. (Ajter Doflein.} 



brane are essential factors. Cells as a rule have a membrane of 

 different degrees of thickness and composition. It may be albuminoid 

 or chitinous (Infusoria], or it may be made up of carbohydrates, as 

 cellulose, pectose, and callose (algae, fungi). Bacteria always have a 

 membrane, but its nature has not yet been definitely determined. 

 Often the cell membrane is able to thicken noticeably, and thus protect 

 the cell from influences of environment; the cell may then be regarded as 

 transformed into a cyst which passes into a state of sluggish existence. 

 Encystment is frequent with protozoa, and is produced when the 

 environment becomes unfavorable (Fig. 14, A). 



The external layer of the membrane frequently undergoes modi- 

 fications, transforming itself into a mucilaginous or gelatinous sub- 



