ELEMENTS OF MICROBIAL CYTOLOGY 



23 



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

 are always formed in the chloroplastids. 



Membkane. — ^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- 



- cm 



Fig. 12. — Metachromatic corpuscles ifim) in S'arcosporidia, Sarcocystis tenella. 

 (After Erdmann.) Fat globules (g) in Trypanosoma rotatorium. {After Doflein.) 



braiie 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 carjbohydrates, as 

 cellulose, pectose, and callose (algse, 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 environing influences; the cell may then be regarded as 

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

 lEncystment is frequent with protozoa, and is produced when the 

 environment becomes unfavorable (Fig. 13, A). 



The externa^ layer of the membrane frequently undergoes modi- 

 fications, ■ transforming itself into a mucilaginous or gelatinous sub- 



