28 ELEMENTARY BIOLOGY. [CHAP, 



c. Structure; an outer more transparent layer 

 enveloping less transparent matter: in the com- 

 pound forms the envelope appears only where 

 two joints come in contact, so that the rod 

 looks as if made up of alternating transparent 

 and more opaque substances. 



d. Movements ; some vital, and some purely phy- 

 sical (Brownian). The former various but pro- 

 gressive: the latter, a rotatory movement round 

 a stationary centre; study it in a drop of boiled 

 infusion in which the Bacteria are all dead. 



4. Treat with iodine only the more opaque parts 

 stain ; probably then we have to do with protoplasm, 

 enveloped in nonprotoplasmic matter. 



5. Resting Bacteria. (Zoogloea-stage.) 



a. Examine the scum from the surface of a hay 

 infusion; it exhibits myriads of motionless Bac- 

 teria, embedded in gelatinous material. 



b. Treat with iodine; the Bacteria stain as before : 

 the gelatinous uniting material remains un- 

 stained. 



6. Mixed with the Bacteria proper, both in the pellicle 

 and the fluid beneath, may be found the following 

 forms of living beings. 



a. Micrococcus. Bodies much like Bacteria, but 

 short and rounded, and occurring singly, or in 

 bead-like rows. They may be found free or in a 

 Zoogloea stage. 



b. Bacillus. Threads composed of straight cylin- 

 drical joints much longer than those of Bacteria 



