IV.J BACTERIA. 27 



b. Size ; breadth, very small but pretty constant ; 

 length, varying, but several times greater than 

 their breadth.: measure. 



c. Structure ; an outer more transparent layer enve- 

 loping less transparent matter : in the compound 

 forms the envelope alone appears 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 physical 

 (Brownian). The former various but progressive : 

 the latter, a rotatory movement round a sta- 

 tionary 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 ; 

 probablj'' then we have to do with protoplasm, enve- 

 loped in non-protoplasmic 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. 



h. 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. 



