BACTERIA. 37 



as illustrated in Fig. 28, in which a is the Micrococcus or spher- 

 ical form ; b the Bacterium or rod-like .form ; c the Bacillus or 

 filiform form, and d the Spirillum or coiled form. In growing, 

 bacteria are often grouped in different arrangements, to which 

 special names are applied, thus Streptococcus, a moniliform or 

 necklace-like grouping of cocci; Diplococcus, cocci in pairs; 

 Staphylococcus, single cocci ; Leptothrix, a filament of bacilli ; 

 Sarcina, a plate of cocci. 



000 ^nO/j 



00 n (? 



V = 



a c 



Fig. 28. 



Make an infusion of fresh hay by steeping it in water warmed 

 to between 40 and 50 C. for one-half hour or more, filter and 

 set aside for 36 hours or more. The liquid becomes cloudy, due 

 to swarms of bacteria. Examine a drop of the liquid under the 

 highest power at command. Focusing must be done very care- 

 fully because of the extreme minuteness and transparency of 

 the cells. The illumination must be somewhat dimmed, else 

 the bacteria will be almost invisible in the glare. A conveni- 

 ent way to find the focal plane is to move the slide about until 

 a coarser object, like a speck of dust, comes into view, and with 

 this as a guide to make the fine adjustment of the focus. 



Note the moving bacteria, elliptical or rod-like, sometimes 

 forming short, jointed rows. The cells have an outer, more 

 transparent wall, enveloping a more opaque substance. 



Apply a drop of iodine solution at the edge of the cover glass. 

 The bacteria are killed, all forward motion ceases, the con- 

 tents of the cells are stained and become more conspicuous. 

 The cell- wall does not stain. Other forms that may be found 

 are micrococcus, bacillus and spirillum. 



Resting Bacteria or Zooglcea Stage. The hay-infusion after 

 a time develops a scum or zooglcea. Mount a little of this 

 and examine with high power. Myriads of bacteria resting in 

 a gelatinous mass will be seen. Although they do not move 

 away from their places, the bacteria will be seen to have a 

 wiggling or oscillatory motion, known as the Brownian move- 

 ment. This motion is not a vital one, but is characteristic of 

 very small bodies, whether dead or alive. Fine clay, pumice, 

 lamp-black, gamboge, show the same motion. The cause is not 

 definitely known. 



Raise the cover glass and add a drop of iodine solution to the 

 scum. The bacteria stain, but not the gelatinous material in 

 which they are imbedded. 



Mount a little of the scum composing "mother-of-vinegar." 

 Nearly the same appearance will be seen as in the case of the 

 scum of the hay-infusion. The scum is known as mycoderma 



