EXPERIMENT STATION BULLETINS 



215 



Fig. 2. Bacilli, 



the length of which is two to ten times the thickness 

 of the rod. The length of any species varies consider- 

 ably, yet it has its limitations. The diameter of the rod 

 is more likely to remain constant, still this will vary 

 slightly. The figures just pointed out indicating the 

 ratio of thickness to length, bearing in mind the con- 

 stancy of the diameter, will illustrate the possibility 

 Size. of variation. The size of these cells is measured by a unit 



called ■* micro millimeter or micron, whicli is equal to y^yVij-th 

 millimeter, or about ^-yfg-oth inch. This unit is indicated by the Greek 

 letter mu. m. The length of a bacillus will vary from one to ten 

 microns. The bacillus which causes tuberculosis measures about 2 

 m. or ^g-fxTTT^i^ inch in length; that is, considering the human red blood 

 corpuscles equal to sVo'o^'^ inch in diameter, it would require nearly 

 four bacilli arranged iu a row end to end to reach across the cell. It is 

 possible to distinguish markings of ^oTT^h inch with the naked eye. In 

 order to make a thread of bacilli as above, that would correspond to the 

 Tofltli inch, sixty-two and one half bacilli would be needed. There may be 

 several hundred thousand of these bacilli confined within a drop of water. 

 Some bacilli are much longer and larger than the tubercle bacillus, and 

 Lbere are others much smaller. A rod, straight, whose length is greater 

 than its diameter, is usually called a bacillus. To a short, thick 

 i^acterinm. rod whose length is about equal to its diameter, the term bac- 

 terium has been applied. In this sense it is used specifically, 

 but is also employed generically as the singular of bacteria. 

 Micrococcus. Coordinate with bacillus in the classification based upon form 

 is the micrococcus, a small sphere. Instead of being a rod 

 shaped organism, as the bacillus, the micrococcus is round and 

 ball shaped. The size of a micrococcus does not 

 vary so much as a bacillus, yet it is not constant. 

 Some micrococci are much larger than others, still 

 the average micrococci have a diameter which is 

 about equal to the diameter of a bacillus. One 

 micron indicates about the average diameter of 

 each. To illustrate, it would take 25,000 micro- 

 cocci arranged as a string of beads to equal in 

 length the diameter of a medium sized plum. These cells are not always 

 perfectly spherical, but may be almost cube shaped, compressed on sev- 

 eral sides, or they may be biscuit shaped or may be pointed on opposite 

 sides. 



Along with bacilli and micrococci is another form which 

 completes thi^ classification. This form is screw shaped 



or spiral; the screw of a corkscrew fairly 

 represents this form. Such micro-organ- 

 isms are known as spirilla, the singu- 

 lar of which is spirillum. The lengths 

 of these bacteria vary more than any 

 of the other forms. They may be much 

 longer than any of the bacilli and 

 again may not exceed some of the 

 shorter in length. There are many of 

 the spirilla made up of segments, one segment consisting of 

 only one curve of the spiral. When a segment of this kind exists by itself, 



Shape. 



Size. 



Fig. 3. 

 Micrococci. 



Spirillum. 

 Siiape. 



^ize. 



Vibrio. 



Spirilla. 



