CHAPTER III. 

 CELL FORMS. 



Though there is apparently a wide variation in the shapes 

 of different bacterial cells, these may all be reduced to 

 three typical cell forms. These are: first and simplest, the 

 round or spherical, typified by a ball and called the coccus 

 form, or coccus, plural cocci^ (Fig. 35). The coccus may be 

 large, that is, from 1 .5 m to 2 ju in diameter. The term macro- 

 coccus is sometimes applied to these large cocci. If the 

 coccus is less than 1 ^t in diameter, it is sometimes spoken 

 of as a micrococcus; in fact, this term is very commonly 

 applied to any coccus. When cocci are growing together, 

 many of the cells do not appear as true spheres but are more 

 or less distorted from pressure of their neighbors or from 

 failure to grow to full size after recent division. Most cocci 

 divide into hemispheres and then each half grows to full 

 size. A few cocci elongate before division and then appear 

 oval or elliptical. 



The second cell form is that of a cylinder or rod typified 

 by a section of a lead-pencil. The name bacillus, plural 

 bacilli, is applied to this type (Fig. 36). The bacillus may 

 be short (Fig. 37), 1 // or less in length, or long, up to 40 ii 

 in rare cases. Most bacilli are from 2 ^u to 5 m or 6 /x long. 

 The ends of the rod are usually rounded, occasionally square 

 and very rarely pointed. It is evident that a very short 

 rod with rounded ends approaches a coccus in form, and 

 it is not always easy to differentiate in such cases. Most 

 bacilli are straight, but some are slightly curved (Fig. 38). 



The third cell form is the spiral, typified by a section of a 

 cork-screw and named spirillum, plural spirilla (Fig. 39). A 

 very short spiral consisting of only a portion of a turn is 

 sometimes called vibrio (Fig. 40). Vibrios when seen under 

 the microscope look like short curved rods. The distinc- 



1 The pronunciation of this word according to Enghsh standards is kok-si. 



(37) 



