CHAPTER III. 

 CELL FORMS. 



Though there is apparently a wide variation in the shapes 

 of different bacterial cells, yet 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. 32). The coccus may be 

 large, that is, from 1.5ix to 2n in diameter. The term 

 macrococcus is sometimes applied to these large cocci. If 

 the coccus is less than lyn in diameter, it is sometimes spoken 

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

 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 ftom 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 typi- 

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

 bacilli, is applied to this type (Fig. 33). The bacillus may 

 be short (Fig. 34), 1/i or less in length, or long, up to 40/i 

 in rare cases. Most bacilli are from 2ju to 5^ or 6/i 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. 35). 



' The pronunciation of this word according to English standards is 

 lc5k-si; the continental pronunciation is k6k-kee; the commonest American 

 seems to be kok-kl. We prefer the latter since it is easier and more 

 natural and should like to see it adopted. (Author.) 

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