III. 



MORPHOLOGY. 



IN the present chapter we shall give a general account of the 

 morphology, modes of grouping, and dimensions of the bacteria. 



The standard of measurement used by bacteriologists is the micro- 

 millimetre, or the one-thousandth part of a millimetre. This is 

 represented by the Greek letter yw. One >M (micromillimetre) is equal 

 to about one-twenty-five-thousandth of an English inch. 



The spherical bacteria, or micrococci, differ greatly in size, and 

 also in the mode of grouping when, as a result of binary division, 

 they remain associated one with another. The smallest may mea- 

 sure no more than 0.1 >w, while some of the larger species are from 

 one to two yw in diameter. The enormous number of these minute 

 organisms which may be contained in a small drop of a pure culture 

 may be easily estimated in a rough way. Compare a single micro- 

 coccus, for example, with a sphere having a diameter of one-twenty- 

 fifth of an inch. If our micrococcus is one of the larger sort, having 

 a diameter of one >w, it would take a chain of one thousand to reach 

 across the diameter of such a sphere, and its mass, as compared 

 to the larger sphere, would be as 1 to 523,600,000. 



The number of cocci in a milligramme of a pure culture of Staphy- 

 lococcus pyogenes aureus has been estimated by Bujwid, by count- 

 ing, at 8,000,000,000. 



Not only do different species differ in dimensions, but consider- 

 able differences in size may be recognized in the individual cocci in a 

 pure culture of the same species. On the other hand, there are 

 numerous species which so closely resemble each other in size and 

 mode of association that they cannot be differentiated by a micro- 

 scopic examination alone, and we must depend'upon other characters, 

 such as color, growth in various culture media, pathogenic power, 

 etc. , to decide the question of identity or non-identity. 



When in active growth the micrococci necessarily depart from a 

 typical spherical form just before dividing, and under these circum- 

 stances may be of a short or long oval. When division has taken 

 place, if the two members of a pair remain associated they are often 

 more or less flattened at the point of contact (Fig. 1, a). 



