48 BACTERIOLOGY. 



In size the bacteria are certainly the smallest living 

 creatures with which we have acquaintance, being visible 

 only when very highly magnified. In order that some 

 conception of their microscopic dimensions might be 



Fig. 1. 







Zooglcea of bacilli. 



formed, it has been comj^uted that of the average size 

 bacteria about thirty billion would be required to weigh 

 a gramme, and that about one billion seven hundred 

 million of the small spherical forms might readily be 

 suspended in a drop of water. 



The classifications of the older authors and of the 

 botanists being usually upon purely morphological pecu- 

 liarities, and, as a rule, taking into consideration all the 

 slight variations that are seen to occur in the size and 

 shape of one and the same species, and of closely allied 

 species, are more or less complicated. The present 

 tendency is to simplify this morphological classification, 

 and to distribute the bacteria into three great groups, 

 with their subdivisions, the members of each group 

 being identified by their individual outline, viz., that 

 of a sphere, a rod, or a spiral. To these three grand 

 divisions are given the names cocci or micrococci, bacilli, 

 and spirilla. 



In the group micrococci belong all spherical forms — 

 i. e., all those forms the isolated individual members 

 of whicli are practically of the same diameter in all 

 directions. (See Fig. 2, a, b, c, d, c.) 



