GENERAL CHARACTERS OF BACTERIA. 9 



Any further classification of yeasts is quite unnecessary for our 

 purposes. 



3. Schizomycetes (Fission Fungi, or Bacteria). This group 

 comprises the bacteria proper; it is certainly the most abundant 

 of the three, and in some respects it is the most important. It 

 is with the bacteria that we are chiefly concerned in this work. 

 Bacteria have sometimes much the same shape as yeasts. The 

 chief distinction between them is their method of multiplication. 

 Instead of budding they multiply by fission. The bacterium 

 elongates a little, and then divides into two equal /~\ ^^ 

 halves at once (Fig. 5). Hence the name fission 

 fungi. Bacteria are also, as a rule, smaller than . , ^"^ 

 yeasts, frequently not more than 1/25000 of an 

 inch in diameter. The size would make it possible 

 for 8,000,000,000 to be crowded into a mass no 

 larger than a pinhead; and we can, therefore, easily 



understand that there may be 100,000,000 in a of division by 

 drop of milk. Occasionally, however, there- are 

 larger bacteria and smaller yeast cells. While the size is no sure 

 criterion between the two, when one finds, under the microscope, 

 rather large round or oval plants, he is pretty safe in calling them 

 yeasts, while the smaller ones he may call bacteria. But it is 

 necessary, in some cases, to study the method of reproduction before 

 one can with certainty distinguish yeasts from bacteria. 



This group of bacteria is of such primary importance to our 

 study that we must learn further facts concerning their classifica- 

 tion and characters. 



GENERAL CHARACTERS OF BACTERIA. 



Colonies. Bacteria are so minute that they cannot be handled 

 as individuals, but must be treated in masses. One of the primary 

 difficulties in the study of these organisms has been to get masses 

 of bacteria that would be large enough to handle, and yet would 

 contain only one kind of bacteria. Such masses are called pure 

 cultures, and it was this difficulty in procuring pure cultures that 



