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 /-\ x N 
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 
