Io THE GENERAL CHARACTERS OF MICROORGANISMS 
frequently not more than 145999 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 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 for a long time prevented the development of the science. 
Bacteriological study to-day commonly begins with cultivating 
the bacteria, z.¢e., allowing them to grow in some medium adapted 
to them until they become abundant enough to be handled in bulk. 
To prevent the mixing of the different kinds that may be in the 
material we are studying, they are usually grown in a solid or 
jelly-like medium, which holds the individuals fast in one spot. 
As the individuals multiply in this solid medium, they are unable 
to separate from each other; so they remain in little clusters 
which in time become large enough to be seen without a micro- 
scope. Such clusters are called colonies, and figures of some of 
