234 CLEAN MILK 



called i fi or mu or micron) to three or four times as much. The 

 diameter of the individuals belonging to any one species varies very- 

 little. The length of bacteria in general ranges from 1/25,000 of 

 an inch to very many times that figure — perhaps to 1/16 of an inch 

 or more amongst the higher bacteria, but these very long forms are 

 twisted and curled, never extended in a straight line. The length 

 of the individuals belonging to any one species varies very greatly, 

 according to age and other conditions. Hence, in examining bac- 

 teria under the microscope, great differences in length do not neces- 

 sarily indicate that more than one species is present; but if differ- 

 ences in diameter are found, the observer may feel pretty certain 

 that he is dealing with a mixture of several kinds ; on the other hand, 

 it is quite possible to have two different kinds of the same diameter : 

 in such cases the shape of the individuals may assist in deciding 

 whether one or more kinds are present. 



Shape. — The bacteria present very simple symmetrical outlines 

 - — so simple that there is little room for great differences between 

 them. The simplest form is spheroidal, some kinds being truly 

 round, others flattened slightly. The rest of the bacteria are more 

 or less cylindrical. Of these cylindrical forms, some are short, 

 others very long, others intermediate. The short cylindrical bac- 

 teria are, in some species, twisted spirally. Some few bacteria 

 branch. 



Structure of Bacteria. — The individual bacterium is composed 

 of practically the same material as are all other living things — 

 protoplasm. Chemical tests cannot distinguish the protoplasm of 

 the animal from that of the vegetable, nor the protoplasm of either 

 of these from the protoplasm of the bacterium. That the protoplasm 

 of the animal varies, nevertheless, from that of the vegetable, and 

 both from bacterial protoplasm, is known, because each grows and 

 acts in a manner peculiar to itself in many details. The minute par- 

 ticle of protoplasm which forms a bacterium is enclosed in a wall, 

 composed of a chitin-like material similar to that which forms the 



