18 NOTES ON MICRO-ORGANISMS. 
IV. Is found associated, as its name implies, with milk that is 
turning sour. It is frequently seen in other liquids under- 
going a similar change. 
V. Can be obtained easily from an infusion of chopped hay 
in warm water. It is obtained with more certainty and 
free from other forms if the liquid be boiled and put 
aside covered up in a warm place. 
VI. Present not unfrequently during decompositions of very 
moist animal matter. In meat infusions, also during 
decomposition of brewers’ grains. 
We have here a considerable variety of shapes, ranging from 
spheres to spirals, and possessing essentially different properties. 
We will now enter into consideration of the structure of these 
organisms. The outer enveloping membrane is a fairly resisting 
and elastic substance of the nature of cellulose. In the case of 
Bact. aceti and an organism called Leuconostoc, it is pretty certain 
that it is cellulose. The inner portion of the Bacterium cells is 
a pasty mass, rich in nitrogen, conveniently called Protoplasm or 
Mycoprotein, varying in density, transparency, and refractive 
power. Many Bacteria enter into a motile or actively moving 
state at some period of their development, and the organs by 
which this movement is effected are hair-like protusions from the 
organisms known by the names of Cilia or Flagella. Having 
probably about the same refractive power as water, it is very 
difficult to view them unless their movement be stopped, and the 
membrane of which they consist be condensed by treatment with 
iodine solution or osmic acid. It is affirmed that some organisms 
have the power of retracting the cilium into the cell. A curious 
fact is mentioned in Zopf’s work, viz., that micro-photography will 
sometimes render visible the cilia that cannot be seen by the eye, 
the sensitized plate being more susceptible than the retina. 
We may now deal with the plan of reproduction or multiplica- 
tion of some typical Bacteria, and for convenience in grouping 
take Cohn’s classification of 1872 :— 
Class I.—Sphcero-bacteria ... ... Dot or Sphere. 
»» II.—Micro-bacteria ... ... Short Rods. 
