130 PHOTO-MICROGRAPHS. 
Micrococcr. (Puate II. Fie. 2.) 
Micrococci are unicellular vegetable organisms, 
which, as regards dimensions, simplicity of struct- 
ure, and mode of reproduction, are at the very foot 
of the scale of living beings. They belong to the 
class of unicellular plants known as Bacteria, — 
a class which includes many rod-like forms, and 
of which all the species are microscopic. These, 
being larger than the Micrococci, which are little 
spheres, first attracted the attention of microscop- 
ists, and gave name to the class (also called 
Schizomycetes ). 
The structure of Micrococci is that of vegeta- 
ble cells generally, viz. a cell-wall, and contained 
protoplasm. They are so minute that the cell- 
wall cannot easily be demonstrated under the 
microscope ; but that it exists, and that it is com- 
posed of cellulose, as in other vegetable cells, is 
shown by the resistance offered to various chemi- 
cal reagents, — strong acids and alkalies, — which 
destroy naked protoplasm or animal membranes. 
The mode of multiplication is the simplest 
known or conceivable. A spherical micrococcus, 
placed in a suitable nutritive medium, becomes 
elongated, and divides in the median line into two 
micrococci. Each of these again divides into two, 
in the same way, by spontaneous fission, as it is called, 
and so on ad infinitum ; that is to say, so long as 
the supply of nutriment holds out, and the condi- 
tions as to temperature, etc., are favorable to the 
growth of the microscopic plant. 
