DESCRIPTION OF PLATES. 133 
species when it is cultivated in blood-serum,; at least 
it is not so marked in other media or in other 
species. 
A careful inspection of the figure will show 
all the stages in the process of multiplication by 
binary fission. Some of the micrococci are seen to 
be quite spherical ; others are slightly oval ; others 
long-oval, and among these some are seen to pre- 
sent evidence of commencing division; others, 
again, show the process of division more or less 
advanced, or quite completed. 
There are different species of Micrococci, which 
vary as to size, color, and physiological characters. 
Cohn describes several species which are chiefly 
distinguished by the presence of different colored 
pigments, and which he calls Chromogenes. One of 
these is of a “ golden-yellow ” color, one a “ pink- 
carmine,” another a “deep-blue,” ete. 
In size, they vary from one-half a micro-milli- 
metre (one-fifty-thousandth of an inch) or less, to 
two micro-millimetres or more. 
As among plants higher in the scale, the size and 
rapdily of multiplication varies, within certain linuts, 
with the richness of the nutritive medium in which they 
are placed, and the temperature at which wt is main- 
tained. A suitable nutritive medium for one species, 
however, is not always suited to the development of an- 
other, and considerable diversity exists as regards 
the most favorable temperature. 
A mitritive medium which is exhausted for one species 
may serve for the development of another. 
By the use of the highest powers and of staining 
