GENERAL CHARACTERISTICS OF BACTERIA. 35 
growth and products to derangements which are known 
as acute or chronic infectious diseases. 
Numerous attempts have been made by various 
authors to classify bacteria systematically, but usually 
with the proviso that the system was only a temporary 
one. The classification of the older naturalists and 
botanists was based generally upon purely morphological 
peculiarities. As this depended, at times, upon slight 
variations that were seen to occur in the size and shape 
of one and the same species, it naturally resulted in a 
more or less complicated arrangement. In this place 
the morphological character of the bacteria will alone 
be given, their classification being left until the general 
characteristics of bacteria have been considered. 
MORPHOLOGY. 
The basic forms of the single bacterial cells are 
threefold—the sphere, the rod, and the segment of a 
spiral. Although under different conditions, the form 
of any one species may vary considerably, yet these 
three main divisions under similar conditions are per- 
manent; and, so far as we know, it is never possible 
by any means to bring about changes in the organisms 
that will result in the conversion of the morphology of 
the members of one group into that of another—that 
is, micrococci always, under suitable conditions, produce 
micrococci, bacilli produce bacilli, and spirilla produce 
spirilla. 
The form of the bacterial cells at their stage of com- 
plete development must be distinguished from that 
which they possess just after or just before they have 
divided. As the spherical cell develops preparatory to 
