258 BACTERIOLOGY. 
placed in one class and under others in another. Thus 
the power to produce spores may be totally lost or held 
in abeyance for a time. 
The relations to oxygen may be gradually altered, 
so that an anaérobic species grows in the presence of 
oxygen. Parasitic bacteria may be so cultivated as to 
become saprophytic varieties, and those which have 
no power to grow in the living body given pathogenic 
properties. 
The possibility of making any thoroughly satisfac- 
tory classification is rendered still more difficult by the 
fact that many necessarily imperfect attempts have 
already been made, so that there is a great deal of con- 
fusion, which is steadily increased as new varieties are 
found or old ones reinvestigated and classified differ- 
ently in the different systems. 
As one of the more successful attempts to classify 
bacteria, the system devised by Migula is here given, 
simply as an example. The morphology of bacteria is 
used as the basis of the divisions : 
FAMILIES. 
I. Cells globose in a free state, not elongat- 
ing in any direction before division 
into 1, 2, or 3 planes. $ . 1. Coccacez. 
II. Cells cylindrical, longer or shorter, 
and only dividing in one plane, and 
elongating to twice the normal length 
before the division. 
(1) Cells straight, rod-shaped, without 
sheath, non-motile, or motile by 
means of flagella . ‘ : . 2. Bacteriaces. 
(2) Cells crooked, without sheath . 3. Spirillaces. 
(3) Cells enclosed inasheath . ~~. 4. Chlamydobacteriacex. 
(4) Cells destitute of a sheath, united 
into threads, motile by means of an 
undulating membrane , ' . 5, Beggiatoacez, 
