Lond., 1908); Park,
_Pathogenic Micro-organisms_ (London, 1906); Sternberg, _Manual of
Bacteriology_ (with full bibliography, 2nd ed., New York, 1896); Woodhead,
_Bacteria and their products_ (with bibliography, London, 1891). The
bacteriology of the infective diseases (with bibliography) is fully given
in the _System of Medicine_, edited by Clifford Allbutt, (2nd ed., London,
1907). For references consult _Centralbl. fuer Bakter. u. Parasitenk._
(Jena); also _Index Medicus_. The most important works on immunity are:
Ehrlich, _Studies in Immunity_ (English translation, New York, 1906), and
Metchnikoff, _Immunity in Infective Diseases_ (English translation,
Cambridge, 1905).

(R. M.*)

[1] Gr. [Greek: bakterion], Lat. _bacillus_, little rod or stick.

[2] _Cladothrix dichotoma_, for example, which is ordinarily a branched,
filamentous, sheathed form, at certain seasons breaks up into a number of
separate cells which develop a tuft of cilia and escape from the sheath.
Such a behaviour is very similar to the production of zoospores which is so
common in many filamentous algae.

[3] Brefeld has observed that a bacterium may divide once every half-hour,
and its progeny repeat the process in the same time. One bacterium might
thus produce in twenty-four hours a number of segments amounting to many
millions of millions.

[4] The difficulties presented by such minute and simple organisms as the
Schizomycetes are due partly to the few "characters" which they possess and
partly to the dangers of error in manipulating them; it is anything but an
easy matter either to trace the whole development of a single form or to
recognize with certainty any one stage in the development unless the others
are known. This being the case, and having regard to the minuteness and
ubiquity of these organisms, we should be very careful in accepting
evidence as to the continuity or otherwise of any two forms which falls
short of direct and uninterrupted observation. The outcome of all these
considerations is that, while recognizing that the "genera" and "species"
as defined by Cohn must be recast, we are not warranted in uniting any
forms the continuity of which has not been directly observed; or, at any
rate, the strictest rules should be followed in accepting the evidence
adduced to render the union of any forms probable.

BACTRIA (_Bactriana_), the ancient name of the country between the range of
the Hindu Kush (Paropamisus) and the Oxus (Am