CHAPTER III. 



MORPHOLOGY OF BACTERIA. 



General grouping. The minuteness of bacteria ren- 

 ders investigation into their structure somewhat difficult. 

 When they are examined microscopically in the fresh or liv- 

 ing condition they appear as colorless, refractive bodies of 

 different shapes and sizes. According to their morphology,, 

 bacteria are divided into two groups, viz., the lower and 

 higher forms. 



Lower bacteria. The lower bacteria consist of unicel- 

 lular, spherical, rod-shaped, thread, or spiral forms, rarely 

 more than three to five microns* in length. The largest 

 forms met with in animals do not exceed ten microns in 

 length and perhaps 2.0 microns in thickness. The smallest 

 are 0.5 micron long and 0.1 to 0.2 micron in width. They 

 are divided into three distinct classes, namely, the spherical, 

 the rod-shaped, and the spiral forms. They are rarely 

 branched. Lower bacteria reproduce in the vegetative form 

 by fission or transverse division. 



Higher bacteria. The higher bacteria are characterized 

 by quite definite branched or unbranched filaments. In most 

 cases the filaments are cut transversely at regular intervals 

 by septa into short rod-shaped or curved elements. Some- 

 have a definite membrane or sheath common to all the ele- 

 ments in the filament. In certain cases the free ends only 

 seem to be endowed with reproductive function. They de- 

 velop bodies known as conidia. In this group belong the 

 Chlamydobacteriaceae and the Beggiatoa. 



The most striking features in the development of the 



*A micron = .001 of a millimeter = 1/25000 of an inch. It is 

 the unit for microscopic measurements. 



