REPRODUCTION AND CELL DIVISION 31 
Usually the size and position of the spore is fairly constant in a given 
type of bacteria. Spore formation is most common among the anae- 
robes, fairly common among the saprophytic bacteria, practically absent 
in the pathogenic bacteria, and rarely or never takes place spontane- 
ously in the intact tissues of the human or animal body. The spiral 
organisms rarely produce spores, and, with the exception of Sarcina 
pulmonum, spore formation has never been observed in the cocci. 
It has never been satisfactorily determined whether spore formation 
is a regular definite stage in the life history of bacteria which produce 
them, or whether spores are produced rather under the stress of 
inifavorable environmental conditions. 
(lermination of Sjwres.— When bacterial spores are placed in an 
environment favorable to the vegetative activity of the cell, they 
germinate: the dense membrane which constitutes the ectoplasm of 
the spore softens, usually at the pole or the equator, and the vegeta- 
tive rod emerges, at first as a small bud, then rapidly assumes the 
typical size and shape of the fully mature cell. The development 
of the anthrax bacillus from the spore is usually in the line of the longer 
axis, polar germination: B subtilis, on the contrary, usually emerges 
at right angles to the larger axis of the spore, equatorial germination. 
Many spores are circular in outline, and in such cases the relation of the 
developing vegetative cell to the axis of the spore is unknown. Fre- 
quently the remnants of the spore membrane remain attached to one 
end of the newly formed vegetative cell, appearing as a cap, as it were. 
Some spores do not appear to rupture as germination takes place— the 
newly forming organism appears to absorb the entire spore and its 
ectoplasm, incorporating the entire structure by solution in the vege- 
tative cell. This is commonly referred to as germination by absorption, 
Arthrospores.— Certain organisms belonging to the coccal group, 
more particularly the streptococci, exhibit from time to time cells 
which are decidedly larger than their fellows. These cells are more 
highly refractile, they usually possess a granular cytoplasm, and fre- 
quently stain somewhat irregularly. They have been designated by 
Hueppe^ as arthrospores. These arthrospores appear to have no 
unusual resisting powers to environmental conditions, and they are 
in no sense to be regarded as true spores. It is very probable that 
thev are involution forms. 
E. REPRODUCTION AND CELL DIVISION. 
Bacteria are structurally the simplest known organisms which 
maintain an independent existence: all their vital functions are 
exhibited in a single asexual cell devoid of a morphologically defin- 
able nucleus. The absence of sexual characters and of a morphological 
1 Die Methoden der Bakterien Forschung, Wiesbaden, 1889. 
