NOTES ON MICRO-ORGANISMS. 21 
It then appears that in the majority of cases it is almost im- 
possible to identify a Bacterium from its mere appearance at any 
one time. Zopf’s statements are not unsupported, for Klebs and 
other workers have seen rod and spirillum forms in the same 
organism. There seems to be a very general tendency for 
Bacteria in the form of rods and threads to become curved or 
crooked, especially with alterations of nourishment. Besides the 
normal forms exhibited by Bacteria, very curious deformities are 
occasionally met with, showing dark-coloured protoplasm and 
peculiarity of form, that were the portions viewed by themselves 
one would not associate them with the original form. Such 
deformities are called retrograde or involution forms, and are 
probably called into existence by deficient nourishment (Plate I., 
Fig. 2, aa). 
Let us return to a closer consideration of the reproduction by 
fission of rodlengths. Zopf says that the membrane by which 
the Bacteria are enveloped is capable of thickening and then 
dividing into layers, one of which (the inner) is capable of re- 
arranging itself, whilst the other (the outer layer) grows for a 
longer or shorter time till finally it may yield to the pressure of 
enclosed cells and some of these last be pushed out. This may 
be seen in Plate III, Fig. 2, Crenothrix Kiihniana, Plate IV., 
Fig. 2, with an enlargement to show the process just described 
more plainly. This organism is somewhat closely allied to the 
moulds ‘in its characteristics, but is classed by most authorities 
amongst Bacteria. It affords a striking example of cell formation 
by internal differentiation of the protoplasm in the threads. We 
can now proceed to consider in greater detail the reproduction of 
Bacteria by spore formation, a process first observed by Cohn in 
Hay bacillus. Spores are formed by a contraction of the contents 
of the cell and condensation into small spherical masses with a 
new and independent membrane. A disintegration of the old 
cell envelope often takes place about the same time, thus freeing 
the spores. The process of sporulation is clearly seen in two 
kinds of Bacteria, the Bacillus subtilis and the Butyric fer- 
ment, or Clostridium butyricum ; also called Bacillus butyricus 
