The matter of spore formation serves as one of the points for distinguishing species.  Some species do not form spores, at least under any of the conditions in which they have been studied.  Others form them readily in almost any condition, and others again only under special conditions which are adverse to their life.  The method of spore formation is always uniform for any single species.  Whatever be the method of the formation of the spore, its purpose in the life of the bacterium is always the same.  It serves as a means of keeping the species alive under conditions of adversity.  Its power of resisting heat or drying enables it to live where the ordinary active forms would be speedily killed.  Some of these spores are capable of resisting a heat of 180 degrees C. (360 degrees F.) for a short time, and boiling water they can resist for a long time.  Such spores when subsequently placed under favourable conditions will germinate and start bacterial activity anew.

Motion.

Some species of bacteria have the power of active motion, and may be seen darting rapidly to and fro in the liquid in which they are growing.  This motion is produced by flagella which protrude from the body.  These flagella (Fig. 15) arise from a membrane surrounding the bacterium, but have an intimate connection with the protoplasmic content.  Their distribution is different in different species of bacteria.  Some species have a single flagellum at one end (Fig. 15 a).  Others have one at each end (Fig. 15 b).  Others, again, have, at least just before dividing, a bunch at one or both ends (Fig. 15 c and d), while others, again, have many flagella distributed all over the body in dense profusion (Fig. 15 e).  These flagella keep up a lashing to and fro in the liquid, and the lashing serves to propel the bacteria through the liquid.

Internal structure.

It is hardly possible to say much about the structure of the bacteria beyond the description of their external forms.  With all the variations in detail mentioned, they are extraordinarily simple, and about all that can be seen is their external shape.  Of course, they have some internal structure, but we know very little in regard to it.  Some microscopists have described certain appearances which they think indicate internal structure.  Fig. 16 shows some of these appearances.  The matter is as yet very obscure, however.  The bacteria appear to have a membranous covering which sometimes is of a cellulose nature.  Within it is protoplasm which shows various uncertain appearances.  Some microscopists have thought they could find a nucleus, and have regarded bacteria as cells with inclosed nucleii (Figs. 10 a and 15 f).  Others have regarded the whole bacterium as a nucleus without any protoplasm, while others, again, have concluded that the discerned internal structure is nothing except an appearance presented by the physical arrangement of the protoplasm.  While we may believe