(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 that they have some internal structure, we must
recognise that as yet microscopists have not been able to make it
out. In short, the bacteria after two centuries of study appear to
us about as they did at first. They must still be described as