separating immediately after division, and thus always appearing
as short rods (Fig. 6), while others remain attached after
division and form long chains. Sometimes they appear to continue
to increase in length without showing any signs of division, and
in this way long threads are formed (Fig. 7). These threads are,
however, potentially at least, long chains of short rods, and
under proper conditions they will break up into such short rods,
as shown in Fig. 7a. Occasionally a rod species may divide
lengthwise, but this is rare. Exactly the same may be said of the
spiral forms. Here, too, we find short rods and long chains, or
long spiral filaments in which can be seen no division into
shorter elements, but which, under certain conditions, break up
into short sections.

RAPIDITY OF MULTIPLICATION.

It is this power of multiplication by division that makes bacteria
agents of such significance. Their minute size would make them
harmless enough if it were not for an extraordinary power of
multiplication. This power of growth and division is almost
incredible. Some of the species which have been carefully watched
under the microscope have been found under favourable conditions
to grow so rapidly as to divide every half hour, or even less. The
number of offspring that would result in the course of twenty-four
hours at this rate is of course easily computed. In one day each
bacterium would produce over 16,500,000 descendants, and in two
days about 281,500,000,000. It has been further calculated that
these 281,500,000,000 would form about a solid pint of bacteria
and weigh about a pound. At the end of the third day the total
descendants would amount to 47,000,000,000,000, and would weigh