14 
which is fiuid when heated, but which solidifies when cooled 
below 42 degrees centigrade. When the agar in the three 
tubes is melted they are cooled down to 43 degrees, anda small 
portion of the material containing spores is introduced ona 
flamed platinum needle into the first tube. The tube is then 
well shaken, the cotton plug is re-inserted and the top burned 
off to kill any organisms on the rim of the tube. One or two 
drops are now poured from this first tube to the next which is 
shaken, and the process is repeated between tubes number two 
and three. It will be seen that of the material taken from the 
insect, most is in tube number one, less in number two and least 
of allin number three. It really amounts to a twice repeated 
dilution, whence the term dilution-culture. The contents of 
the tube must have been kept all this time in a water-bath at 
42 degrees centigrade. Each tube is now removed, and its con- 
tents poured into asterilized shallow, flat dish of glass, called a 
Petrie-dish, which has an overlapping cover of glass. The agar 
on being poured into the cool Petrie-dish quickly hardens, and 
any organisms present are fixed in position in the thin layer of 
jelly-like agar which contains nourishment for their growth. 
The Petrie-dishes are now labeled and placed in a moderately 
cool place for a day or less. They may be placed on the stage 
of a microscope and studied at any time, and the germination 
and growth of the spores thus watched to the time of fruiting, 
when new spores are produced. In the course of one or two 
days minute growths of a filmy radiate form are usually to be 
seen scattered over the plates. Hach separate spot is pro- 
duced ordinarily by asingle spore or germ, so that each growth 
is pure in itself. These spots or colonies grow in a definite 
manner, which is constant for any one species. Sometimes 
they produce beautiful white circular growths of fluffy threads 
which in some species turn green with the production of conidia; 
in some species the growth issmalland star-shaped, or occasion- 
ally it is flat, but more often raised in relief and rounded, while 
not uncommonly the growth stops at any of these stages and 
can not be induced to proceed, a sure sign that the method of 
growing them is wrong. (See figure 8, plate 2.) 
Scattered through the plates, and often occupying nearly 
the entire surface, will be seen growths of common moulds and 
bacteria, which have been introduced. with material from the 
