196 The American Naturalist. [February, 
down over the drum JD, so as to firmly hold f down upon P; then 
moisten the central exposed part of f, that is not saturated with 
paraffine, with a little water with a fine-nozzled pipette; then pick up 
one of the little disks of elder pith by one edge with a fine forceps and 
lay it down onthe moist centre of f, with the convex side down, when 
it will at once flatten out and adhere to f and just neatly cover the cen- 
tral area not saturated with paraffine. The apparatus is now in readi- 
ness to begin operations. 
Place a drop of water (d) swarming with animalcules from a vigor- 
ous culture of infusoria on e, when it will be found that the water will 
be rapidly drawn through e and f into P in the direction of the arrows. 
In this way several drops of water may have a large part of their ani- 
malcular population separated out and caught upon the surface of e. 
To kill the contents of D it isonly necessary to add a little saturated 
corrosive solution or osmic acid, one per cent., with the help of a thin, 
slender wooden rod or toothpick to the drop. Enough of either of 
these killing and fixing agents can thus be added as a minute drop by 
simply thrusting the charged end of the wooden rod or toothpick ¢ into 
the drop d. The animalcules are at once precipitated by the killing 
agents upon the upper surface of e, where, strange to say, they are 
caught and held in the meshes formed by the pith cells. The filter e 
may now be gently removed and lifted off f by means of a needle and 
forceps. With gentle handling I find that Ciliates will remain attached ` 
to e, and may be passed through a dozen reagents without their becom- 
ing detached, and that the pith disk, with its adherent objects, may be 
embedded in paraffine very readily by the watch-glass method. 
Care should be exercised that the edges of the opening in m m do not 
come too close to the edge of e, else the water of the drop d will run 
off edgewise between m and f, and thus not pass through e alone. The 
paraffined portion of f should project a little beyond the free edges of 
m. Under such conditions the drop charged with organisms will round 
itself off as in the figure, and be kept from spreading by the greasy 
circular inner margin formed by the paraffine that saturates the margin 
of f. In lifting off e, raise its free edge slightly at one point with a 
needle; then catch the edge thus raised between the blades of a sharp- 
pointed forceps and transfer to a watch-glass containing 50 to 60 per 
cent. alcohol; then through the other solutions in succession. 
Even orientation may be very easily effected by means of this 
method, either by sketching the outline of the whole disk and the posi- 
tion and direction of the axes of the very minute objects on it under a 
low power of the microscope, or else by shaving down the block after 
