1916] GRA VES: CHEMOTROPISM IN RHIZOPUS 341 
marked out on a clean slide by means of a brush and a very small 
quantity of vaseline. Then on a warming table were placed a 
small beaker in which were water, a thermometer, and a small vial 
holding the agar for the films. When the agar, previously melted . 
in hot water, had fallen to 40° or slightly below, the requisite num- 
ber of drops of a suitable suspension of spores in distilled water was 
added. With a small pipette, marked to indicate the amount of 
agar solution it should contain for the making of a film, the correct 
quantity was drawn up, quickly dropped and spread evenly on the 
area marked out on the glass slide by the vaseline lines. In order 
to facilitate an even spreading of the agar, the slides were laid on a 
large glass plate fixed in an exactly horizontal position with the 
aid of screw supports and a spirit level. It was found that it was 
very essential to keep the agar agitated during the process of pipet- 
ting it from the vial; otherwise the spores would not be distributed 
evenly. Before the agar on the glass slide had had time to set 
completely, the mica plate was placed carefully upon it in such a 
way that the perforated area covered the film, care being taken not 
to press the mica down and thus force the agar up through the 
holes. Next, the same amount of agar of the required composition, 
from another vial, was dropped and spread on the surface of the 
mica plate up to the boundaries of the perforated area. Transverse 
grooves in the mica, cut for this purpose, in most cases confined 
the agar to the perforated area. In case no spores were needed for 
this upper film, several drops of distilled water had previously been 
added to the agar, which were equal in volume to that of the spore 
mixture added to the agar for the lower film. After the upper film 
was set, the whole preparation, consisting of the mica plate between 
the two films of agar, was carefully pushed off the slide with the 
help of a scalpel. It was on account of this last operation that it 
was found impracticable to use gelatin, since this adhered so closely 
to the slide that it was impossible to remove it without breaking. 
The preparation was now rested in a horizontal position,? by 
means of the extensions at each end, on cork supports in a Petri 
* In the earlier experiments, some of the preparations were suspended vertically 
by means of a thread passed through one of the holes in the mica plate. This vertical 
Position was found to have no influence on the result. 
