512 The American Naturalist. [May, 
diameter ; then covered with about 5 drops of the prepared white of 
egg, and torn open with two dissecting needles; or, after puncturing 
with one needle, cut with a small curved pair of scissors. The out- 
flowing parts of the egg are then cautiously reduced in size by a few 
movements of the needles. The circular plate is then placed in a 
round glass dish (4-5 cm. in diameter) with a rim 1 cm. high, contain- 
ing 10-15 drops of water—just enough to fill the space between the 
edge of the object-plate and the rim of the dish, but not enough to 
come in contact with the white. The purpose of the dish and the 
water is to check the evaporation of the medium in which the egg lies, 
and thus to guard as far as possible against concentration of the 
medium and currents in the same. 
The dish offers the advantage that one, on interrupting the observa- 
tion, can cover it and so protect the preparation against evaporation. 
Thus protected, cells may be kept alive in a suitable medium for one 
or two days. ! 
The preparation should be immediately examined while in the dish 
with a low objective (e. g. Zeiss A). It is important that the table of the 
microscope and the object-plate bearing the preparation should be per- 
fectly level. 
The examination of isolated cells in an uncovered medium has the 
advantage that one can easily change the position of the cells with 
needles or other means. But it is indispensable for checking results to 
examine also preparations covered with a cover-slip. The cover-slip 
for this purpose must be large enough, so that at least two of the wax 
feet (} mm. high) supporting it may fall on dry points of the object- 
plate, where they will firmly adhere and not allow the cover to slide. 
A still more complete protection against currents in the medium 
may be had by having a moist chamber ground into the object-plate 
and covered with a large cover-slip. The bottom of the chamber must 
be flat and horizontal. 
After separating the cells of an egg, one searches at first with a 
low power (Zeiss A) to find two cells separated from each other by a 
distance equal to, or less than, the radius of the smaller cell, and from 
all other cells by a distance not less than about double the diameter 
of the cells. No yolk substance should lie between or beneath the 
cells, Such a pair of cells having been found, higher objectives (Zeiss 
C or D) may be turned upon them and the cell so adjusted under the 
ocular micrometer that the line connecting their centres will fall length- 
wise of the micrometer. In this position, one can easily see whether 
the cells move towards, or away from, each other. 
