1884] Microscopy and Histology. 1069 
grades of alcohol (“ weak, strong, and absolute”) are used in hard- 
ening, in each of which the object should be left at least forty- 
eight hours before staining. Borax-carmine (probably the alco- 
holic solution) is recommended as a staining agent; a dilute solu- 
tion is used in preference to the full strength, and allowed to act 
from three to four days. 
For preservation as museum specimens, they arekilled with strong 
nitric acid (about fifty per cent), in which they die fully extended. 
Freparation of the Ova—The egg-capsules of fresh-water Pla- 
narians are generally attached to water-plants by means of a white 
secretion. The ova are very small and few in number, and are 
Scattered among an immense number of yolk-cells. The ova are 
completely naked, and a little smaller than the yolk-cells, and are 
not easily isolated. When cleavage begins, a large number of 
yolk-cells surround the ovum, and form with it a mass large 
enough to be seen with the naked eye. Mr. Iijima adopts the 
following mode of isolation and preparation: By the aid of two, 
sharp dissecting needles, the egg-capsule is opened on an object- 
slide in dilute acetic acid (two per cent). The contents flow out, 
and the empty capsule is then removed.. The slide is next shaken 
in order to isolate the ova so far as possible from the yolk-cells. 
This process detaches many of the yolk-cells, but not all; eac 
ovum will still have yolk-cells adhering to it, and will now appear 
to the naked eye as a minute white mass. A cover-glass supported 
by wax feet, or by slips. of paper is now placed over them. After 
about thirty minutes the acetic acid is carefully removed by the 
aid of small pieces of blotting paper placed at one side of the 
cover, and replaced by alcohol (seventy per cent). The with- 
drawal of the acetic acid must be as slow as possible, otherwise 
the ova will be lost. After an hour the alcohol is replaced by a. 
Stronger grade (ninety per cent), in which the ova shonld remain 
two hours. . Finally, the alcohol is replaced by a mixture of gly- 
cerine and water in equal parts, and this in turn by pure glycer- 
ine. The preparation is now complete, and the cover-glass may 
be fixed in the usual way by means of lac. 
In order to obtain sections of embryos which are too small to 
be treated individually, the contents of the capsule may be hard- 
ened in toto in chromic acid (one per cent), which renders them 
less brittle than corrosive sublimate. 
laid capsules, which showed the germinal vesicle still unchanged ; 
ers were found to have two nuclei, supposed to be derivatives , 
certain when fecundation takes place, whether in the cocoon or 
before its formation. , 
