ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
587 
or a flask of similar capacity, with a long narrow neck, which is marked 
off in divisions of 0 * 1 ccm. The powdered material and the hot solu- 
tion are placed together in one of these vessels and then water added 
drop by drop, the vessel being carefully shaken after each drop. 
In this way the fluid is very slowly diluted, and the specific gravity 
lowered so gradually that it is possible to separate substances from one 
another the specific weight of which differs by 0*01 p.c., or less. As 
soon as the density corresponding to that of the mineral or of the 
organisms is reached, the particles sink slowly to the bottom of the 
neck of the funnel. When the supernatant fluid is perfectly clear, the 
tap is turned, and the sediment allowed to flow very slowly into watch- 
glasses. This takes from 12-24 hours, and as the fluid has cooled, more 
water must be added, and fresh sediments removed as they form. As 
diatoms consist of the same chemical substance, silicic acid, with but 
slight admixture of other chemical compounds, as soon as the density of 
silicic acid is reached, all siliceous bodies are deposited, and it might be 
supposed that all the diatoms will come down together. This, however, 
is only theoretically the case ; for, as a matter of fact, the larger diatoms 
are deposited sooner than the smaller sorts ; so that an expert manipu- 
lator has it in his power to collect the different kinds by themselves, aud 
unmixed with other varieties. The material collected is washed with 
water, and further purified by sedimentation. The Klein’s fluid which 
has been used is separated by the addition of some cadmium and by eva- 
poration, and may be used over and over again. 
Modification of Golgi’s Method.* — The energetic action of aldehyds 
on silver salts suggested to Dr. F. Kopsch a favourable modification of 
Golgi’s method. The fixative employed is a mixture of 40 ccm. of a 
3*5 per cent, solution of potassium bichromate and 10 ccm. of formalin. 
After 24 hours’ immersion in this fluid, the objects are removed to pure 
bichromate solution, and in 2-3 days transferred to 0*75 per cent, nitrate 
of silver solution for 3 to 6 days. Nervous tissue stains well without 
suffering from precipitates. 
Schaper’s Reconstruction Method.f — The main difference between 
Dr. A. Schaper’s method and that of Born is that the base line of the sec- 
tions is not at a distance from the section of the object, but is on the edge 
of the section itself. The embryo is first saturated with paraffin to prevent 
its drying and shrinking afterwards. It is then taken from the bath and 
fixed to a piece of Bristol board, after which it is sketched or photo- 
graphed. It is then transferred to the bath, and a line drawn on the 
sketch or photo just touching the head, thus including the figure in a 
right angle. A similar angle is drawn on a piece of cardboard that fits 
into the imbedding-box. The latter is filled with melted paraffin and 
the embryo oriented so as to correspond with the position of the figure 
in the sketch. After hardening, the mass is sectioned. In sectioning, 
the plane of the section must be perpendicular to the median plane of 
the embryo. The sections should be 20 //, thick. Sketches of the mag- 
nified sections are made on paper and transferred to wax sheets ; but 
before doing so a pencil-point is made on the dorsal side of the sketch 
* Anat. Anzeig., xi. (1896) pp. 727-9. 
f Zeitschr. f. wiss. Mikr., xiii. (1896) pp. 446-59 (10 figs.). See also Amer. 
Natural., xxxi. (1897) pp. 746-8 (2 figs.). 
