1894], Microscopy. . 825- 
MICROSCOPY? 
Notes on Gold Impregnation Technique.—The following 
method of using formic acid and gold chloride is a modification, or 
adaptation of a method used by Miss Julia B. Platt and kindly sug- 
gested by her to me. She refers it to Professor Mark of Harvard 
University. I have used it in tracing the nervous system of Nephelis 
lateralis and have found it reliable. In leech tissues, it differentiates 
all nerve tissue, though the histology of other tissues is poor. After 
more than a year's use of this method without a complete failure 
among my preparations, I feel that Lee's characterization of the other 
methods of gold staining does not apply to this method. 
It has been used successfully on larval vertebrate material as well as 
on leech tissue, by varying the strength of the formic acid, or the time 
of its applieation. "The other factors are to a great extent indifferent 
as to strength used or time employed. If maceration occurs, lessen 
the action of the formie by weakening or by shortening the time. If 
the impregnation is slight, increase the action. The thickness of the 
piece stained should not exceed 5 mm., and the tissue must be living. 
The following i is the process euiployed with Nephelis : 
The leech is put into twenty or thirty times its bulk of 10% formie 
acid and left from 3 to 5 minutes. It dies well extended. Transfer 
without washing to 1% Gold chloride (of commerce) for 25 minutes; - 
then without washing into 1% formic acid for 24 hours, or until reduc- 
tion is complete. This is indicated by a rich purple color over the 
whole specimen. Wash slightly in tap water; run up through the 
alcohols to chloroform; to chloroform saturated with hard paraffine. 
My sections are usually cut 16% thick. * When the impregnation 
appears to be very light—almost a failure, stain the sections on the 
slide with erythrosin or some other deep red anilin stain for contrast. . 
These sections will often show the most exquisite details. | 
Transparent larve 5 to 10 mm. long require a milder treatment, 
— such as the following: 5% formic acid 2 or 3 minutes, 1% or }% gold 
chloride 10 minutes, weak formic 1 to 4 hours. If the specimens are 
= watched from time to time under the dissecting lens, it will be seen : 
that the central nervous system stains first and then the peripheral. — 
The reduetion of the gold chloride: may POUE of course, at any 
point by transferring to alcohol. 
‘Edited t by C. O. Whitman, University of Chicago. 
