20 BRISTOL. [Vol. XV. 



sitely delicate in some of its effects that it deserves extended 

 use. It may be used with equal success on vertebrate or 

 invertebrate, adult or larval tissue. It must be adapted to the 

 tissue studied ; but this can easily be done after a few experi- 

 ments. The formic acid appears to be the variable factor, and 

 upon its strength and the time it acts depends the measure of 

 success. I give the procedure applicable to Nephelis. 



The leech is killed in a lo or 15 per cent solution of formic 

 acid, left from 5 to 10 minutes, and then put without washing 

 into a I per cent solution of gold chloride for 25 minutes. From 

 this it is transferred, without washing, into a large volume of 

 I per cent formic acid, and left for 12 or 18 hours, or until 

 reduction has taken place. It is next washed, passed through 

 the alcohols to chloroform, and then imbedded in paraffin. 

 The sections were cut 18 micra thick. The specimen will 

 appear a rich purple when the reduction has taken place under 

 the best conditions. The precautions are : to use small pieces 

 of material, not thicker than 5 mm., to avoid maceration by 

 reducing the strength of the formic acid and the time of action. 

 My solutions were all well sunned, but no especial precautions 

 were observed. 



In tracing out the innervation of the somites it was neces- 

 sary to examine long, continuous series of sections, and some- 

 times it was necessary to check results found in one somite by 

 comparison with the next somite. The following method was 

 used which would apply to other purposes. An ordinary library 

 reference card, about 8 cm. by 10 cm., is ruled so as to include 

 as many small rectangles in the same number of rows as the 

 slide to be examined contains sections. The unused margin 

 serves for making notes. An ordinary check mark denotes 

 that the section occupying the same place on the slide that the 

 rectangle does on the card has been examined but does not 

 contain the element under examination. Initials, symbols, 

 different colored pencils, etc., may be used to indicate various 

 details, and each card is numbered the same as the slide. After 

 a number of slides have been carefully plotted in this manner, 

 the cards may be arranged in series and studied as a map. It 

 furnishes an excellent reference-card system for any set of 



