ZOOLOGICAL STATION IN NAPLES. 105 



6. Chromic Acid.— Chromic solutions have, in common with 

 osmic acid, the pecuHarity of hardening by virtue of the chemical 

 combinations which they form with cell-substances, and all the 

 consequent disadvantages with respect to staining. The use of 

 chromic acid in the Zoological Station of Naples may be said to 

 have been largely superceded hyJ)icro-suIphuric add, corrosive subli- 

 mate, and MerkeVs fluid, for it is now seldom used except in com- 

 bination with other fluids. '^ It is sometimes mixed with Kleinen- 

 berg's fluid, for example, when a higher degree of hardening is 

 required than can be obtained by the use of the latter fluid alone. 

 It is a common error to use too strong solutions of chromic acid, 

 and to allow them to act too long. Good results are in some cases 

 obtained when the objects are treated with a weak solution (^ to 

 y2 per cent.) and removed soon after they are completely dead. 



7- Merkel's Fluid.— 



Platinum chloride dissolved in water ... 1:400. 



Chromic acid „ „ . . . 1 400. 



Professor Merke],J who employed a mixture of these two 

 solutions in equal parts for the retina, states that he allowed from 

 three to four days for the action of the fluid. Dr. Eisig has used 

 this fluid with great success in preparing the delicate lateral organs 

 of the Capitellidce for sections, and recommends it strongly for 

 other annelids. Dr. Eisig allows objects to remain 3 — 5 hours in 

 the fluid, then transfers to 70 per cent, alcohol. With small 

 leeches I have found one hour quite sufficient, and transfer to 50 

 per cent, alcohol. 



8. — Corrosive Sublimate. — Prompted by a statement found in 

 an old paper by Blanchard,f Dr. Lang began experimenting with 

 corrosive subHmate as a medium for killing marine Planarians, 

 and his marked success led him and others to employ the same 



* Dr. Pfitzner (" Morph. Jahrb.," B. xvii., p. 731, 1882) has recently made use 

 of chromic acid followed by (i) osmic acid, or by (2) chloride of gold, formic acid, 

 and safranin (or hcematoxylin) for the demonstration of nerve-terminations. 



Flemming (see his method on a following page) believes that chromic acid is 

 one of the most reliable fixing reagents for the karyakinetic figures, and has proved 

 that objects hardened in this acid can be beautifully and durably stained. 



t " Ueber die Macula lutea des Menschen," etc., Leipzig, 1870, p. 19. 



+ " Recherches sur I'Organisation des Vers," by Emile Blanchard. Ann. dea 

 Sci. Nat, Zool. Ser. 3, t. viii., 1847, p. 247. 



