6 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
is a deep brown. If pyroligneous acid is used subsequently to the 
platinic-chloride mixture, all parts of the animal become so intensely 
black as to be useless, and the tissue is rendered extremely brittle as 
well. ‘The material may, however, be very readily decolorized to any 
extent by the use of peroxide of hydrogen. Very good preparations 
may be obtained in this way, either from tissue which is too dark from 
the action of the picric-osmic mixture alone or from that which has in 
addition been treated with pyroligneous acid; the method may be 
applied to the entire animal, or to sections upon the slide. Decolori- 
zation may be controlled by watching the tissue during the process. 
I have used the pure commercial peroxide without visible injury to 
the tissue, but the rapidity of decolorization depends upon the strength 
of the solution employed. 
The stains I have used have been chiefly the iron-haematoxylin of 
Heidenhain, Mallory’s (:00) connective-tissue stain, and the platinic- 
osmic-acetic mixture of vom Rath, which was also the fixing reagent. 
The three stains are about equally valuable; each supplements the 
others. ‘The haematoxylin is best for nuclear structures; vom Rath’s 
fluid is excellent for nerve-tracts and medullated fibres. ‘The con- 
nective-tissue stain of Mallory is unexcelled for general contrast 
and for extreme delicacy in many structures, such as the nerve- 
endings in the cells of the eye. It is very serviceable in any tissue 
unless the nuclei are to be particularly studied. It differentiates 
nerve tracts in the brain as well as, or better than, vom Rath, since 
the color contrast is greater, but for following the course of single 
fibres it is in general not as satisfactory as the latter. Mallory’s stain 
has the disadvantage, possibly, that it is rather capricious; at any 
rate preparations in which it has been used vary greatly in value and 
unaccountably so. But when successful, the stain gives pictures 
which are most excellent in the matters of delicacy and sharpness. 
In addition to this there is the great advantage of simplicity and 
rapidity of procedure in its use. 
The methylen-blue method has, in my hands, been uniformly 
unsuccessful in the case of the marine copepods, though it has been 
tried many times. It is, of course, impossible to inject the stain, and 
if the animals are immersed in a solution the result has been negative. 
The marine forms do not respond to such treatment in the way that 
the fresh-water forms do (Esterly :06). 
No attempt has been made to impregnate nervous structures accord- 
ing to the method of Golgi, and I have not obtained good results from 
the Bielschowsky method. 
