INTRODUCTORY

gradual by placing the oil of cloves (or other clearing medium)
under the alcohol.     This is done as follows : A     sufficient
quantity of alcohol is placed in a tube (a watch-glass will do,
but tubes are generally better), and then with a pipette a
sufficient quantity of clove oil is introduced at the bottom of
the alcohol. The two fluids mingle but slowly. The objects
to be cleared being now quietly put into the supernatant
alcohol, float at the surface of separation of the two fluids,
the exchange of fluids takes place gradually, and the objects
slowly sink down into the lower layer. When they have
sunk to the bottom, the alcohol may be drawn off with a
pipette, and the objects examined in the clearing medium, or
mounted in balsam. (It may be noted here that this method
of making the passage from one fluid to another applies to
all cases in which objects have to be transferred from a
lighter to a denser fluid, for instance, from alcohol, or from
water, to glycerine.)
3. The Naples Methods.'-It may be useful here to give an
account of the methods actually employed in the Zoological
Station of Naples, both in order to bear out the statements I
have made regarding the general method of research, and in
order to call attention to one or two points which it is neces-
sary to take into consideration in the study of marine
organisms.
Fixation is always carefully attended to. Pure alcohol is
not very suitable as a fixing agent, as it precipitates most of
the salts of the sea-water adhering to the surface of marine
animals, thus giving rise to a crust that prevents the pene-
tration of the alcohol to the interior (thus allowing macera-
tion of the internal structures to be set up), and also hinders
1 Mayer, in 'Mitth. Zool. Stat. Neapel,' ii (1881), p. 1, et seq. See also
the abstract in ' Journ. Roy. Mic. Soc.' (N.S.), ii (1882), pp. 866-881, and
that in ' Amer. Natural,' xvi (1882), pp. 697-706, in which two last some
improvements are mentioned which have been worked out since the pub-
lication of Mayer's paper.

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