G. H. F. Nuttall, W. F. Cooper and L. E. Robinson 163 
picric acid, immediately before the final clearing. Absolute phenol has 
been of great use in clearing entire specimens, the procedure being to 
melt a small quantity in a bottle or a watchglass in the embedding 
oven; drop the specimen in, leaving it until sufficiently cleared; transfer 
to xylol and mount in Canada balsam. The advantages of this method 
are the saving of time, the clean results, and the fact that the specimen 
is completely cleared before the soft parts are disintegrated—a useful 
feature in the study of the relationship between the exoskeleton and the 
internal structures. 
Glycerine, glycerine jelly and especially monobromide of naphthalene 
have been used with advantage as mounting media in the study of the 
structure of the almost colourless, highly refractiye chitinous parts, such 
as the digits of the chelicerae and the pulvilli. Minute dissections were 
made, as a rule, in oil of cloves, a method which facilitates manipulation 
with mounted needles. In the preparation of material for section cutting, 
all the better known fixing reagents have been tried, but the majority 
rendered the chitin so brittle that success was impossible. The reagent 
which has given satisfactory results is picro-sulphuric acid (Kleinenberg’s 
formula). This is used in the undiluted condition and is heated to 
about 90° C. before immersing the material. The time required for 
complete penetration varies, but gorged females of H. punctata have 
been found to be thoroughly permeated with the reagent after an 
immersion of 1—2 minutes. 
The authors intend to consider the subject in greater detail in the 
section dealing with the internal anatomy and histology. 
For many purposes, photomicrography has lent assistance, but with 
few exceptions drawings alone have been used for illustration. 
In the work on the structure of the external parts, considerable 
difficulty was experienced in obtaining proper orientation and fixation 
of the more minute parts for the purpose of drawing: this led the authors 
to try various plastic materials in which the part to be investigated 
could be lightly embedded and then orientated under the microscope, by 
means of mounted needles. The preparation known as Plasticine has 
proved an admirable medium for this purpose, and its use can be 
recommended in all cases where it is desirable to examine opaque 
objects which, on account of their form, are difficult or impossible to fix 
in proper position without resort to some such means. 
