_EXAMINATION OF MARINE OBJECTS--DISSECTION_
An enthusiastic observer of nature will learn much concerning the
structure of natural objects with the unaided eye, but there are times
when he will desire some kind of magnifier to reveal more perfectly the
structure of minute parts, or to enable him to observe the small
creatures that are invisible to the naked eye. Further, one may learn
many interesting and instructive facts relating to animal and plant life
by cutting sections for close examination, or by making such simple
dissections as will enable one to observe the more salient features of
internal structure; we therefore propose in the present chapter to make
a few remarks and suggestions regarding work of this kind.
A pocket magnifier is of great value to the young naturalist, both for
the inspection of natural objects while engaged in out-door work, and
for the subsequent examination of the specimens collected for study. It
is often necessary to enable one to identify and classify small animals
and plants, and will be in constant demand for the purpose of studying
the less conspicuous external features. Such an instrument should be
regarded as an essential companion of the naturalist, and should
accompany him on every ramble.
There are several different forms of pocket lenses, but for general work
there is, perhaps, nothing more convenient and serviceable than the
‘triplet’ magnifier. It is a combination of three lenses, enclosed in a
pocket case, and so arranged that they may be used separately or in
combination, thus supplying a variety of powers. The three lenses of the
triplet are themselves of different magnifying powers, and these powers
may be increased by combining two or all of them.
For work at home a ‘dissecting microscope’ is very useful. This consists
of a magnifying lens, mounted on a support over a surface on which small
objects may be examined and dissected, the height of the lens being, of
course, adjusted according to its focal distance. Lenses ready mounted
on adjustable stands may be purchased for this purpose, but no one ought
to experience much difficulty in designing and constructing some simple
stand that will give every satisfaction.
The arrangement just described is, of course, suitable for the
dissection of only small objects, and these are placed on a material
adapted to the nature of the work to be done. Thus it is sometimes
convenient to place the object to be examined on a small sheet of cork,
in order that it may be secured by means of pins while the dissection
proceeds, while at other times it is essential that it be laid on a hard
and unyielding surface, such as that of a slip of glass. But whatever be
the nature of the substance on which the dissection is made, its colour
may be regulated according to that of the object. If, for example, we
are dissecting a small white flower on a piece of cork, we should
naturally blacken the cork, or cover it with a piece of dead black
paper; or, if we are to dissect a small, light-coloured object on a
glass surface, we lay the glass on black paper.
[Illustration: FIG. 45--THE TRIPLET MAGNIFIER]
The advantage of dissecting objects under water does not seem to be
generally appreciated by beginners, who often allow their specimens to
become dry and shrivelled, almost beyond recognition, during the
progress of their examination. This mode of dissection is certainly not
necessary with all objects, but may be generally recommended for soft
and succulent vegetable structures, as well as for almost all animal
dissections.
This being the case, arrangements should certainly be made to provide a
miniature dissecting trough as an accessory to the dissecting
microscope, and the following instructions will enable the reader to
construct a highly satisfactory and inexpensive one:--
Procure the flat lid of a cylindrical tin box, or the lid of a glass or
porcelain pomade pot, such lid to be about two inches in diameter and
about half an inch in depth. Cement the flat side of this lid to a small
slab of hard wood, or to a square piece of sheet lead, by means of
acetic glue--ordinary glue or gelatine dissolved in glacial acetic
acid--to give it the necessary steadiness during the dissection. When
the cement is quite hard, pour into the lid some melted paraffin
(paraffin wax) which has been blackened by the admixture of a small
quantity of lamp-black in the form of a fine powder. The paraffin should
be melted by putting it into a beaker or wide-mouthed bottle, and
standing it in hot water, and the lamp-black should be added, with
stirring, as soon as it is entirely liquefied. The quantity of the
mixture used must be sufficient to half fill the lid, thus leaving a
space to contain water to the depth of about a quarter of an inch. The
blackened wax provides a good background on which to work, and provides
a hold for pins when these are necessary in order to fix the object
under examination.
[Illustration: FIG. 46.--A SMALL DISSECTING TROUGH]
The complete trough is represented in fig. 46; and will be found to
answer its purpose admirably, except that it occasionally displays one
fault, but one that is easily remedied. The wax contracts on cooling,
and may, therefore, detach itself from the trough; and, being lighter
than water, will float instead of remaining submerged. This may be
prevented by securing the disc of wax in its place by means of a ring of
brass wire, or by weighting the wax with two or three small pieces of
lead pushed down into it while it is yet soft.
With such a dissecting microscope and trough as we have described one
may do a great deal of exceedingly useful work, both hands being quite
free to manipulate the object under examination.
The dissection may be conducted with the aid of a small scalpel or other
very sharp knife, the parts being arranged or adjusted by means of a
needle, mounted in a handle, and held in the left hand. Sometimes,
however, the object to be dissected is so minute that even a small
scalpel is too large for the purpose, and in such cases nothing is
better than little dissecting instruments made by mounting large sewing
needles in suitable handles, and then grinding down the points of the
needles on two opposite sides, on a hone, so as to produce little
pointed, two-edged blades. Bent needles are often useful, too, and these
may be prepared by heating the points to redness in a gas-flame, bending
them as desired while hot, and then hardening them by suddenly thrusting
them, at a red heat, into cold water.
The compound microscope will often prove useful for the examination of
very minute objects, as well as for the study of the structure of the
principal tissues of the larger species; but since detailed instructions
for the management of the microscope, and for the preparation of objects
for microscopic examination would occupy much more space than we can
spare, we shall content ourselves with nothing more than a few general
hints on this portion of the young naturalist’s work, dealing more
particularly with those points which commonly present difficulties to
the amateur.
If it is desired to examine some minute living object, such as a
protozoon, place the object in a drop of the water in which it lived
just in the middle of a clean glass slip, and cover it with a
cover-glass. The quantity of water should be just sufficient to fill the
space between the two glasses. If less than this has been used, a little
more applied to the edge of the cover by means of a glass rod will
immediately run in between the glasses; while if an excessive amount was
employed, the surplus may be removed by the application of a strip of
blotting paper. Place the glass slip on the stage of the microscope, and
reflect light through it from the mirror below.
Examine it first with a low power; and, after having observed as much as
possible of the creature’s movements and structure with this aid, repeat
with a higher power. This rule applies not only to such small objects as
we have now under consideration, but to all objects, and parts of them,
in which minute details are to be observed.
Beginners with the microscope often find prolonged examination very
tiring to the eyes, but this, we believe, would seldom be the case if
right methods were followed. Both eyes should always be open, and the
microscopist should train himself to use both eyes equally for the
actual observation.
The higher the magnifying power used, the nearer must the objective (the
lower combination of lenses) be brought to the object itself, and it is
no uncommon thing for the amateur, in his attempts to focus his object,
to lower the body of the microscope beyond its proper position, causing
the objective to crush the object, break the thin cover-glass, and
become wetted with the liquid, if any, in which the object was being
examined. All this may be avoided by lowering the body of the microscope
until it nearly touches the cover-glass before attempting to view the
object through it, and then, with the eye above the object-glass, to
gradually raise the body until the object is in focus.
[Illustration: FIG. 47.--CELL FOR SMALL LIVING OBJECTS]
The top of the cover-glass should always be perfectly dry; and if by any
chance the objective becomes wet it should be wiped perfectly dry with a
piece of old silk or with chamois leather. Also, if permanent mounting
is attempted, and the preservative liquid is allowed to come in contact
with the objective, such liquid must, of course, be washed off with some
suitable solvent before any attempt is made to wipe the lens dry.
If the object under examination is of such dimensions that the
cover-glass has a tendency to rock on it, or if it is a living object of
such a size that it is unable to move freely in the exceedingly thin
film of water between the cover and the slip, it should be placed in a
cell. The cell may be made by cementing a ring of glass or vulcanite to
the middle of a slip, or it may be a little circular cavity prepared in
the slip itself. In either case the cell must be quite full of water
before the cover-glass is applied, so that no air-bubbles are included.
Hitherto we have spoken only of mounting small objects in water, and
this is advisable when the object is moist, whether it be animal or
vegetable, alive or dead. But dry objects may be examined in the dry
state, in which case they need not be covered. If they are composed of
transparent material they are to be dealt with in the manner recommended
before, as far as the management of the light is considered; that is, a
moderately strong light is sent through them by the reflector below the
stage; but opaque objects are best examined on a dead black ground, the
light being directed on to them by means of a condensing lens placed
between them and the source of light.
A collector who has done only a few days’ work on the sea shore will
probably find himself the possessor of a host of interesting objects
that will afford much pleasure and instruction when placed under the
microscope--objects, many of which have been somewhat hastily deposited
in a bottle of spirit or other preservative for study in his future
leisure moments. These objects, if small, may be examined as above
described, simply placing them under a cover-glass, or in a cell, with a
clear drop of the same liquid in which they have been kept.
The general characters of the larger objects may also be observed by
means of some kind of hand lens, but even these are generally best
examined under water or other suitable liquid.
A great deal may be learnt of natural objects by preparing very thin
sections for microscopic examination; and although special works should
be consulted if one desires to become proficient in the different
methods of cutting and preparing such sections, yet a great amount of
good work may be done with the aid of a sharp razor, manipulated with
nothing more than ordinary skill.
Some objects, especially certain of those of the vegetable world, are of
such a nature that suitable sections may be cut, either from the fresh
or preserved specimen, without any preliminary preparation. All that is
required is to hold the object firmly between the finger and thumb of
the left hand, previously securing it in some kind of holder if
necessary, and pare off the thinnest possible slices with a horizontal
movement of the razor, both razor and object being kept very wet during
the process. As the sections are cut they may be allowed to drop into a
shallow vessel of water; and, the thinnest then selected for examination
in water as previously described.
Other objects are so soft that the cutting of sections becomes
impossible without previously hardening them. Methylated spirit is a
good hardening reagent, and many of the soft structures that have been
preserved in this fluid, especially if it has been used undiluted, will
be found sufficiently hard for cutting thin sections. Among the other
hardening reagents used by microscopists may be mentioned a solution of
chromic acid--one part by weight of the solid acid dissolved in from one
hundred to two hundred parts of water, and a solution of bichromate of
potash--one part of the bichromate to about forty parts of water. In
either case the hardening of the object takes place slowly, and it
should be examined from day to day until the necessary consistence has
been obtained.
The structures of many soft animals can never be satisfactorily hardened
for section-cutting by either of the above reagents, and thus it becomes
necessary either to freeze or to imbed them. In the former case the
object is first soaked in gum water--a thin solution of gum arabic--and
then frozen by an ether spray or by a mixture of ice and salt. The
sections should be cut with a razor just as the object is beginning to
thaw, and they may then be examined under a cover-glass, in a drop of
the gum water.
The other method is conducted as follows:--The soft object is first
soaked in absolute alcohol to extract all the water it contains, and is
then transferred to paraffin that has been heated just to its
melting-point by standing it in warm water. After the object is
thoroughly permeated with the paraffin, the whole is cooled quickly by
immersion in cold water. Sections are now cut, the paraffin being sliced
away with the substance it contains. These sections are placed in warm
turpentine, where they are allowed to remain until the whole of the wax
has dissolved, and they may then be mounted in a drop of turpentine, and
covered with a cover-glass.
We have given brief instructions for temporary mounting only, but most
amateur microscopists would undoubtedly prefer mounting their objects
permanently, so that they may be set aside for study at any future
period. Hence we append a few directions to this end, advising the
reader, however, to consult a work dealing especially with this subject
if he desires to become proficient in the preparation of microscopic
slides.
Moist objects, including those which have been preserved in dilute
spirit, may be soaked in water, then transferred direct to the glass
slip, and covered with a drop of glycerine. Any excess of the glycerine
should then be absorbed from around the cover-glass by means of a strip
of blotting-paper, and the edge of the cover cemented by gold size
applied with a small camel-hair brush.
Glycerine jelly is also a valuable mountant for permanent work. When
this is used the object should first be soaked in glycerine, and then in
the melted jelly. It is then transferred to a drop of melted jelly which
has been placed on a _warm_ slide, and covered as before. The jelly soon
solidifies, so that a ring of cement is not absolutely necessary, though
it is advisable, as a rule, to cement the cover-glass all round with
gold size or black varnish.
Sections cut while frozen are best mounted in glycerine, to which they
may be transferred direct.
Canada balsam is one of the best media for permanent mounting; and, as
it becomes very hard after a time, it serves the purposes of both
preservative and cement. When this is used the object must be entirely
freed from water by soaking it in absolute alcohol. It is then put into
turpentine for a minute or two, transferred to a warm slide, and covered
with a drop of the prepared balsam. Sections that have been imbedded in
paraffin may be mounted in this way, the turpentine acting as a solvent
for the paraffin in which it was cut.
Although the compound microscope is absolutely necessary for the study
of the minutest forms of life and of the minute structure of the various
tissues of larger beings, yet the young naturalist will find that a vast
amount of good work may be done without its aid. Thus the general
structure of the larger species may be made out by means of simple
dissections requiring no extraordinary skill on the part of the worker,
and with appliances that may be obtained at a low cost. Certain of the
marine animals, however, require special treatment that can hardly be
described in a short chapter devoted to general instructions only, but
hints with regards to these will be given in future chapters in which
the animals referred to are described.
The appliances referred to above include nothing more than a simple form
of dissecting trough, a few dissecting instruments, and one or two minor
accessories that may always be found at hand as required.
The dissection of animals is always best performed under water, for by
this method the object examined may not only be kept clean as the work
proceeds, but the parts, having a tendency to float, readily separate
from one another and therefore become more distinctly visible when
submerged.
[Illustration: FIG. 48.--SHEET OF CORK ON THIN SHEET LEAD]
A very convenient form of trough may be made by taking any kind of
rectangular, flat-bottomed dish, one made of zinc being, perhaps, the
best of all, and covering the bottom with a slab of good cork carpet
which has been weighted with sufficient lead to prevent it from
floating. Or, instead of cork carpet, a sheet of cork may be used. In
either case, a piece of thin sheet lead, a little larger than the slab,
should be cut, the corners of which are then snipped off as shown in
fig. 48, and the edges finally turned over as represented in the next
illustration. The size of the trough must be regulated according to the
nature of the work to be done, but one measuring ten inches long, seven
wide, and two inches deep will answer most purposes.
[Illustration: FIG. 49.--WEIGHTED CORK FOR DISSECTING TROUGH]
The object to be dissected is placed in the trough, secured in position
by means of a few ordinary pins, and then completely covered with
water.
We need hardly impress upon the reader the great importance of
thoroughly examining all external characters--all those structures that
are visible without actual dissection--before attempting to remove
anything; and we have already insisted on the importance of carefully
examining all creatures while alive before anything else is done. The
value of this latter stipulation can hardly be overestimated, for in
many instances it is almost impossible to detect the use of an organ
unless it has been observed in action; and the enthusiastic student will
go even further than this, for he will make it an invariable rule to
sketch everything he sees, and to make full notes on all his
observations.
When pins are used to fix the object under examination--and it is
generally essential that the object be fixed--their heads should be
turned outwards; for then the object will not slip from its position,
nor will the pins tend to get in the way of the work.
Some objects are of such a nature that they are not easily secured by
means of pins, and yet require to be fixed in some way or other. Thus,
one may desire to examine the structure and appendages of a prawn or
small crab, or to investigate the nature of a chiton. In such instances
as these it is a good plan to make a cake of paraffin wax of suitable
size by pouring the melted substance into a mould, then secure the
object in proper position in the wax while still fluid, and pin the
latter to the cork of the dissecting trough.
It is often necessary to trace the courses of internal passages that
open on the surface of the body, or of tubes that are revealed during
the progress of dissection, and this may be done by means of a little
instrument called a seeker. It is simply a blunted needle, bent into a
large angle, and mounted in a handle; or, it may consist of nothing but
a moderately long and stiff bristle, rendered blunt at one end by
tipping it with melted sealing wax. This is not always sufficient,
however, for it frequently happens that certain tubes and passages in
animal forms are disposed in such a complicated manner that it is
impossible to send even the most flexible seeker through them. For
instance, suppose one desires to trace the course of the digestive tube
of some large bivalve mollusc with its many reflections, the seeker is
useless except that it will penetrate to the first sharp bend. The
arrangement of such a tube must be traced by dissecting along its
course, but this may be aided considerably by first filling it with some
coloured substance to enable its direction to be more easily followed.
In fact, the injection of some brightly coloured fluid, forced through
the tube by means of a fine-nozzled glass syringe will often enable the
course of such a tube to be seen without any dissection at all, the
colour of the fluid used being detected through the semi-transparent
tissues surrounding it. A mixture of Berlin blue and water, or a mixture
of plaster of Paris and water coloured with carmine is well adapted to
this purpose; and if the latter is employed it may be allowed to set,
and thus produce a permanent cast from the tube that is being dissected.
Perhaps it should be mentioned that if either of the injection mixtures
be used for this purpose it must be previously strained through muslin,
and that, in the case of the plaster, the mixing and straining should
occupy as little time as possible, or it may begin to set before the
injection has been completed.
A very considerable insight into the structure of animals may be
frequently obtained by cutting sections through the body with all its
organs _in situ_, but, generally speaking, they are too soft to allow of
this without danger of the displacement of those very parts, the
relations of which we desire to determine. To avoid this the body should
be previously hardened by a somewhat prolonged soaking in methylated
spirit, or in a solution of chromic acid prepared as before directed.
Then, with the aid of a good razor, very interesting sections may be
prepared with the greatest of ease, and the true relations of the
various organs throughout the body may be exactly determined by cutting
a succession of slices, not necessarily very thin, from end to end, or,
transversely, from side to side.
Even those crustaceans that are protected by a hard, calcareous
exo-skeleton, and the molluscs that cannot be removed from their stony
shells without injury to their soft structures, may be studied in the
manner just described, and this may be done by first soaking them in
dilute hydrochloric acid, renewed as often as may be necessary, until
all the mineral matter has been dissolved completely, and then hardening
the softer tissues in one of the reagents mentioned above. Hydrochloric
acid may also be used to dissolve the calcareous shells of foraminifers,
the vegetable corallines, and other small forms of life, previous to
microscopic examination of the soft parts.