98 Histology and Embryology. By C. S. Minot. 
the course of their transformations into the components of the 
tissues of the adult, and to investigate in detail the final results of 
these metamorphoses. It is much to be desired that America should 
assist more in this work, and it is with the hope of stimulating 
some persons to do so that this article is written. 
In the tissues of the adult we find the cells arranged in a defi- 
nite manner, and we have consequently to examine the shape and 
character of the single cells, and then their relation to one another. 
Simply placing a small piece of an organ underneath the micro- 
scope is not sufficient to enable us to do this, but we are obliged in 
every case to subject the preparation to a special treatment. The 
first thing to be done is to make the object transparent enough to 
let the light pass through it to the objective, which is usually done 
by mounting it in glycerine or in Canada balsam, both of which 
substances have a hi»h index of refraction, and therefore when they 
penetrate the interstices of a tissue do away with the refraction 
inside of it, so to speak ; for in every tissue the different parts 
refract the light so variously that a ray passing through frequently 
changes its path, thus confusing the final image which reaches the 
observer’s eye. A layer of powdered glass lets the light pass 
through, but nothing distinct can be seen ; if, however, the whole 
is immersed in Canada balsam, it immediately becomes beautifully 
transparent, because the balsam fills up the spaces between the bits 
of glass, and since balsam and glass refract light to about the same 
degree, the mass becomes optically nearly uniform, and a ray of 
light can pass through it without being deviated from its course or 
destroying the image. The action on the tissues is identical — and 
this should be carefully remembered, because balsam renders objects 
more transparent than does glycerine, so that in some cases one 
liquid is better than the other. It is a sign of inexperience to 
assert that balsam is better than glycerine, or vice versa , for they 
are both useful, but for different purposes. 
In order to observe the cells well it is necessary not to have too 
many superposed layers in the field of view, but to make the object 
as thin as possible. This is usually accomplished by making 
sections. So important and so useful are such very thin slices that 
probably nine-tenths of every histological collection consist of them. 
The first thing, therefore, is to acquire skill in making sections, 
and the perfection reached will mainly decide how far the progress 
of the student shall continue. The importance and benefits of 
making sections have led to the invention of a great many mecha- 
nical contrivances for cutting them. One form of cutter or micro- 
tome well adapted to its object was described in the April number 
of the ‘American Naturalist’ of this year. Numerous other forms 
have been suggested, but those with which I am acquainted all 
have some defects. Free-hand cutting still remains absolutely 
