and one portion would closely resemble another. So staining is resorted to, and, where practicable, it is preferable to stain before cutting the sections. Of stains for microscopic purposes there are many, and the value of each depends upon the fact that the different elements of cells and tissues will absorb it in varying quantities. Most used of all is some preparation of carmine which stains certain portions red, leaving others uncolored. Of these carmine solutions the student has no less than twenty to choose from. Next in order comes hæmatoxylon, or extract of logwood, which, when combined with alum, stains certain portions blue or purple. Osmic acid also stains a brown or a black, according to the structure and the length of exposure. Nitrate of silver is also frequently used for certain purposes, while of the anilines only eosin and Bismarck-brown have any great value.
In order to section the egg we must employ some means to hold it firmly, and for this purpose various substances are employed, paraffin or celloidin being the most common. The requisites of an imbedding substance are that it be possible to make it thoroughly impregnate every part of the egg, and also that it be of such a consistency as to be readily cut into the thinnest sections. The egg is imbedded in paraffin by completely replacing all the water in it by alcohol, this in turn by some solvent of paraffin, as turpentine or oil of clove, and then by keeping it for a time in melted paraffin. Then egg and paraffin may be cut as if only paraffin were present. In the case of celloidin (a solid form of gun-cotton) the intermediate reagents are alcohol and a mixture of alcohol and ether. The process involves some time to accomplish thoroughly, and here, as elsewhere, neglect of details is sure to result in failure.
In order to cut the sections, special instruments (microtomes they are called) have been devised, and are now made of a high degree of accuracy and excellence. So delicately are they made, that it is possible to cut an egg into a series of sections so thin that it would require twenty-five or even more of them to equal in thickness the paper on which this magazine is printed. In the early days of section-cutting no such facilities were available, and the apparatus described in the hand-books of microscopy even five years ago were utterly inadequate to produce good results. Of modern microtomes there are now four distinct types in use, two having the knife stationary, the other two having it moved through a fixed and definite plane. It is not necessary to describe these here; those who wish may find accounts and figures of them in recent works, like Whitman's "Methods of Research in Microscopical Anatomy and Embryology."
Very recently a new "kink" has been introduced into section-cutting which has relieved the student from a great deal of drudgery. It has been found that by trimming the block of paraffin square, and by having the edge of the section-knife at right angles to the line of stroke, the successive sections would adhere together by their edges,
