HOW A NATURALIST IS TRAINED. 637 



and one portion would closely resemble another. So staining is re- 

 sorted to, and, where practicable, it is preferable to stain before cut- 

 ting 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 hamiatoxy- 

 lon, 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. Xitrate 

 of silver is also frequently used for certain purposes, while of the ani- 

 lines 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 ac- 

 curacy 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 neces- 

 sary 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-cut- 

 ting 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, 



