A WORD ON MICROTOMES. 133 



CHAPTER XIY. 



IMBEDDIN'G METHODS ! INTRODUCTION. 



263. A Word on Microtomes, It is no part of the purpose of 

 this work to discuss instruments, yet a word on this subject 

 may be very helpful to the student. The freezing microtome 

 so generally employed in England is less than any other form 

 adapted to the wants of the zoologist. Very thin sections can 

 be obtained by it more readily than with any other microtome, 

 but they are of little use when obtained. The relations of the 

 parts of the organs are deranged by the freezing and by the 

 thawing, and the aqueous nature of the process prevents it 

 from being readily applicable to the mounting of series of sec- 

 tions. The microtome of the zoologist, therefore (I am not 

 writing for pathologists or for dilettanti), must be an imbed- 

 ding microtome. The two most important points to be 

 attended to in the choice of such a microtome are the object- 

 holder and the knife motion. The object-holder should never, 

 as is usual in the English forms, be a well in which the 

 imbedded object is raised by a screw ; the principle of construc- 

 tion should always be that the object-holder be raised in its 

 entirety by the screw, not the object alone. The knife motion 

 should always be mechanical; cutting of sections with a free 

 knife held in the hand is a primitive process, by which only 

 coarse sections can be obtained, and that at the expense of 

 much time and attention. Remember that the mean thickness 

 to which sections are now cut is 5ju, half a hundredth of a 

 millimetre, or less than 6o 1 o0 th of an inch. Clearly the free 

 hand is not capable of producing regular series of sections of 

 such a degree of fineness. The student should therefore be 

 careful to provide himself with an instrument in which the 

 knife is guided by a mechanism giving the required precision 

 of stroke. Cylinder-microtomes with free knife motion should 

 be unhesitatingly condemned by him. 



Amongst microtomes fulfilling the conditions I have laid 



