CHAPTER VII. 



IMBEDDING METHODS INTRODUCTION. 



124. 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 helpful to 

 the student. The freezing microtome so generally employed in England, 

 and no doubt highly useful for the pathologist, 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 sections. The microtome of the zoologist, therefore, must be an 

 imbedding microtome. 



Now there are two methods of imbedding in general use the paraffin 

 method and the celloidin method. In the paraffin method the object is cut 

 dry, with the knife set square ; whilst in the celloidin method the object is 

 usually cut wet, and in a softer and more elastic state than paraffin objects, 

 and always with an obliquely-set knife. It so happens that the most precise 

 and beautiful microtomes that have been constructed are designed in view 

 of the paraffin method, and cannot be applied, or at all events are much less 

 adapted, to work with celloidin objects. A thoroughly equipped laboratory 

 should therefore possess two microtomes, one for paraffin work, and one for 

 celloidin material, or other material that has to be cut in the wet way. If 

 the anatomist cannot afford two instruments, he will perhaps do well not to 

 choose one of those that are adapted only for paraffin, but to choose an all- 

 round instrument, one that without being absolutely of the highest attain- 

 able precision in paraffin work will yet give sufficiently good results in that 

 way, and will also cut in the wet way. 



Amongst microtomes fulfilling these conditions various forms will be 

 found almost equally convenient. Zeiss makes a good one; Schanze, of 

 Leipzig, makes a good one; Reichart, of Vienna, makes a good one. All 

 these are relatively cheap, and, being at the same time perfectly efficient 

 for easy work, may be recommended. Amongst more precise instruments 

 of this class I particularly recommend the THOMA sliding microtome, as 

 made, in several sizes, by R. Jung, Mechaniker in Heidelberg. For 

 zoological and general histological work I recommend the medium size 

 (No. 4), with the newest Naples object-holder and newest form of knife and 

 knife-holder. 



