6 INTRODUCTORY 



for dissection, whilst its viscosity serves to lend support to delicate 

 structures. Clove oil has a tendency to make tissues that have 

 lain in it for some time very brittle. The brittleness is, however, 

 sometimes very helpful in minute dissections. Another property 

 of clove oil is that it does not easily spread itself over the surface 

 of a slide, but has a tendency to form very convex drops, and this 

 also makes it frequently a very convenient medium in which to 

 make minute dissections. 



If it be desired to dissect in a watery fluid, such as glycerin, it may 

 be well to prepare the slide by spreading on it a thin layer of Mayer's 

 albumen, and on this to place a small drop of glycerin, or other dissecting 

 medium. As soon as the dissection has been accomplished, a cover 

 may be let fall, horizontally, on to the preparation to keep the parts in 

 place, and a weight placed on it. Then the mount may be filled up 

 with glycerin, or other moimting mediiun, run in under the cover, and 

 closed, if desired, or instead of the albumen a solution of gelatin may be 

 taken, and hardened in formol with the objects on it. For a balsam 

 moimt, after clove or cedar oil, Schallibaum's collodion may be taken, 

 and the organs fixed in situ on this by adding xylol. 



9. Microtomes are instruments for the accurate production of 

 thin slices of tissues. They are used both for cutting tissues that 

 have acquired a certain favourable consistency through having 

 been imbedded in paraffin, and also for cutting tissues that have 

 been imbedded in softer masses, such as collodion, and tissues that 

 have not been imbedded at all. Not all microtomes are equally 

 well adapted for all these three classes of work. The microtome 

 of the zoologist should at all events be one that is well adapted 

 for cutting imbedded material. 



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, frequently with the knife set square 

 to the line of section. In the celloidin method, as in the cutting 

 of unimbedded tissues, it is generally cut xvet, and always with a 

 knife set slanting. Some microtomes that are well adapted for 

 the paraffin method are ill adapted for the celloidin method or the 

 cutting of unimbedded material, and vice versa. It may be well 

 to possess the two sorts of instruinent ; but if only one can be 

 afforded it should be such as will give good work in either way. 



Microtomes fall further into two classes according as the knife and 

 the surface of section of the object are (a) in a horizontal plane, or 

 (b) in a vertical plane. The former offer greater facility for the orienta- 

 tion of the plane of section, which is an important point for the zoologist 

 and embryologist. Amongst these may be mentioned (a) The " Slid- 

 ing " Microtomes, in which the knife is carried on a sledge and moved 

 against the object. They work equally well with paraffin or celloidin, 

 and some can be adapted as a freezing microtome. But this (as is the 

 case with the others above mentioned) will not always furnish work 

 of the highest accuracy ; for the knife being only clamped at one end 

 is liable to spring, and to give sections of unequal thickness. This 



