488 PREPAKATION, MOUNTING, AND COLLECTION OF OBJECTS 



in the fixed state. Hence, by penetrating amongst these highly 

 refractive elements, they render the tissues transparent arid clear,, 

 which is the reason of their being called ; clearing agents.' 



The best clearing agent for general use is oil of cedar wood. Oil 

 of cloves is a very good one ; it should be known that it make& 

 objects brittle, which is sometimes to be desired, sometimes the 

 reverse. Oil of bergamot is useful ; it will clear from alcohol of no 

 more than 90 per cent, strength. 



It should be noted that the proper stage for performing minute 

 dissections in is the one at which the objects have now arrived, a 

 drop of clearing agent being a most helpful medium for carrying 

 out such dissections in. Oil of cedar is very good for this purpose. 

 But oil of cloves is sometimes to be preferred, not only on account of 

 its property of making tissues brittle, which is often very helpful, 

 but also on account of the property it has of forming very convex 

 drops on the slide. 



Staining. Good histological stains can in general only be 

 obtained with properly fixed tissues. But it is possible to obtain with 

 unfixed and even with living tissues a stain which though imperfect 

 and not ' fast ' may be of considerable utility in research, either as a 

 means of controlling the results obtained by the examination of fixed 

 and prepared specimens, or as a means of revealing delicate traits of 

 structure that may be masked or destroyed by the action of fixing 

 and preserving reagents, and only visible in the living or perfectly 

 fresh object. 



It goes without saying that staining is performed by immersing 

 the tissues in the colouring solution employed. After the tissue has 

 become duly stained, all superfluous colour is removed from it by 

 ' washing out ' with an appropriate liquid. 



Stains for Living Objects (Intra Vitam Stains). The most 

 widely used of these stains is methylen-blue (to be obtained from 

 Griibler and Hollborn, 1 and not to be confounded with methyl-blue, 

 which is a totally different dye). Small aquatic organisms (such as- 

 rotifers, infusoria, small annelids, tadpoles) are stained by adding 

 a small quantity of the dye (best previously dissolved in distilled 

 water) to the water in which they are kept, and leaving them till 

 the stain has taken effect. Enough of the dye should be added to- 

 make the water of a good blue, the proportion required varying 

 roughly between 1 part of the dye to 10,000 of the water, and 

 1 part to 100,000. Most aquatic organisms will live in the 

 coloured water for many hours, some for days or weeks. They 

 should be examined as soon as the required intensity of stain has 

 been attained. For if they are allowed to remain longer the 

 elements that have taken up the dye will begin to yield it up again 

 to the water, and the objects may become quite pale again even 

 though they have riot been removed from the coloured water. The 

 stain is an imperfect one, being mostly confined to certain granules 

 of the protoplasm of cells, and taking effect capriciously now on one 

 tissue and now on another. It is difficult to preserve the stain in a 



1 63 Bayerische Strasse, Leipzig ; or through Mr. C. Baker, 243 High Holborn. 



