26 



medium will answer best ; for others, such as some scales and wings 

 of lepidoptera, the dry method of mounting is more suitable. 



Most animal tissues require to be hardened before sections can be 

 cut from them, and nearly all require some special individual treat- 

 ment. The general procedure for soft tissues is as follows : fixing, 

 hardening, staining, dehydrating, and mounting. By fixing is meant 

 the rapid killing of the tissue, so that it does not have time to 

 change its form before undergoing further treatment. The agents 

 most generally used are perchloride of mercury, chromic acid, and 

 picric acid. For hardening animal tissues preparatory to staining 

 and section-cutting, a good method is by the employment of Muller's 

 fluid, the formula for which is potass, bichromate, 25 grammes; 

 sodium sulphate, 10 grammes ; water, 1000 c.c. Small portions of 

 the tissue should be kept soaking in this fluid for several weeks, after 

 which they are well washed in running water and placed in methylated 

 alcohol, frequently changing. Some tissues harden rapidly, others 

 require two or three weeks. As this method is unsuitable for some 

 staining processes, perhaps, for general purposes, nothing is so good 

 as saturated perchloride of mercury. Small pieces of tissue are 

 soaked in this for one or two days ; they are then washed for several 

 hours in running water, and afterwards placed in gradually increasing 

 strengths of alcohol up to 70 per cent., in which they may be kept 

 until ready for cutting sections. 



There are several methods of section-cutting. One formerly much 

 in use was Valentin's knife, which consists of two thin parallel blades 

 fixed in a handle, the distance between the blades, and consequently 

 the thickness of the resulting section, being regulated by a screw. It 

 is not now much used. The simplest method, and one which I can 

 recommend, is the freezing process, in which a Cathcart's microtome 

 is used ; this is not an expensive apparatus. Prior to cutting a small 

 portion of the tissue is soaked for two or three days in thick mucilage 

 of acacia five parts, syrup three parts : it may be kept in this inde- 

 finitely. The tissue is now placed on the bed of the microtome and 

 frozen by means of the ether spray. After elevating to the desired 

 position by means of the screw underneath, sections are cut with a 

 sharp razor which is made to slide along two parallel glass plates. 

 By this means sections thin enough for most purposes can be 

 expeditiously cut. Another method is by imbedding the tissue in 

 melted hard paraffin previous to cutting. Some workers prefer this 

 method to the last, and probably the thinnest sections are obtained in 

 this way. If the very best class of work is required and expense is 

 no object, the finest microtomes in the market are the Minot and the 

 Thoma ; they may be obtained of the best makers of apparatus. For 

 precise details of these methods the worker is referred to the various 

 treatises on microscopical manipulation. 



Staining is an art requiring much experience and care in manipula- 

 tion. Its value lies in the perfection with which the various tissues 

 are differentiated and their structure exhibited, apart from the mere 



