The Microscope. 127 



With this development of the microscope and its applica- 

 tions, many methods of preparations of objects intended for 

 its use have been elaborated, and it will be our purpose in these 

 pages to give the recent and valuable processes of modern 

 investigators. 



THE PREPARATION OF ANIMAL TISSUES. 



The successful preparation of animal tissues for microscopic 

 study requires great care and a knowledge of the action of the 

 reagents used, for different methods are used in preparing ob- 

 jects to show different characters. The process may be divided 

 into five steps, killing, hardening, embedding and sectioning, 

 staining, and mounting. 



Killing. — The tissue is killed so that it shall present the 

 appearance and structure when examined that it had at any 

 desired stage of development or physiological action. This, 

 except where the freezing microtome is used, is done by im- 

 mersing it in a fluid which destroys the life of the cells. This 

 however is not the only end sought ; a fluid to be of value for 

 this work must have the following four properties: It must 

 quickly replace the water or other fluids in the object, it must 

 immediately kill the cells so that no vital action or metastatic 

 alterations go on, it must not induce any structural change, and 

 lastly, its action must not interfere with the subsequent harden- 

 ing and staining of the specimen. For these ends a number of 

 substances have been employed, and below is given a list of the 

 more valuable ones with the peculiar uses for which each is 

 fitted. 



Osmic Acid. — This is purchased in small sealed glass 

 tubes containing one grain of the crystallized acid. This tube 

 may be placed in 100 c. c. of distilled water and broken with a 

 glass rod, the acid quickly dissolving. The one per cent, solu- 

 tion obtained is too strong except for a few tissues. Usually a 

 one-fifth to one-tenth per cent, solution, which can be quickly 

 made by adding one part of the one per cent, solution to four 

 or to nine parts of water, is sufficient. The greatest care should 

 be taken to procure absolutely pure water, as the acid is quickly 

 reduced in the presence of organic matters, and it is well in all 

 cases to keep the bottle containing the stock solutions in a dark 



