HISTOLOGY AND BACTERIOLOGY. 5 



2nd. Hardening the elements of a tissue so that their 

 structure will remain as nearly normal in appearance as 

 possible, after the various reagents for their preparation 

 have been used. 



The fixing of a tissue is of the greatest importance, 

 and final success or failure with the sections depends 

 very largely upon this step. 



One must know what fixing agent to use for a par- 

 ticular tissue; how long to allow it to act; with what to 

 wash out the fluid, and what strength of alcohol to use 

 after washing. 



Secure perfectly fresh tissue if possible. Use 

 small pieces, and 15 to 20 times their volume of the 

 fixing fluid. 



There are many good fixing agents and the choice of 

 one depends upon the kind of tissue and the result de- 

 sired. For most purposes the following will be found 

 satisfactory fixing fluids, and among them, corrosive 

 sublimate, absolute alchohol, Flemming's fluid and Per- 

 enyi's fluid are particularly recommended. 



FIXING AGENTS IN GENERAL USE. 



1. PERENYI'S FLUID. Formula on page 32. 



Objects are left in the fluid from 3 to 5 hours for small 

 embryos and 4 to 12 hours for the tissues of vertebrates, 

 and then transferred directly to 70-75 per cent, alcohol 

 for at least 24 hours. They may then be placed in 80 

 per cent alcohol and left in this until wanted, or they 

 may be dehydrated at once by carrying them through 

 the alcohols, including absolute. This is a very valua- 

 ble fluid for both embryonic and adult tissues. No seri- 

 ous results follow when a tissue is left in the fluid for a 

 number of hours. Borax carmine may be added to 70 

 per cent alcohol so that the hardening and staining is 

 done at the same time. 



