CH. X ] STAINING AND PERMANENT MOUNTING 313 



natural waters. One could use distilled water, adding a few drops 

 of a saturated solution of lithium carbonate. 



Dehydrate in 95% alcohol and absolute if necessary ; clear and 

 mount in balsam as described in the next section (§ 464). 



Hematoxylin is so nearly a pure nuclear stain for most tissues 

 and organs that the cell bodies are not very evident with this alone, 

 hence some counter stain is generally used also. 



S 464. Counterstaining with Eosin. — One of the solutions 

 of eosin (§ 401) is dropped upon the sections after the hematoxylin 

 has been washed away with water. This stains almost instantly. 

 One rarely needs to stain with eosin over 10 or 30 seconds. The 

 excess stain is then washed away with a pipette or by dipping the 

 slide into water. 



§465. Dehydrating, Clearing and Mounting. — Puttheslide 

 directly into 95.% alcohol after it is rinsed with water.* Leave it 

 in the alcohol a short time and transfer to fresh 95 % alcohol or to 

 absolute alcohol a few seconds, 10-20. One must not leave the 

 sections too long in the alcohol or the eosin will all dissolve out. 



Remove the slide from the alcohol and put it into ajar of clearer 

 (§ 39 2 ) or put it on the rack (Fig. 248, 251) and add enough clearer 

 to cover the sections. Soon the clearer will displace the alcohol and 

 make the sections translucent. It usually requires only half a 

 minute or so. The clearer is drained off and balsam put on the sec- 

 tions, and then a clean cover-glass is added. One soon learns to 

 use the right amount of balsam. It is better to use too much than 

 too little. It is usually better to press the cover down very gently. 

 With some delicate objects like embryos in the early stages this is 



*In the past the plan for changing sections from 95% alcohol to water, 

 for example, has been to run them down gradually, using 75, 50 and 35% 

 alcohol, successively. Each percentage may vary, but the principle of a grad- 

 ual passing from strong alcohol to water was advocated. On the other hand I 

 have found that the safest method is to plunge the slide directly into water 

 from the 95% alcohol. The diffusion currents are almost or quite avoided in 

 this way. There is no time for the alcohol and water to mix, the alcohol is 

 washed away almost instantly by the flood of water. So in dehydrating after 

 the use of watery stains, the slide is plunged quickly into a jar of 95% alcohol. 

 The diffusion currents are avoided in the same way, for the water is removed 

 by the flood of the alcohol. This plan has been submitted to the severe test 

 of laboratory work, and has proved itself perfectly satisfactory ( 1895-1908). 



