CHAPTER III. 27 



Use liberal quantities of liquid for washing. 



Change the liquid as often as it becomes turbid, if that should 

 happen. 



The process of washing out is greatly facilitated by heat. Picric 

 acid, for instance, is nearly twice as soluble in alcohol warmed to 

 40 C. as in alcohol at the normal temperature (Fol). 



32. Fixation of Marine Animals. --The tissues of marine organisms 

 are as a general rule more refractory to the action of reagents than 

 those of corresponding fresh-water or terrestrial forms, and fixing 

 solutions should in consequence be stronger (about two to three 

 times). 



Marine animals ought to be freed from the sea water adherent to 

 their surface before treating them either with alcohol or any fixing 

 reagent that precipitates the salts of sea water. If this be not done, 

 the precipitated salts will form on the surfaces of the organisms a 

 crust that prevents the penetration of reagents to the interior. 

 Fixing solutions for marine organisms should therefore be such as 

 serve to keep in a state of solution, and finally remove, the salts in 

 question. If alcohol be employed, it should be acidified with hydro- 

 chloric or some other appropriate acid. Picro-nitric acid is a fixing 

 reagent that fulfils the conditions here spoken of. (On this subject 

 see MAYER, in Mitth. Zool. Stat. Neapel, ii (1881), pp. 1 et seq., and 

 ALLEN and BROWNE in " Science of the Sea," John Murray, 1912). 



33. Hardening.- -The process of hardening is distinguished from 

 that of fixing as being directed to the attainment of a degree of 

 consistency sufficient to allow of soft tissues being cut into sections 

 without imbedding. It is an after-process, and only ranks as a 

 special method. 



Methods of imbedding have now been brought to such a degree of 

 perfection that the thorough hardening of soft tissues that was 

 formerly necessary in order to cut thin sections from them is, in the 

 majority of cases, no longer necessary. But there are some excep- 

 tions. Such are, for instance, the cases in which it is desired to cut 

 very large sections, such as sections of the entire human brain. 



The reagents employed for hardening are for the most*part of the 

 same nature as those employed for fixing. But it does not follow 

 that all fixing agents can be employed for hardening. Corrosive 

 sublimate, for instance, would be most inappropriate as a hardening 

 agent. 



34. The Practice of Hardening. Employ in general a relatively 

 large volume of hardening liquid, and change it very frequently. If 



