28 CHAPTER III. 



tissues of corresponding fresh- water or terrestrial forms, and 

 fixing solutions should in consequence be stronger (about two 

 to three times stronger, according to Langerhans). 



Marine animals ought to be freed from the sea water adhe- 

 rent 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 penetra- 

 tion of reagents to the interior, thus allowing maceration to 

 be set up, and hindering the penetration of staining fluids. 

 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. As a general rule, they 

 should not l>e made with sea water as a menstruum, as some 

 workers have inconsiderately proposed. If, however, for 

 any particular purpose, it is deemed desirable so to prepare 

 them, care should be taken to remove the sea salts after- 

 wards by appropriate washing, or to mount the objects in 

 glycerin (MAYEK). If alcohol be employed, it should be 

 acidified Avith hydrochloric or some other appropriate acid. 

 Picro-nitric acid is a fixing reagent that fulfils the conditions 

 here spoken of. (On this subject see Paul Mayer, in Alittli. 

 Zoo]. Stut. Neapel, ii (1881), p. 1, et seq. See also the 

 abstract in Journ. Buy. Mic. Soc. (N. S.), ii (1882), pp. 866- 

 881, and that in Aun-r. Natural, xvi (1882), pp. 697706.) 



31. Hardening. The process of hardening was above ( 26) 

 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 also distinguished from fixing in that it does not include 

 the killing of the elements. Nerve tissue, for instance, is 

 daily hardened after having come into the hands of the 

 anatomist some twelve or twenty-four hours after the death 

 of the subject, under which conditions there can of course 

 be no question of fixing. Hardening is an after process, and 

 onlv r;inks ;is a *i>t'<-il method. 



.Methods of imbedding have now been brought to such a 

 degree of perfection that the thorough hardening of soft 

 tissues that \vas formerly necessary in order to cut thin 

 sections from them is, in the majority of cases, no longer 



