70 CYTOLOGICAL TECHNIQUE 



If gelatine be maintained for a long time at a temperature ex- 

 ceeding 60° C, and especially if it be boiled, the shortening of the 

 protein chain goes so far that the capacity to form a gel on cooling 

 is destroyed. The resulting substance is called metagelatine. 

 Gelatine gels that are intended for embedding should not be 

 melted frequently, since this also has a softening effect. 



A piece of fixed tissue that has been washed in water may be 

 transferred directly to a gelatine gel melted at 37° C. There is no 

 reason, either theoretical or practical, why tissue should first be 

 placed in a melted gel of low gelatine content and then transferred 

 to a more concentrated one. A suitable gel may be made by plac- 

 ing 25 g of powdered gelatine in 100 ml of distilled water, and 

 leaving this in an incubator at 37^ C until the gelatine has 

 dissolved. 



If the gel is to be kept in stock, it is necessary to take pre- 

 cautions against the growth of bacteria and moulds. A good 

 disinfectant for the purpose is sodium /7-hydroxybenzoate, since 

 this is particularly effective in preventing the growth of the bac- 

 terium that liquefies gelatine gels. It should be dissolved at 0-2 °o 

 in distilled water, and the gel made with this in place of distilled 

 water. 1^ 



A gelatine gel that has been formed by simple cooling is 

 scarcely hard enough to give the necessary support to embedded 

 tissue, and sections cut from it are inconveniently sticky. It is 

 possible to harden the gel by slow evaporation,^^ but the usual 

 method is to link the molecules more securely together by the 

 action of fixatives. Formaldehyde is suitable. It will be remem- 

 bered that it acts by forming methylene bridges between protein 

 chains (p. 42). When the covalent bonds of these bridges have 

 added their effect to that of the hydrogen bonds, the gel is harder; 

 it no longer melts on being warmed, and the tendency to swell in 

 acid solutions is greatly reduced. Sections are still slightly sticky, 

 however. The salts of aluminium are able to fix gelatine. Potas- 

 sium alum (p. 119) is suitable. It abolishes stickiness and gives 

 hard blocks; but it cannot be used alone, instead of formalde- 

 hyde, because sections of gelatine hardened in this way roll up 

 instead of remaining flat. 'Formalum' ^^ is a suitable solution for 



