THE SHELLAC METHOD. 175 



usual way, they are put into turpentine, which dissolves the resin, and sets 

 free the plate of collodion with the sections. 



For sections that are to be stained the procedure is as follows. Well-sized - 

 paper is prepared with a thick solution of gum containing 10 per cent, of 

 glycerin, and dried. It is then prepared for use with a surface of the thin 

 collodion mixture, on which the sections are arranged and painted over with 

 the thick mixture. The whole is then brought into rectified turpentine 

 until the paraffin is dissolved and the collodion hardened, and then (in order 

 to remove the last traces of castor oil) into a second bath of clear turpentine. 

 The preparation is then pressed in a press between sheets of filter paper in 

 order to get rid of the turpentine mechanically (alcohol is not applicable 

 here), and brought for a quarter to half an hour into a mixture of equal parts 

 of chloroform and 95 per cent, alcohol. It is then passed through successive 

 alcohols into the staining fluid. 



For ordinary work this process is evidently too elaborate to have any 

 chance of supplanting the classical methods of preparing the series on the 

 slide. Strasser recommends it for very large sections of the central nervous 

 system ( entire pons of the adult). But it seems to me that he will hardly 

 have many followers in his belief that as good sections of such large objects 

 can be obtained by the paraffin method as by the celloidin method ; for which 

 reasons the method appears to me superfluous for ordinary purposes. 



317. The Shellac Method (GIESBEECHT, Zool Anz., 1881, p. 484). 

 Prepare a stock of slides covered with a' thin and even film of shellac. This 

 is done as follows : Make a not too strong solution of brown shellac in abso- 

 lute alcohol, filter it thoroughly ; warm the slides, and spread over them a 

 layer of shellac by means of a glass rod dipped in the solution and drawn 

 once over each slide. Let the slides dry. 



Just before beginning to cut your sections take a prepared slide and brush 

 it over very thinly with kreasote applied by means of a brush ; this forms a 

 sticky surface on which the sections are now arranged one by one as cut, 

 care being taken to bring them on to the slide with as little surrounding 

 paraffin as possible. 



When all the sections are arranged the slide is heated on a water-bath for 

 about a quarter of an hour at the melting point of the paraffin ; this causes 

 the paraffin to run down into a thin layer, and allows the sections to fall 

 through it and come into close contact with the shellac film, whilst at the 

 same time it evaporates the kreasote. 



The slide is allowed to cool, and the sections are now found to be firmly 

 fixed in the shellac. The paraffin is dissolved away by dropping turpentine 

 on to the sections, which are then mounted in Canada balsam. There is no 

 danger of the sections being floated away by the turpentine, because turpen- 

 tine does not dissolve shellac. 



In the note in the Zool. Anz. above quoted, the shellac solution is stated 

 to be prepared with common brown shellac (choosing, of course, by prefer- 

 ence the paler sorts), on account of the insolubility of white shellac in 

 alcohol. In the Mitth. d. Zool. Stat. of Naples, of the same year, " bleached 

 white shellac " is recommended to be dissolved as before, in absolute alcohol. 



