4 02 APPENDIX. 



thoroughly under the tap, with occasional separation and agitation until all 

 traces of the acid are removed. The slides and covers are then placed in 95 

 alcohol and carefully dried by wiping with a clean linen cloth of appropriate 

 thickness, from which all sizing and starch have been removed by previous 

 laundering. Neglect to provide properly cleaned slides and covers mars 

 many valuable preparations. 



The method of fixing to the slide should be made, if possible, to serve 

 an additional and important purpose, namely, to expand and to flatten out 

 the paraffin sections, which very often are slightly compressed and wrinkled. 

 To mount them in this condition may seriously interfere with their later use, 

 as in the case of serial sections of embryos in making reconstructions. When 

 the fixed sections are not to be treated with watery solutions, a convenient 

 and satisfactory means is the gum method, carried out as follows: Of a satu- 

 rated aqueous solution of best gum arabic (a crystal of thymol being added 

 to prevent the growth of fungi) about 12 drops are added to 30 cc. of dis- 

 tilled water and thoroughly shaken. The clean slide is flooded with the 

 solution, care being taken that the solution does not run over the edges, and 

 the sections are floated on the liquid, all parts of the sections being separated 

 from the slide by a stratum of the solution. When all the sections are 

 arranged, the slide is placed on a warm metal plate and very cautiously 

 heated, the temperature never being allowed to rise to the melting point of 

 the paraffin, the object being to secure the expansion of the sections while 

 swimming on the gum solution. They expand in a few minutes. The excess 

 of fluid is then drained off, the sections are finally rearranged with a needle, 

 and the slide, protected from dust, is set aside to dry. The latter must be 

 thorough, and requires, therefore, some hours, a good plan being to leave 

 the sections undisturbed over night. 



The albumen-glycerine mixture is another excellent fixative, which has 

 the advantage over the gum solution of being unaffected by water. This 

 consists of equal parts of the strained white of a fresh egg and glycerine, 

 thoroughly stirred with a glass rod. A small drop of the fixative is placed 

 on the slide and spread out as evenly and thinly as possible. A number of 

 slides may be prepared, dried, and suitably stored for subsequent use. The 

 albumen-coated slide is now covered with a thin stratum of water, the 

 sections arranged, and the slide then placed on the top of the oven, or 

 other appropriate warm location, where the sections expand and the water 

 evaporates. 



Removing the paraffin is the next step preparatory to staining sections 

 on the slide, whatever method may have been employed to secure their at- 

 tachment. To accomplish this the slides with the securely affixed sections 

 are immersed for a few minutes in xylol, which promptly dissolves the par- 

 affin and leaves the cleared sections in place, freed from the embedding ma- 

 terial. The slides are then transferred to 95 alcohol to displace the xylol, 

 after a few minutes passed to fresh alcohol, and then carried through 70 per 

 cent, spirit into the staining fluid, if that be an alcoholic solution, or into 

 water, in case the stain be chiefly aqueous. The most convenient receptacles 

 for exposing a number of section-loaded slides to the various solutions are 

 the square glass " staining-jars," provided with grooves and covers, which 

 allow several pairs of slides, placed back to back, to be stood on end and im- 

 mersed at one time. The disadvantages of the square jars in common use 

 are the danger of damaging the sections by contact with the grooves and the 

 lack of a ground-joint cover to prevent evaporation, as when containing xylol 

 or absolute alcohol. Although less readily procured, the small cylindrical 



