212 STRONG. [Vol. X. 



chloroform or 80 % alcohol. A partial infiltration with celloidin and harden- 

 ing the latter is apt to crack the tissue inside. With the tadpole and similar 

 tissues it is necessary to fill up the interstices with celloidin in order to 

 obtain complete and coherent sections. When this is done, chloroform is, 

 in my opinion, the best fluid to harden the celloidin. It does this quite 

 rapidly, and does not, for some time at least, seem to affect the impregna- 

 tion. Specimens may be often left in it over night, apparently without 

 injury. 



The sections, 50 to 70 /x thick, usually, are removed from the knife 

 with a camel's-hair brush or piece of tissue paper, and arranged on the 

 slide. 95% alcohol is used in the cutting. The alcohol is then blotted off 

 by gently pressing a piece of filter paper on the slide, and a few drops of 

 absolute alcohol are put on. This is then carefully drained off, — not 

 blotted as before, — and all superfluous alcohol allowed to evaporate, though 

 the sections should not be allowed to dry. The celloidin, softened by the 

 absolute alcohol, will then adhere to the slide during the remaining treat- 

 ment. The sections are then cleared by means of ol. origanum cretici^ and 

 the latter is washed off with xylol. They are then mounted in dammar 

 balsam, without a cover slip. The dammar is used in the condition in which 

 it is obtained — a thick fluid. After it is spread over the sections it must 

 be watched for a while, as it tends to run off the sections and accumulate 

 around the edges of the mount, probably owing to diffusion currents. The 

 aim should be, in covering with balsam, to get an even layer of balsam, as 

 thin as is consistent with covering the sections. If too thick it does not 

 dry rapidly, even in the oven, and where yellowing takes place subsequently 

 it is most liable to occur where the balsam is thickest. When mounted, the 

 slides are put in an oven, at about 50° C, for a day or two, care being taken 

 that they shall be level. Heat does not seem to affect the preparations, and 

 the sooner the balsam is dried the better. If the balsam should dry off the 

 sections in spots leaving them exposed, the dry places should be first 

 moistened with xylol and then balsam added as required, and the slide 

 dried again in the oven. In my experience, origanum and xylol are much 

 preferable to creosote and turpentine, which are recommended by Golgi. 

 This is especially true with the central nervous system, where turpentine 

 tends to crack the sections. The origanum does not allow the balsam to 

 dry so readily as does xylol and will also, after a while, affect the impreg- 

 nation, hence the washing with xylol. 



The best way to mount loose sections is to transfer them from the xylol 

 into a dish of quite thick balsam instead of immediately to the slide. In 

 this way the above-mentioned diffusion currents upon the slide are avoided. 



Note. — Since the above writing, I have found that the period of hardening 

 may be reduced to a day or so and yet good impregnations of adult brains be 

 obtained also by adding formalin {e.g., 4 %) to a potassium bichromate solution. 

 How this will compare with the lithium bichromate method, I can hardly, as 

 yet, say. 



