40 ' HAl'lER IV. 



specimens, and f"imd it admirable for these purposes. But preparations 

 i u ,d.' to fcesl it- value tV.uu :i cytological point of view have given me 



( ,nlv -...ad-rat'' results. It is m\v little used. 



49. Chromic Acid and Platinum Chloride (MERKEL'S Macula 

 luh.i des Menschen } Leipzig, 1870, p. 19). Equal volumes of 

 ] I i i - .liuion of chromic acid and l'-i-00 solution of platinum 

 chlorM''. Obje--:- -hould remain in it for several hours or 



n da vs. After washing out with alcohol of 50 per cent. 

 to 7o per cent., objects stain excellently. If objects that 

 have been fixed by osmic acid be put into it for some hours, 

 blackening 1 is said to be effectually prevented. 



This is an excellent hardening medium for delicate objects. 

 Merkel allowed from three to four days for the action of the 

 llnid for the retina; for Annelids Eisig employs an immersion 

 of three to five hours, and transfers to 70 per cent, alcohol; 

 for small leeches Whitman finds one hour sufficient, and 

 transfers to 50 per cent, alcohol. 



A similar mixture, with the addition of 0'25 to O'l per 



nt. of acetic acid, is recommended by BRASS for Protozoa; 

 and LAVDOWSKY has used for nuclei a mixture of 10 parts of 

 1 per cent, chromic acid, 5 of 1 per cent, platinum chloride, 

 and 100 of 5 per cent, acetic acid. 



Whitman recommends for the hardening of pelagic fish 

 ova, a stronger mixture (due, I believe, to Eisig), viz. 



0"25 per cent, solution of platinum chloride . 1 vol. 



1 per cent, solution of chromic acid . . 1 ,, 



The ova to remain in it one or two days (WHITMAN, Methods 

 in Micro. Anal., p. 153). 



Salts. 



50. Chromates. The chromates are amongst the oldest and 

 best tried of hardening agents. The bichromate of potash 



IH ciallv was at one time universally employed for hardening 

 all sorts of tissues. 



FU;.MMIN<; (Arch. mik. Anat., xviii, 1880, p. 352) pointed 

 out that though it preserves cytoplasm well it causes 

 chromatin to swell, and therefore should not be employed 

 J<>r flu study of nuclei. But, duly corrected with acetic acid, 

 it affords a correct and fine fixation of nuclei; whilst pro- 



