135 



142. Directions for use. Tissue of a (preferably) young ani- 

 mal is placed in a mixture of 4 parts of 3% potassium dichromate 

 and i part of i% osmic acid. The amount of the fluid should be at 

 least twenty times the bulk of the tissue and should be changed as 

 soon as it grows turbid or loses the strong characteristic odor of the 

 osmic acid. 



After the hardening has proceeded to the right degree ( 143), 

 rinse the tissue in water for about 5 minutes and place for 15 min- 

 utes in a l /^% solution of silver nitrate, and then for 2 or more days 

 in a y% solution of silver nitrate, preferably keeping it in the dark. 



Without washing, imbed rapidly in collodion as follows : 



(a) Dehydrate 2-3 hours in 95% alcohol, changed two or three 

 times ; () place in thin collodion for 20 minutes, in thick collodion 

 for 20-30 minutes ; (c) imbed in thick collodion, on a cork or block 

 of wood (best; 55, a) ; (d) harden the mass in chloroform for 

 20-30 minutes, and (<?) place the block in clarifier and cut, sections 

 being 50-100 // thick, according to the nature of the tissue and the 

 character of the impregnation. 



(/) Place the sections in 95% alcohol for a few minutes ; clear 

 in carbol-xylene and mount in balsam by placing the section on the 

 slide, absorbing the clearer thoroughly by means of tissue paper and 

 spreading over it thick xylene balsam. Do not cover. L,ater,when 

 the balsam has hardened somewhat, it may be melted by heat and 

 much of the superfluous balsam drained from the section and scraped 

 away with a knife. 



143. Time of hardening. The following periods will prob- 

 ably be found approximately correct. In general : The best results 

 are to be obtained with kittens 3-20 days old, puppies 2 weeks old, 

 rats 8-10 days, rabbits 8 days, (a) For cerebral cortex (and hip- 

 pocamp) : New-born kitten, 1-2 days ; kitten half grown (3-4 

 months), 3-4 days ; new-born rabbit, 24 hours ; rabbit one month 

 old, 2-3 days, etc. 



(0) For spinal cord : Chick of 5-6 days' incubation, 24 hours ; 

 chick, 14-15 days' incubation, 3 days ; new-born kitten or puppy, 

 2-3 days. 



(c) Cerebellum : New-born kitten, 1-2 days ; kitten half grown, 

 4 days. 



144. Methylene blue. Imbed in paraffin tissue that has 

 been hardened in 95% alcohol ; cut sections rather thick, 15-20 //, 



