378 METHODS. 



with the other reagents. Specimens preserved with Bouin's fluid have the advantage 

 of staining sharply. To study the medullary sheaths of nerve-fibers, as is necessary 

 to follow the development of the fiber tracts in later stages, the specimens may 

 be preserved in Miiller's fluid. Flemming's, Hermann's, and Bouin's fluids are valua- 

 ble, especially for cytological study, but are applicable only to small pieces. 



.1. ZENKER'S FLUID. 



Formula: Corrosive sublimate ... 5 gm. 



Potassium bichromate i gm. 



Sodium sulphate ' i gm. 



Water 100 c.c. 



Add 5 c.c. of glacial acetic acid to the fluid immediately before using. 



The fluid does not have great penetrating power, but may be used for embryos 

 of 25 mm. The amount of fluid should be from ten to twenty times the volume 

 of the specimen, and better results are obtained if the fluid is changed after a few 

 hours. Chicks of the first and second days are hardened in from two to four hours; 

 embryos of from 6 to 8 mm. in from eight to ten hours; embryos of 12 mm. in twenty- 

 four hours; larger embryos in from thirty to forty hours. After the proper interval in 

 Zenker's fluid the specimens must 'be removed and washed in running water for from 

 twelve to twenty-four hours. Transfer to 50 per cent alcohol for from one to three 

 hours, then to 60 per cent, 70 per cent, and 80 per cent. It is indispensable to remove 

 now the excess of corrosive sublimate by adding sufficient tincture of iodine to 

 give the alcohol the color of port wine; if the iodine disappears, it must be renewed. 

 After from one to three days, according to the size of the specimen, transfer 

 it to fresh 80 per cent alcohol, which must be changed until it no longer extracts 

 any iodine from the specimen. 



2. TELLYESNICKY'S FLUID. 



Formula: Bichromate of potassium 3 gm. 



Water 100 c.c. 



Immediately before using add 5 c.c. glacial acetic acid. 



This reagent is employed in the same manner as Zenker's fluid, except that the 

 treatment with iodine is omitted. 



3. FORMALIN. 



Formula: Formalin 10 c.c. 



Water 7 90 c.c. 



Specimens are placed in the fluid, which should be changed in a few hours. On 

 account of its extreme simplicity this method is used especially for human embryos. 

 If the fluid is used in large quantity embryos even of 80 mm. may be well pre- 

 served. They may be kept indefinitely in the solution, and transferred to alcohol 

 when needed for sectioning. The method has the disadvantage that the transfer 

 to alcohol, even if made very gradually, always causes a considerable shrinkage. 



