THE NERVOUS SYSTEM. 569 



portant, but must be specially determined for each specimen. In general it varies from 12 

 hours (chick embryos of two to three days incubation) to six or seven days (e.g., human 

 cerebral cortex at birth). 



2. The specimen is next rinsed in a 3/4 per cent, to i per cent, solution of silver nitrate 

 (a previously used solution will answer for this rinsing) and changed until the solution no 

 longer becomes turbid with silver chromate. The specimen is then transferred to a fresh 

 silver nitrate solution of the same strength for 24 hours, preferably in the dark. It is often 

 well to keep the specimen at this stage in a warm chamber at a temperature of from 25 

 to 30 C. 



3. The specimen is next brought directly into 95 per cent, alcohol for an hour or more, 

 the alcohol being changed several times, and then into equal parts alcohol and ether for 

 from 1/4 to 1/2 an hour. It is next placed in thin celloidin for about 1/2 hour and then for 

 the same length of time in thick celloidin. The specimen is blocked and the celloidin 

 hardened quickly in chloroform. Thick sections (50 to 70 microns) are cut, cleared in oil 

 of origanum Cretici, followed by xylol, and mounted in xylol-balsam or damar, without a 

 cover-glass. It is often of advantage to transfer the sections from the xylol to a dish of xylol- 

 balsam or damar and from this to the slide, thereby avoiding diffusion currents. The slides 

 are then placed in the paraffin bath to hasten the hardening of the balsam. Sections may be 

 mounted under a cover glass if melted hard balsam or damar is used instead of a solution of 

 the same. If the balsam in uncovered specimens becomes wrinkled or cracked, heat 

 applied until the balsam just melts (but does not bubble) will render it smooth again. 



Material which is to be subjected to the Golgi technic should be cut into small pieces, not 

 exceeding 2 to 3 mm. in thickness The method shows a few of the neuroblasts and spongio- 

 blasts as black objects, they being filled or encrusted with a black silver compound. Internal 

 structure is of course not shown. The method is capricious and is best used when there is 

 considerable material available. 



Methods of Cajal. Of these the most generally useful is as follows: 



1. Specimens not more than 5 or 6 mm. in thickness are fixed and hardened for twenty- 

 four hours in strong alcohol or in strong alcohol to every 100 c.c. of which from four to 

 twelve drops of strong ammonia have been added. 



2. The specimen is next rinsed in distilled water or simply placed in the water until it 

 sinks and then transferred to a i if 2 per cent, aqueous solution of silver nitrate. The strength 

 of the silver solution may be varied from 1/2 per cent, to 4 per cent., 11/2 per cent, being the 

 strength most commonly employed. While in the silver solution the specimens should be 

 kept in the dark and at a temperature of 32 to 38 C. The exact time the specimens should 

 remain in the silver solution must be determined by experiment in each case, but is usually 

 from three to six days. 



3. The specimen is next rinsed quickly in distilled water and then placed for twenty- 

 four hours at room temperature in 



Pyrogallol, i to 2 grams 

 Water, 100 c.c. 



Formalin, 5 to 10 c.c. 



The solution should be freshly made up and changed after the specimen is put in it if it 

 becomes turbid. 



4. The specimen may be embedded in celloidin or paraffin and cut in the usual way 

 and about the usual thickness. Dehydrate, clear in carbol-xylol and xylol, and mount in 

 xylol -damar. 



