75 



The age of the animal; some regions of the nervous system give bet- 

 ter results in young or fetal animals ; other parts take the stain 

 better in older animals, etc. (c) The time of mordantage; it is neces- 

 sary that the tissue be mordanted a certain length of time, constant 

 (relatively) for a certain kind of tissue under the conditions above 

 (a and b). It is necessary that the best amount of dichromate mor- 

 dantage be given, (d) Different organs and regions of the central 

 nervous system vary greatly in the ease with which they can be made 

 to furnish satisfactory impregnations. Almost certain impregna- 

 tions of hippocamp can be gained; cerebral cortex is likewise quite 

 easy to stain. With the olfactory bulb the action is not constant 

 though fairly complete. The optic lobes and retina of birds and 

 large reptiles are more satisfactory than those of mammals. The 

 my el (spinal cord) of embryo birds (7 to 14 day chick best) is gen- 

 erally more satisfactory than that of mammals; in any case, fetal or 

 new-born animals should be employed. Difficult are satisfactory 

 impregnations of sympathetic ganglia, organs of special sense and 

 the intrinsic nerves of the viscera. 



The important forms of the method are: (a) the Slow Method; 

 mordantage in dichromate solutions of preferably increasing 

 strength, 2, 3, 5%, for 1 to 4 months depending on the temperature, 

 strength of solution, etc. (b) The Rapid Method; in this another 

 oxidizer, osmic acid, is combined with the dichromate with a reduc- 

 tion in the duration of the treatment to a few days. Combination of 

 (a) and (b) are sometimes serviceable; (c) Double (or triple) Im- 

 pregnations, obtained by repeating (b). Important modifications 

 are: (1) substitution of formalin for the osmic acid in (6), (2) 

 mercuric chlorid instead of silver nitrate (Cox's Method). The 

 Golgi methods have been widely applied and for details the individual 

 papers may be consulted [6]. 



206. Golgi's Rapid Method. This is the most generally 

 serviceable of the different methods. 



Directions jor use. Tissue of a (preferably) yoiing animal is 

 placed in a mixture of 4 parts of 3% potassium dichromate and 1 

 part of 1% 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 action has proceeded to the right degree ( 207), rinse 

 the tissue in water for about 5 minutes and place for 15 minutes in 



