On CajaVs Formalin-Silver Nitrate Impregnation Method. 323 



being controlled with the microscope. In his later paper Cajal 

 apparently omits toning, which is usually unnecessary. Should it 

 be indicated, use equal parts of solutions A. and B. : — 



A. Sodium hyposulphite ... 3 grm. 

 Ammonium sulphocyanato . . 3 „ 

 Aq. desfc 100 c.cm 



B. Gold chloride 1 grm. 



Aq. dest .99 c.cm. 



If desired, the sections may be counterstained with Ehrlich's 

 haematoxylin, methylen-blue, or safranin. 



The method gives the best results with young Mammals ; also, 

 according to Cajal, the most constant results are given by the 

 cat and rabbit. Nevertheless the Grolgi apparatus has been 

 demonstrated in various Invertebrates with success by many 

 workers — Gatenby, Helix ovotestis (Ji) ; Sanchez, nervous system 

 in Hirudinea (IS). 



3. Observations and Peecautions. 



Fixation. — The length of time tissues are left in the uranium 

 nitrate-formol is an important factor in making for successful im- 

 pregnation. This fixative apparently acts after the manner of a 

 mordant with regard to the Golgi apparatus, preparing it, so to 

 speak, for combination with the silver salt. Thus, underfixed 

 tissues will not impregnate, whereas overfixation induces a coarse 

 deposit of silver throughout the material. In the case of tissues 

 which resist penetration the time in the fixative may be increased 

 with advantage up to twenty-four hours. Such at any rate has 

 been the experience of P. Del Eio Hortega (11) for Mammalian 

 ovary. The general quality of the fixation is somewhat variable ; 

 thus while the preservation of intestine is excellent testis often 

 shows marked shrinkage. 



It is advised by Cajal that the formol should not contain free 

 acid; this he obviates by keeping commercial formol in contact 

 with granulated chalk. Although I usually neutralize the formol 

 with caustic soda solution, I have never observed any harmful 

 effects after using a fairly acid formol. 



The Silver Bath. — Generally thirty-six to forty-eight hours in 

 this will impregnate most tissues. Sometimes however the time 

 may be considerably prolonged with advantage. Material over- 

 impregnated with silver shows a fine deposit of the metal throughout 

 the cells. It is of course essential to make up the solution in 

 distilled water, ordinary tap water causing a precipitate of silver. 



Reduction. — In my Experience it is usually useless to reduci 

 for longer than two hours at the most. Working with pieces 



z 2 



