and Laboratory Methods. 15(>3 



bichromate in an oven kept at 60°C. for twelve hours. Change the fluid fre- 

 quently. Continue the saturation with potassium bichromate in the dark box for 

 five to seven days ; the proper duration must be determined by tests made from 

 time to time. Then transfer to the solution of silver nitrate. 



From this point, the procedure is the same as for the " rapid " method of 

 Golgi previously described. This plan is especially successful for the forebrain 

 and spinal cord. In any case where there is good impregnation, the beauty of 

 the preparation is enhanced by the perfect clearness of the field. 



It may be worth noting here that this same method is particularly desirable 

 for demonstrating the bile-capillaries of the mammalian liver. A shortening of 

 the bichromate-bath is, of course, necessary in this case. 



5. JVeigerfs Myelin Stain. 



The study of the central nervous system cannot be complete without 

 preparations to illustrate the course of medullated nerve-fibres. The many 

 schemes extant for staining the myelin sheath depend upon the formation in the 

 nervous elements of either a chrome- or copper-lake of hiEmatoxylin. This lake 

 combines so firmly with myelin that the decolorizing process does not remove it. 

 Doubtless the chrome-lake yields the more delicate results, and hence some plan 

 for securing it is to be recommended for purposes of •special investigation. But 

 for general classwork, the writer has found a copper method preferable because 

 of its entire certainty under all conditions. 



Harden the brain and spinal cord of the animal chosen for study in a bichro- 

 mate solution. This will, of course, require several months if the well-known 

 fluid of Miiller, or a simple solution of a bichromate salt be used. The time may 

 be shortened to a more convenient length by preliminary hardening with formal- 

 dehyde ; or, formaldehyde may be added directly to the bichromate solution. In 

 any case, the hardening should take place in the dark. 



When the hardening has been completed, cut slices from the regions desired, 

 and, without washing in water, transfer them to graded alcohols for dehydration, 

 still in the dark. 



After the pieces have been dehydrated, imbed them in celloidin, allowing 

 plenty of time for thorough penetration. Harden the celloidin in eighty per 

 cent, alcohol. 



The celloidin blocks are now to be mordanted for two days in a half-saturated 

 solution of neutral acetate of copper. At the end of this time, place the blocks 

 back in eighty per cent, alcohol for a day, and then cut sections at once. 



Sections should have a thickness of 25//. Rinse the sections rapidly with 

 distilled water, and bring them directly into the stain : 



. \ Distilled water - - - 93 c. c. ) q , 



( Lithium carbonate, sat. aq. sol. - 7 c. c. ^ 

 ^ < Absolute alcohol - - - 10 c. c. ) .. , 

 ( Haematoxylin crystals - - 1 gram. ) 



Mix A and B just before using. 

 The duration of the staining will depend upon the region involved. For sections 

 of the spinal cord, two hours will be sufficient ; but for sections of the brain, 

 twenty-four hours will be required. 



