Ken YON, JJie Brain of the Bee. 139 



Enough of this was added to a dish of water to give the 

 latter a black appearance, or it was diluted about i to 5. From 

 a quarter of an hour to an hour is required for staining. The 

 sections hardened in the potassium bichromate mixture require 

 a longer time than the others. 



After staining, the sections were washed off with 70% 

 alcohol and dehydrated or, if too deeply stained, left in the 70% 

 alcohol for some time. With the dilute solution, however, there 

 is not much danger of overstaining, if the process is watched. 



For impregnation with bichromate of silver the rapid 

 method of Cajal was at first employed, but only later to be set 

 aside when it was discovered that one in which the osmic acid is 

 replaced by formol gives a much more transparent background 

 for the darkened fibers and cells, thus allowing much thicker 

 sections to be cut, while at the same time retaining the 

 rapidity that has made the osmic acid mixture so excel- 

 lent. At first the osmic acid was replaced by the same 

 amount (one-fifth) of pure formol, later Strong's suggestion 

 of equal parts of 10% potassium bichromate and formol 

 was followed. This was then modified to 5 % bichromate 

 and a smaller amount of formol as follows : 



5^ bichromate, 80 cc. 

 Formol, 20 cc. 



This gave impregnations as good as the stronger and a lit- 

 tle better than the weaker combination while at the same time 

 lessening the formation of a black precipitate that is always 

 found in formol-bichromate mixtures after they have stood some 

 hours. This precipitate was avoided as much as possible by 

 changing to fresh fluid every twenty-four hours. Impregnations 

 may be obtained in brains left unchanged in the mixture until 

 ready for the silver solution. The precipitate must however weak- 

 en both the formol and the bichromate and hence operate disad- 

 vantageously. I think my failures were more numerous with 

 brains left in the unchanged fluid, (though no experiments were 

 undertaken to settle the matter definitely,) and hence I am not 

 inclined to agree wholly with Kopsch ( 90) when he asserts that 

 the precipitate makes no material difference. 



