THE STRUCTURE OF TRYPANOSOMA LEWISI. 759 
slide forms an air-tight cell for the enclosed blood. To 
ensure against the evaporation the coverslip is further luted 
all round the edge.! The blood is thus removed from the — 
osmic chamber with no other alteration than the amount of 
evaporation which it suffers during the rapid passage through 
the air from the glass ring to the clean slide. Preparations 
made in this way keep without perceptible alteration for 
months; the trypanosomes can be examined with any power 
of the microscope and drawn to any scale; it is possible to 
make out all details of structure in them and even some which 
cannot be made out in other ways (figs. 5-8). The trypano- 
somes have undergone no changes during the treatment than 
such as result from the fixation with the vapour of osmic 
acid on the one hand, and such as may be due, on the other 
hand, to the shght amount of evaporation of the blood-serum 
which is inevitable, however rapidly the manipulation is 
carried out; hence these preparations represent, in my 
opinion, the nearest approach possible, in the present state 
of our technique, to the living condition, and may therefore 
serve as a standard for comparison with the results of other 
methods. I shall therefore refer briefly in the sequel to 
these preparations as “the standard,’ whenever I have to 
speak of them. 
In making the standard preparations I introduced one addi- 
tional complication into some of them, namely, that on the clean 
slide a small drop of acidulated methyl-green solution was 
placed (§ per cent. methyl-green in 1 per cent. acetic acid), 
and the blood, after exposure to the osmic vapour, was placed 
on the stain. In the course of a few hours the stain mixes 
with the blood and tinges the nuclei of the trypanosomes. 
Comparison of trypanosomes, treated with the acidulated 
methyl-green (figs. 5-8), with those in which no stain was 
used (figs. 3, 4), revealed no perceptible difference in size, 
‘IT employ for this Czokor’s mixture of beeswax and Venetian 
turpentine, heated together until the mass is hard and smooth (not 
sticky) when cool, and applied to the edge of the coverslip with a 
piece of heated wire. 
