STRUCTURE AND LIFE-HISTORY OF COPROMONAS SUBTILIS. 79 
methylene blue. Neutral red has been the most generally 
useful. It does not stain the living nucleus, but will colour 
the cytoplasm a faint pink, so that the nucleus appears more 
distinctly by contrast. The food masses are coloured various 
shades of red, orange, and yellow (see p. 86).  Brillant- 
kresylblau has also been very useful. It stains the cytoplasm 
a pale bluish or purplish colour, leaving the nucleus as a very 
distinct grey globule. Food masses take up the colour very 
strongly, many of them staining red or purple in a meta- 
chromatic manner. Methylene blue has been of but little 
service. 
In making permanent preparations I spread out a small drop 
of the culture solution on a coverslip and then fix the moist 
film so made as quickly as possible. The most suitable 
fixatives are Schaudinn’s sublimate-alcohol (2: 1), used hot, 
and formalin (Schering—40 per cent. formaldehyde). The 
former has been especially useful. Osmic vapour is very 
useful for displaying the flagella, and good preparations can 
also be made after fixation in Hermann’s solution. 
By far the most useful stain is Heidenhain’s iron hema- 
toxylin. I use this alone, or sometimes counterstain with 
eosin or orange G. Other stains which have occasionally 
been of use are Delafield’s hematoxylin, used very dilute, and 
Giemsa’s stain. ‘The latter is difficult to use, and untrust- 
worthy, but it has proved of some service on special occasions 
—e.g.in cyst formation. Methyl green (used in the manner 
described on p. 83) has also been useful. 
The permanent preparations were always mounted in 
balsam, except those stained by Giemsa’s method, which 
were mounted in cedar-wood oil. 
For investigating the minute anatomy of the animals I 
have successfully used the method devised by Schewiakoff (45) 
for ciliates. It consists in killing the organisms with osmic 
vapour and examining them in 10 per cent. soda solution. 
Many structures are rendered very clear by this method. 
In examining the living animals I always used the 2°5 mm. 
apochromatic water-immersion of Zeiss (apert. 1°25), cor- 
