1909. } BLOOD-PARASITES OF FRESHWATER FISHES. 33 
question of the transmission of these parasites, I thought it worth 
while to publish my investigations at the point they have reached, 
leaving the subject in a condition in which it can be taken up 
again and carried further by myself or by anyone else working in 
the same region, 
Methods and Technique. 
In examining the blood of fishes for the parasites, it 1s not 
difficult, when dealing with a fish of fair size, to take the blood 
from the gills of the fish without materially injuring it. There 
is always a risk, however, that the blood obtained in this way may 
contain a certain amount of water mixed with it. To obtain 
good smears I found it best to sacrifice the fish. Having killed it 
by a smart blow on the head with a blunt instrument (best a 
stout rod of hard wood) I opened up the pericardial region and 
took the blood from the heart by means of a fine capillary glass 
tube, thrust through the wall of the heart; the blood was allowed 
to run up into it, and then blown on to the slide or coverslip. 
The whole process has to be done quickly, since fish-blood 
coagulates very rapidly. Fish which are used in this way for 
making blood-smears suffer no detriment in respect to their 
culinary properties and need not be wasted. 
Much has been written lately about methods of fixation of these 
blood-parasites, especially with regard to the procedure most in 
vogue, of drying smears of the blood. That the process of drying 
affects the minute structural details cannot be doubted and can be 
demonstrated easily. It should, however, be pointed out that the 
effects of drying, so far as trypanosomes are concerned, differ 
greatly according as the drying is done before, or after, fixation 
with some histological reagent. Very instructive in this respect 
are the species of the genus Zrypanoplasma. ‘Their soft proto- 
plasmic bodies become greatly deformed if dried before fixation ; 
in this respect they contrast strongly with the species of 7rypano- 
soma. In some fishes, for example the pike, tench, and bream, 
it is common to find both trypanosomes and trypanoplasms in 
the blood; in smears dried before fixation, the trypanosomes 
may be found quite satisfactory in form and generai structure, 
while the trypanoplasms side by side with them are deformed 
almost beyond recognition; compare figs. 25, 26, and 59, show- 
ing trypanosomes and trypanoplasms of the pike, from the same 
slide; figs. 32, 53, 54, showing similar conditions in the blood 
of the bream. In some cases, however, a trypanoplasm may, 
apparently, flatten down evenly and thus give a fairly reliable 
representation of its natural form and structure (compare figs. 37 
and 38, from the same slide); such cases, however, are in my 
opinion to be regarded as accidental and exceptional, and the rule 
is that trypanoplasms when dried become much deformed. It is 
therefore a matter of astonishment to me that Keysselitz should 
have relied so much on material dried before fixation for his 
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