332 
A Critical Review, etc. 
problem in connection with the transmission of the pathogenic 
trypanosomes of Africa will not be completely solved. 
We now propose dealing shortly with the question of the develop¬ 
ment of trypanosomes of fishes, frogs, eels and leeches. It has been 
accepted as an incontestable fact that the trypanosomes, mentioned 
above, undergo developmental changes in leeches. The first important 
work on this point is that of Keysselitz (1906) who claims to have 
followed the development of Trypanoplasma borreli in Piscicola 
geometra. This investigator, recognising the fact that leeches caught at 
large harboured flagellates, raised them from the egg, and on feeding 
them on fish infected with Trypanoplasma borreli, he observed certain 
developmental changes. He however failed to infect fish either by placing 
leeches on them or by injecting them with the gut-contents of infected 
leeches. We have pointed out above that in the case of a frog leech it 
was found that it harboured a flagellate which was transmitted heredi¬ 
tarily, and that it was not possible to trace any connection between this 
flagellate and frog trypanosomes. In the case of this particular leech it 
is not possible to exclude its natural flagellates merely by raising leeches 
at random ; it is necessary first to make certain that the parent leech 
has no flagellates. Keysselitz makes no mention of this, and without the 
rigid exclusion of a natural flagellate we cannot accept this author’s 
developmental cycle of Trypanoplasma as being free from error. It is 
also not at all clear how Trypanoplasma borreli passes into the 
developmental forms described by Keysselitz ; in fact, this is exactly 
similar to what we have remarked in the case of the developmental 
forms of trypanosomes in tse-tse flies, lice, etc. Leger (1901) has 
also described the development of trypanosomes and trypanoplasms 
in leeches, and Brumpt (1901-1908) has extended this work, describing 
the development of trypanosomes of fresh water fishes, eels and frogs in 
various leeches. He also discovei’ed that some of these leech flagellates, 
particularly those found in leeches from frogs, are transmitted to the 
progeny of the leech. Franca (1907) has also recently described the 
development of a frog trypanosome in a leech. In every instance, and 
we have looked carefully through the literature, these authors never 
mention the possibility of these flagellates being harmless parasites of 
leeches. It would appear that in each case leeches are fed on 
frogs etc. infected with trypanosomes; flagellates are later found in 
their crop diverticula and intestines and these are then described as 
developmental forms of the trypanosomes. In each particular leech 
the transition from the vertebrate trypanosome to the so-called 
