8 PROF. E. A. MINCHIN ON PROTOZOAN [Jan. 12, 
between them makes it difficult, if not impossible, to distinguish 
the varieties parva and magna. 
The significance of this dimorphism or polymorphism is not 
clear, and must be explained from the life-history. Two possible 
explanations present themselves : first, that the difference between 
small and large forms is one of growth ‘and development ; secondly, 
that itis a manifestation of sexual differences, small male and large: 
female forms being differentiated from an indifferent or inter- 
mediate form. The trypanosomes of the pike and the perch rather: 
favour the sexual hypothesis, but the state of affairs in the eel- 
trypanosome strongly suggests stages of growth merely. Certain 
facts that I have observed in the perch have awakened in me the 
suspicion that these fish-trypanosomes have some form of multi- 
plication in the internal organs of the fish, and that fission of the 
type familiar in other trypanosomes perhaps only occurs after a 
new infection, just as in Zrypanosoma lewisi fission is only found 
in the first week or ten days after inoculation. Fission has very 
seldom been seen in fish-trypanosomes ; so far as I am aware, it has. 
only been seen in 7”. remaki immediately after inoculation into a 
pike (Laveran and Mesnil) and in 7. granuloswm in cultures in 
vitro (Lebailly, Franca). The absence of fission-stages in the blood 
of fish infected naturally is very striking. The subject is one 
requiring renewed investigation. 
In the trypanoplasms of the tench (figs. 40-43) and pike (figs. 
33-37, 56) I have also observed smaller and larger forms, sharply 
distinct not only in size but even in nuclear structure; the large 
forms are much less common than the smaller ‘ ordinary” forms, 
and both types are divisible into two categories by differences in 
the nuclear apparatus, especially the kinetonucleus. According 
to Keysselitz the large forms are gametes destined to conjugate in 
the intestine of the leech; those with larger kinetonuclei are 
regarded by him as gametes of male character, while the forms 
with smaller kinetonuclei are regarded as female. In one of the 
ordinary forms of the trypanoplasm of the pike I have seen the 
only example that has come under my notice, of what is apparently 
nuclear division (fig. 62). 
It is a moot point, how far different species of trypanosomes. 
can be distinguished by morphological characters. As I have 
already said, the trypanosomes of the five species of fish studied 
by me belong, in my opinion, to four species, namely (1) the try- 
panosomes of the bream and tench, (2) of the pike, (3) of the 
perch, (4) of the eel. Hach of these four putative species can be 
easily: distinguished from the other three by its structural cha- 
racters, considered as a whole; I need only refer to my figures. 
There is, however, always the possibility to be borne in mind that 
one and the same species of trypanosome, when: inoculated into 
distinct species of fish, might vary in character in response to: 
differences in the environment; this is a point on which experi- 
mental evidence is needed. The trypanoplasms of the tench, 
bream, and pike seem to me also to constitute three well- 
