1909. ] BLOOD-PARASITE OF THE SEA-TROUT. 401 
It is impossible to say exactly what brings about the exit from 
the envelope. It is not a question of size, as quite small specimens 
may be found fixed at what appears to be the moment of 
escape. 
The nuclei in Ichthyosporidium are, relatively speaking, very 
small and very numerous. They consist each of a single compact 
mass of chromatin surrounded by a clear space which is bounded 
by a delicate sharply-defined membrane. Very fine rays pass 
from the mass of chromatin to the membrane. The mass of 
chromatin or karyosome is dense and compact, and shows no 
structure, no matter what the stain used. I[ron-hematoxylin, 
Delafield’s hematoxylin, cavm-alum and picronigrosin, picro- 
carmine and picronigrosin, and Twort’s neutral red and light 
green stain were all tried and all gave very fair results. Iron- 
hematoxylin and Twort’s stain, however, gave the best pictures. 
The rays passing from the karyosome to the outer membrane are 
exceedingly fine and stain rather faintly with the chromatin 
stains. The membrane takes up the chromatin colours faintly when 
stained with iron-hematoxylin, Delafield’s hematoxylin, &c., but 
takes up the green of the Twort’s stain—that is to say, it does 
not take the chromatin stain of this combination. This is a 
rather interestmg point, as it indicates that whatever its nature 
it is not the mere condensation of chromatin which generally 
constitutes the so-called “ membrane” in the Protozoa. It may 
be noted that the red colour in Twort’s stain, when properly 
applied, appears to be a pretty good test for chromatin. 
No stages showing nuclear division were observed nor was 
there any indication of spore-formation. 
As has already been stated, the parasite invades practically all 
the organs of the trout, but seems always to be found in the 
greatest numbers in the muscles of the heart. 
The reaction on the part of the host in the way of forming 
connective-tissue cysts round the parasites is comparatively slight. 
In the liver and spleen, it is true, cysts composed of many layers 
of connective-tissue cells are formed, especially in the case of the 
larger parasites; but in other parts of the body, such as the heart- 
muscles, the cysts consist only of two or three thin layers of cells, 
and may sometimes not be formed at all. The envelope secreted 
by the parasite in these cases lies in direct contact with the 
striped muscle-fibres. This relatively slight development of 
connective tissue may have a certain importance in explaining 
the fatal nature of the disease. 
Three years ago, in January 1906, I came across an Ichthyo- 
sporidium* identical, as far as I can see, with the one found in the 
trout, in a smal] flounder (Plewronectes flesus). 
The fish was also infected with a trypanosome, which was the 
parasite I was in search of, and I unfortunately did not examine 
% Proc. Roy. Phys. Soc. of Edinburgh, vol. xvii. No. 5, 1908. 
