16 
BULLETIN OF THE BUREAU OF FISHERIES 
shortly after encystment, followed by the division of the nuclei, blepharoplasts, and 
axostyles (fig. 16). Since this occurs so quickly after encystment, the great 
majority of the cysts always contain two pairs of nuclei, either close together or 
some distance apart (figs. 17 and 18). Later the cysts become more elongate, 
with a pair of nuclei at each end (fig. 19). It is probable that the contents of the 
cyst is always divided into two distinct individuals at this time, but they are so 
closely crowded together that in most cases it is impossible to determine this with 
certainty. An exceptional case is shown in Figure 20, where two individuals can 
be clearly distinguished owing to the fact that they do not entirely fill the cavity 
of the cyst, as is usually the case. 
Advanced stages of the cysts, such as are shown in Figures 19 and 20, are found 
only in the extreme posterior end of the intestine. They evidently pass out with 
the excrement at this stage, and their further development has not been followed. 
Presumably they are accidentally ingested by another fish and in this way start a 
new infection. It is very probable that this is the usual method by which the 
parasite gains entrance to a new host, although when the fish are crowded closely 
together it is not impossible that the flagellated forms may set up a new infection. 
Experiments have shown that the flagellates can live outside the host for 15 to 30 
minutes, but that they are unable to survive in water for a much longer period. 
However, even should the flagellates gain entrance to a new host while still in active 
condition, it is by no means certain that they could survive in the stomach for any 
length of time. 
INTRACELLULAR STAGES 
In addition to the flagellates that are found only in the lumen of the intestine, 
intracellular stages are always present in the intestinal epithelium of infected fish. 
Ordinarily these forms are comparatively rare, but under certain conditions may 
become very abundant. They occur only in the epithelial lining of the pyloric 
caeca and the anterior end of the intestine. 
The earliest stage of the intracellular parasite appears as a small rounded cell 
containing a relatively large deeply staining nucleus (figs. 21 to 24). They are 
usually found in the distal end of the cells, as shown in Figures 21 and 23, but in 
some cases may lie in the proximal portion of the cell below the nucleus. The para- 
site is almost invariably surrounded by a distinct vacuole, which is probably, in large 
part at least, the result of shrinkage. They grow rapidly to many times their 
original size (figs. 25 to 30), causing hypertrophy of the host cell, which in probably 
all cases is eventually destroyed. At first the nucleus is filled with a diffuse 
chromatic network (figs. 24 to 26), but as the organism grows there is a tendency 
for the chromatin to become aggregated in several large masses on the nuclear 
membrane (figs. 27 to 30). At first there are five or six large chromatic masses, 
but at a little later stage these bodies are smaller and more numerous (figs. 31 
and 32). A careful study of a number of cells at this stage shows that there are 
always about 12 of these masses of chromatin in each nucleus. It is believed that 
they are formed by the division of the larger chromatic bodies that were present at 
an earlier stage and that they are, in reality, chromosomes that have divided pre- 
