2

         Rat 8 . Examined at intervals Parasites always very abundant.
     for 12 days.

       Fresh preparations.—In fresh preparations of the peripheral blood the
characteristic parasites are seen lying quite motionless in spaces in, what appear
to be, enlarged pale red cells. The enlarged corpuscles are oval, corresponding
to the shape of the parasite and the cavity which it occupies. The affected
corpuscles average 10µ in greatest length and 7µ in shortest diameter. Un-
affected corpuscles in the same preparations vary from 5.5µ to 6.5µ. The
substance of the corpuscle is of lighter colour than that of the normal red cell
and stains less deeply with eosin. In some cases the enclosing red cell is hardly
distinguishable. The space occupied by the parasite is clearly outlined and oval
in shape. The parasite appears as a vermicule, possessing a short "tail" sharply
flexed upon the body. A refractile mass (nucleus) is seen lying close to the beat
end of the parasite, and one or two refractile granules are usually visible in the
"tail." No pigment grains are present. (Fig. 1.)

       In addition to the motionless intracellular forms, especially in certain
preparations, a number of free bodies, exhibiting active movements, are conspi-
cuous. These resemble the encysted forms, with the exception that the "tail"
is no longer curved inwards. They traverse considerable distances by a rapid
gliding motion, in which the "head" end is situated forwards. They also twist and
turn among the corpuscles, using the "tail" extremity as a fixed point. When
red cells are approached they cling to the body of the parasite and are
forced quickly backwards. On reaching the hinder extremity they remain
attached and are often dragged a considerable distance in this manner.
Eventually they are liberated, but usually only after fresh cells have been similarly
attached. (Fig. 7) In many instances constricting waves are seen passing
backwards along the body of the parasite, and it happens very commonly that
forms in which all movements have ceased possess a moniliform shape, apparently
due to the action of such constricting waves. Not infrequently forms were seen in
the act of leaving the red cell. This process was accompanied by a bulging of
the anterior portion of the parasite, which always protruded first from the cell.
After exhibiting movement for a certain time the vermicules became quiescent and
did not appear to become active again. In preparations kept under observation
for considerable periods, both under the coverglass and as a hanging drop, only
a minority of encysted forms were observed to exhibit any change.

       Blood containing a very large number of parasites was also taken by means
of a sterile syringe, wet with solution of sodium citrate, from the heart of a freshly
killed rat and afterwards kept at a temperature of 24°C. Up to the third day
no appearances suggesting development were seen.