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144 Drs Fantham and Porter, Further Experimental 
Similar parasites occurred in the heart. The spleen contained some 
small, oval, non-flagellate forms, and in the lungs small flagellates 
and oval, non-flagellate dividing parasites occurred. The kidneys 
did not appear to be infected. 
IV. Some Remarks on the Foregoing Researches. 
A number of interesting points follow from the experiments 
just recorded and may now be considered, apart from others of 
economic significance which are dealt with separately in the next 
section. 
The infections obtained in the adult experimental animals were 
not heavy, thereby differing from those recorded in young animals 
in our previous paper. Also, in adult hosts the number of observed 
flagellate forms was relatively few, indeed, much fewer than in the 
case of young hosts. On the other hand, adult hosts contained 
more leishmaniform parasites. Doubtless the parasites are more 
virulent in young hosts, as is the case with Mediterranean Kala- 
azar in children. 
The term “incubation period” has not been used in detailing 
the experiments because of the difficulty of its exact definition. 
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During the first day after inoculation or feeding with insect 
flagellates, various stages of them may be observed in the peri- 
pheral blood of the adult vertebrate host. These forms are usually 
rounded or non-flagellate, and differ from flagellate stages which 
may have been directly introduced. Further, the non-flagellate 
forms may be seen occasionally in process of division during the 
first day. After about 18 hours the organisms often disappear, 
and no parasites may be seen in the blood for a few days. These 
observations are in complete agreement with those published in 
1911 by one of us (H.B.F.) on Trypanosoma gambiense and T. rho- 
desiense. Hence, during the so-called incubation period, a few 
rounded forms of these flagellates (Herpetomonas, Orithidia, Try- 
panosoma) may be found in the peripheral blood. The “incubation — 
period ” is often short in young hosts, as was shown in our former 
experiments. 
The appearance and relative position of the blepharoplast in 
leishmaniform elements vary. . It may be either bacilliform or 
dot-like in shape, and thus resembles that of Leishmania tropica 
more than that often seen in LZ. donovani, though variations occur 
in all these parasites. J. tropica thus seems to have retained 
more of the variation seen in the original insect flagellate stock — 
from which it was derived. 
The ability of species of Herpetomonas and Crithidia to live in 
