introduced into Vertebrates 47 
_ It must be remembered that the stages in the life-history of 
A. jaculum, as with other insect flagellates, vary in different parts 
‘of the insect host. While the fore-gut of the Nepa used con- 
tained non-flagellates (pre-flagellates) in a condition suitable for 
growth in the Nepa in which they were found, they were rela- 
‘tively non-resistant, and the flagellates present were not mature. 
On the other hand, the intestine of the Vepa contained flagellates, 
all of which were mature and some of which were dividing, and, in 
jaddition, the highly resistant post-flagellate forms, which are well 
‘adapted both for resisting unfavourable conditions and for adapting 
themselves to new environmental differences. 
_ Thus we conclude that the post-flagellate stages of H. jaculum 
have greater powers of growing and multiplying successfully in 
‘the vertebrate host to which they were introduced. Also, 
‘these post-flagellate forms, together with the multiplying flagel- 
lates, were responsible for the greater number of flagellates found 
‘in their hosts, compared with those found in mice fed on the fore- 
guts of the Nepa. Consequently there seems evidence to show 
‘that the form of insect flagellate best adapted for transference 
from insect to insect, is also the form most effective in producing 
disease when introduced by way of food or intraperitoneally into 
the vertebrate, in the case of H. jaculum in mice. Probably the 
‘same will be found to be true in herpetomoniasis of the dog 
produced by swallowing dog-fleas infected with Herpetomonas 
ctenocephalr. 
VI. Comparison of the forms of H. sAcuLum found 1m the 
| Vertebrate with those present in the natural Insect Host. 
| 
There are striking resemblances between the forms of H. 
jaculum in the mouse and in Nepa cinerea. The non-flagellates 
present the same morphology in each case. The flagellates show 
practically the same structure. The main differences appear to be 
‘in details. Some of the very long forms of H. jaculum have not 
yet been found in mice, but they might be found were more 
experiments undertaken. The average size of H. jaculum flagel- 
lates in the mice and in the Nepa is the same. The herpetomonads 
‘in mice also appear to have somewhat more granular cytoplasm 
containing a rather larger number of chromatoid granules than do 
‘the corresponding forms in the insect host. In all other respects, 
the parasites keep the facies of H. jaculum as it occurs naturally 
in Nepa cinerea. 
VOL. XVIII. PT. Il. 4 
