42 
Leucocytozoon canis etc. 
The formation of the large cysts containing numerous sporocysts and 
the packing of these in the body cavity exactly recall the conditions 
seen in ticks fed on dogs infected with L. canis. Not only so but in 
many of the details of the cycle described for H. perniciosum I recognise 
appearances seen and studied in the development of L. canis. Miller 
has gone so far in the case of H. perniciosum as to produce infection by 
feeding rats on mites whose bodies contained cysts. 
In each of these cases then the same developmental cycle is clearly 
concerned, and the true or supposed cycles in H. gerhilli, L. canis, 
H.jaculi, and H. perniciosum all stand or fall together. 
Proof that the developmental cycle described in the tick is 
really that of Leucocytozoon canis 
Wenyon does not discuss the possibility that the forms described in 
the tick may be a natural parasite. His belief that they were actual 
developmental stages of L. canis was probably based, as were my own 
earlier observations, on finding them so regularly whenever ticks were 
fed on infected dogs, and on the fact that the various stages could be 
followed step by step from the encysted vermicules taken into the gut 
of the tick. 
More than one error has, however, been made in the past in tracing 
what appeared to be consecutive steps in the development of parasites 
in external hosts, and when Miller published a complete cycle for 
H. perniciosum, which was clearly that with which I was dealing in the 
case of L. canis and H. gerhilli, it seemed to me that the most important 
question at issue then was whether by any possibility some commonly 
occurring type of arthropod protozoal parasite had not been concerned in 
each case. The following expeiiments which were undertaken some 
time ago, but have not yet been published, bear upon this aspect of the 
case. It was intended to repeat Miller’s experimental infection of the 
host by feeding it upon infected specimens of the carrier, but the experi¬ 
ments were unfortunately cut short by other duties before this could be 
done. 
1. Tick larvae fed on dogs with haemogregarines and on dogs free from infection. 
20. 8. 07. Larvae fed on dog 22 (uninfected). 
8 examined showed nothing. 
20. 8. 07. Larvae fed on dog 20 (heavy infection). 
9 examined showed nothing. 
Other experiments with larvae fed on infected dogs not recorded 
were also negative. 
