463 
reveals remarkable variations or oscillations in the numbers of the 
parasites from day to day, the variations being of the nature of a 
periodic increase and decrease at more or less regular intervals. 
4. The periodicity is to be explained by (a) variations in 
resistance on the part of the host, probably due to the formation 
of anti-bodies, accompanied by (£) the formation on the part of the 
trypanosomes of rounded, latent, non-flagellate (relatively resistant) 
forms in the internal organs of the host during the fall in numbers 
of the flagellate parasites in the peripheral blood. 
There is a life-cycle of trypanosomes in the Vertebrate host, 
in addition to stages of the parasite in the Invertebrate carrier (for 
example, Glossind). 
