304 Major Ronald Boss [March 2, 



It will be evident that this fact was the crucial test both as regards 

 the parasitic nature of these cells and as regards their development 

 from the haemocytozoa of the birds ; and it was not accepted by me 

 without very close and laborious experiment. 



The actual results obtained were as follows : Out of 245 Culex 

 fatigans fed on birds containing H. relicta, 178, or 72 per cent., con- 

 tained "pigmented cells." But, out of 41 Culex fatigans fed on a 

 man containing crescentic gametocytes, 5 on a man containing im- 

 mature tertian parasites, 154 on birds containing H. danilewskii, 25 

 on healthy sparrows, and. 24 on birds with immature R. relicta — or 

 a total of 219 insects, all carefully examined — not one contained a 

 single " pigmented cell." 



Another experiment was as follows : Three sparrows, one con- 

 taining no parasites, another containing a moderate number of H. 

 relicta, and the third containing numerous H. relicta, were placed in 

 separate cages within threa separate mosquito curtains. A number 

 of culex fatigans, all bred simultaneously from larvae in the same 

 breeding bottle, were now liberated on the same evening partly 

 within the first mosquito netting, partly within the second, and partly 

 within the third. Next morning many of these gnats were found to 

 have fed themselves on the birds during the night. Ten of each lot 

 of gnats were dissected after a few days, with the following result : 

 The ten gnats fed on the healthy sparrow contained no " pigmented 

 cells." The ten gnats fed on the sparrow with a moderate number of 

 parasites were found to contain altogether 292 "pigmented cells"; 

 or an average of twenty-nine in each gnat. The ten gnats fed on the 

 sparrow with numerous parasites, contained 1009 " pigmented cells " ; 

 or an average of 100 cells in each gnat. These thirty specimens 

 were sent to Manson in England, who made a similar count of the 

 cells. 



I may mention one more out of several experiments of the same 

 kind : A stock of Culex fatigans, all bred from the larva, were fed on 

 the same night partly on two sparrows containing H. relicta, and 

 partly on a crow containing H. danilewskii (placed, of course, under 

 separate mosquito-nettings). Out of twenty-three of the former lot, 

 twenty-two were found to have pigmented cells ; while out of sixteen 

 of the latter, none had them. 



Hence no doubt remained that the " pigmented cells," really 

 constitute a developmental stage in the mosquito of these parasites ; 

 and this view was accepted both by Laveran and Manson, to whom 

 specimens had been sent. In June 1898, Manson published an illus- 

 trated paper concerning my researches, and showed that the pigmented 

 cells must in fact be the zygotes resulting from the process of 

 fertilisation discovered by MacCallurn. 



It remained to follow out the life-history of the zygotes. For 

 this purpose it was immaterial whether I worked with the avian or 

 the human parasites, since these are so extremely like each other. I 

 elected to work with the avian sjiecies, chiefly because the plague- 



