624 Insects and Disease 



never found in insects. of the same species when fed on healthy birds or on 

 birds containing the other parasite, called Hcemamaeba danilewskii. 



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

 parastic 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 /a%asfedon birds containing H. relicta 178, or 72 

 per cent., contained 'pigmented cells.' But, out of 41 Culex jatigans 

 fed on a man containing crescentic gametocytes, 5 on a man containing imma- 

 ture tertian parasites, 154 on birds containing H. danilewskii, 25 on healthy 

 sparrows, and 24 on birds with immature H. relicta or a total of 249 insects, 

 all carefully examined not one contained a single 'pigmented cell.' 



"Another experiment was as follows: Three sparrows, one containing 

 no parasites, another containing a moderate number of H. relicta, and the 

 third containing numerous H. relicta, were placed in separate cages within 

 three separate mosquito-curtains. A number of Culex jatigans, 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 para- 

 sites were found to contain altogether 202 'pigmented cells,' or an average 

 of 29 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 23 of the former lot, 22 were found to have pigmented 

 cells; while out of 1 6 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 illustrated paper concerning 

 my researches, and showed that the pigmented cells must in fact be the 

 zygotes resulting from the process of fertilization discovered by MacCallum. 



"It remained to follow out the Kfe-history of the zygotes. For this purpose 

 it was immaterial whether I worked with the avian or the human parasites, 



