Malarial Parasites 473 



painstaking investigation, the details of which are given in a paper 

 which can be found in the International Medical Annual,* 1890, 

 made the second great discovery intheparasitologyof malarial fever. 

 He found that, as Manson thought, the mosquito is the definitive 

 host of the parasite, but that the matter is much less simple than was 

 imagined, for the organisms taken up by the mosquito undergo a 

 complicated life cycle requiring about a fortnight for completion, 

 after which, not the water into which the mosquito might fall and into 

 which its contained organisms might escape, but the mosquito it- 

 self becomes the agent of infection. In other words, the parasites 

 taken up by the mosquito, after the completion of the necessary de- 

 velopmental cycle, are returned by the mosquito to new. human 

 beings, who thus become infected. Thus it was shown that malaria 

 is not a miasmatic disease at all, but that it is an infectious disease 

 whose parasites divide their life cycle between man and the mosquito, 

 each becoming infected by the other. The only role of the swamp is 

 to furnish the mosquitoes, and since these are only more numerous 

 where swamps are numerous, but may occur without swamps, the 

 not infrequent occurrence of malarial fevers apart from swamps is 

 also explained. Ross further discovered that all mosquitoes are not 

 equally susceptible of infection, and, therefore, not all able to spread 

 the infection. Indeed, he so carefully studied the mosquitoes as to 

 narrow the infectability and infectivity of mosquitoes down to one 

 single family, the Anophelinae, and to one single genus, Anopheles. 



There remained, however, one more important fact to be eluci- 

 dated, and one more mysterious body to be accounted for, viz., the 

 "flagellated" body that had misled Manson. This was found by 

 MacCallumf to be but the spermatozoit of the male parasite. While 

 observing one of the malarial parasites of birds Plasmodium dan- 

 liewskyi he saw one of these "flagella" swimming away from its 

 parent parasite, and followed it carefully, moving the slide upon 

 the stage of the microscope. It, and others of its kind, approached a 

 large globular parasite, to which one effected an attachment and into 

 which it entered. MacCallum realized that he had observed the 

 sexual fertilization of the organism. In 1900 two demonstrations 

 of momentous importance were made. First, Sambon and Low 

 went to Italy, to one of the most pestilential parts of the Campagna 

 Romana, and lived there during three months of the most malarious 

 4ime of the year in a mosquito-proof house, taking every precaution 

 to avoid mosquitoes, and escaped infection; second, anopheles 

 mosquitoes infected in Italy, by biting malarial patients, were taken 

 to England, where they were permitted to bite Dr. P. J. Manson 

 and Mr. George Warren, both of whom, after a period of incubation 

 suffered from malarial paroxysms and showed plasmodia in their 

 bloods. What may perhaps be regarded as the final step in the per- 



* E. B. Treat & Co., New York. 



t "Journal of Exper. Med.," 1898, m, 117. 



