112 



Insects and Disease 



cycle of these parasites were at a loss to know 

 where the sexual stage took place. In the mean- 

 time studies of other parasites more or less closely 

 related to Plasmodium showed that the sexual stage 

 occurred outside the vertebrate host. The remark- 

 able work of Dr. Smith on the life-history of the 

 germ that causes the Texas fever of cattle had 

 a strong influence in directing the search for this 

 other stage of the malarial parasite. Another thing 

 that indicated that this sexual generation must 

 take place outside the body of the vertebrate host 

 was the fact that the investigators found that the 

 parasites in certain of the cells did not sporulate 

 as did the others. When these individuals were 

 drawn from the circulation and placed on a slide 

 for study it was found that they would swell up and 

 free themselves from the inclosing corpuscle and 

 some of them would emit long filaments which 

 would dart away among the corpuscles. 



Many men have worked on this problem, but 

 perhaps the most credit for its solution will always 

 be given to Sir Patrick Manson, the foremost au- 

 thority on tropical diseases, and to Ronald Ross, a 

 surgeon in the English army. There is no more 

 interesting and inspiring reading than that which 

 deals with the development of the hypothesis by 

 Manson and the persistent faith of Ross in the 



