236 ENTOMOLOGY 



After several successive asexual generations, there are produced 

 merozoites which develop no longer into schizonts but into sexual 

 forms, or gametes. These occur in red blood corpuscles either as macro- 

 gametes (female, 7, 8) or as micro gametocytes (male, /a, 8a), in which 

 forms the parasite is introduced into the stomach of a mosquito which 

 has been feeding upon the blood of a malarial patient. The macro- 

 gamete now leaves its blood corpuscle and becomes spherical (9) , as does 

 also the microgametocyte (pa); but the latter puts forth a definite 

 number (six, in P. prcecox, pb) of flagella, or microgametes, which separate 

 off as motile male bodies, capable of fertilizing the macrogametes. A 

 microgamete penetrates a maciogamete (10) and the nucleus of the one 

 unites with that of the other. The fertilized macrogamete now becomes 

 a migrating cell, or ookinete (u), which penetrates almost through the 

 wall of the stomach of the mosquito (12) and then becomes a resting cell, 

 or cyst. This oocyst (15) grows rapidly and its contents develop, by 

 direct nuclear division, into sporoblasts (14, 15), which differentiate into 

 spindle-shaped sporozoites (16, 17). The sporozoites are liberated into 

 the body cavity of the mosquito, carried in the blood to the salivary 

 glands (as well as elsewhere) and thence along the hypopharynx into the 

 body of a human being, bird or other animal attacked by the insect. 



The role of the mosquito as the intermediary host of malarial organ- 

 isms was discovered by Manson and Ross and confirmed by Koch, Stern- 

 berg and others. It has been found repeatedly that certain mosquitoes 

 (Anopheles) after feeding on the blood of a malarial patient can transmit 

 the disease by means of their "bites" to healthy persons. Thus, Anoph- 

 eles mosquitoes were fed on the blood of malarial subjects in Rome 

 and then sent to London, where a son of Dr. Manson allowed himself 

 to be bitten by the insects. Though previously free from the malarial 

 organism, he contracted a well-marked .infection as the result of the 

 inoculation. 



Furthermore, it is highly probable that malaria cannot be trans- 

 mitted to man except through the agency of the mosquito. This appears 

 from the oft-cited experiment of Doctors Sambon and Low on the Roman 

 Campagna, a place notorious for malaria. There the experimenters 

 lived during the malarial season of 1900, freely exposed to the emanations 

 of the marsh and taking no precautions except to screen their house 

 carefully against mosquitoes and to retire indoors before the insects 

 appeared in the evening. Simply by excluding Anopheles mosquitoes, 

 with which the Campagna swarmed, these investigators remained per- 

 fectly immune from the malaria which was ravaging the vicinity. 



