PARASITIC PROTOZOA 361 



sexual stage of the development of these parasites takes place 

 in the stomach of the mosquito. 



It was found that certain of the parasites in the blood do not 

 go on with their development there. When these particular 

 parasites are taken into the stomach of most mosquitoes they 

 are digested with the rest of the blood. But when they are 

 taken into the stomach of mosquitoes belonging to the genus 

 Anopheles, or other closely related genera, they are not digested 

 but go on with their development. Conjugation takes place, 

 resulting in the production of a new form of the parasite, the 

 zygote, that makes its way through the walls of the stomach on 

 the outside of which it forms a little nodule. Within these 

 nodules further division and development takes place, until 

 finally the nodules burst open and many thousand minute rod- 

 like organisms, sporozoites, formed by the repeated division 

 of the zygote, are turned loose into the body cavity of the 

 mosquito. Owing to some unknown cause these little organ- 

 isms collect in the large vacuolated cells of the salivary glands 

 of the mosquito, and when the mosquito bites a man they pour 

 down through the ducts with the secretion into the beak 

 and are thus again introduced into the human circulation. 



The nodules, or cysts, on the walls of the stomach of the 

 mosquito may contain as many as ten thousand sporozoites, 

 and as many as five hundred cysts may occur on a single 

 stomach. 



It takes ten, twelve, or more days from the time the parasites 

 are taken into the stomach of the mosquito before they can 

 complete their transformations and reach the salivary gland, 

 the time depending on the temperature. So it is ten or twelve 

 days, or sometimes as long as eighteen or twenty days, from the 

 time an Anopheles bites a malarial patient before it is dangerous 

 or can spread the disease. On the other hand, the sporozoites 

 may lie in the salivary gland alive and virulent for several 

 weeks. It does not give up all the parasites at one time, so that 

 three or four or more people may be infected by a single 

 mosquito. 



The findings of Ross have been verified many times, and 

 many experiments have proven beyond any doubt that this is 



