190 MOSQUITOES OF NORTH AMERICA 



f er from one host to the other. When a mosquito sucks the blood of an individual 

 harboring gametes in a state of complete maturity, these, finding themselves in 

 a temperature favorable to their further development, copulate. The male 

 gamete or microgametocyte throws off prolongations in the form of flagella, 

 called micro gametes, and these seek to unite with the female gametes or macro- 

 gametes. Upon contact with the microgamete the macrogamete prolongs its pro- 

 toplasm, forming the so-called cone of attraction. This first stage, brought about 

 by the union of microgamete and macrogamete, is designated as the ookinete and 

 can be easily observed in any mosquito, or even in blood transferred from the 

 human body directly to the microscope slide. However, the succeeding stages can 

 only take place in the stomach-wall of an Anopheles, as demonstrated by Grassi. 

 The ookinete, to undergo further development, penetrates into a cell of the 

 stomach-wall of the transmitting insect and there transforms to the oocyst. 

 In the process of growth of the oocyst further stages occur, first, by its division 

 the sporohlasts, and from these, by further division, the sporozoits. When the 

 oocyst is mature it bursts, liberating the sporozoits which thus pass into the 

 general body-cavity of the host. The sporozoits now find their way into the 

 salivary glands of the host and there they remain until the mosquito, in biting, 

 forces them, along with the saliva, through its proboscis into a human being. 

 Then the other, sporogonic, or asexual, cycle begins. 



The sporozoits when introduced into the blood at once enter the red blood- 

 corpuscles and are now called schizonts. The schizont grows within the blood- 

 cell and finally undergoes a process of division. The products of this division 

 are called merozoits. These merozoits are liberated but at once enter another 

 blood-cell where they again go through the stages of schizonts and merozoits. 

 Parallel with this development the schizonts begin also to produce gametes or 

 sexual elements. These sexual elements, however, can not copulate within the 

 human organism on account of the unfavorable temperature. To enable them to 

 carry out this function, and to develop further, they require the intervention of 

 an intermediary host, as already described. 



In the macrogametes there is furthermore to be observed a curious phe- 

 nomenon first determined by Schaudinn ; we refer to the parthenogenesis which 

 is nothing less than a stage which insures the survival of the plasmodium. The 

 macrogamete when mature, in order not to disappear from the organism, as 

 happens with the microgametocyte, divides in the same manner as the schizonts. 

 Probably each product of this division then again becomes a macrogamete. In 

 this parthenogenesis of the macrogametes lies the explanation of the cases of 

 malaria with periodical attacks recurring at great intervals of time. This phe- 

 nomenon has already been determined for Plasmodium vivax and Plasmodiura 

 falciparum, and there is every reason to believe that the same occurs with 

 Plasmodium malarice. An ancient Roman imprecation " Quartana te teneat " 

 is significant in this connection as it can be given no other interpretation. 



As already stated there exist three well-defined species of Plasmodium which 

 cause human malaria: Plasmodium malarice, P. vivax and P. falciparum. For 

 the diagnosis of the species two methods are employed. One is to examine a drop 



