442 



BIOLOGY OF THE PROTOZOA 



digestive tract of some blood-sucking arthropod, insects in par- 

 ticular. The prevaihng opinion is that arthropods were the primary 

 hosts and that parasitism in the blood is the result of adaptation. 

 One such adaptation, and a very essential one, is the absence of 

 protective capsules about the sporozoites. The latter are always 

 formed in the primary or invertebrate host and are transmitted to 

 the vertebrates at the time of drawing blood. A sporozoite pene- 

 trates an erythrocyte and grows to an agamont which forms multiple 

 aga metes after a definite interval ; these agametes are liberated into 

 the blood where other erythrocytes are entered and the asexual 

 cycle is repeated. The parasites thus multiply rapidly by geo- 

 metrical ])rogression until enough blood elements are destroyed to 



Fig. 185. — Type of Haemogregarines. A, Hcemogregarina stepanowi; B, and C, 

 Lankesterella ranarimi. (Original.) 



produce the first marked symptoms of the infection. Hegner and 

 Taliaferro (1924) estimate about 150,000,000 parasitized blood 

 elements at this time in the case of human malaria, all parasites, if 

 derived from a single infection, undergoing sporulation at practically 

 the same time and liberating their toxin simultaneously into the 

 blood. The pyrexial attacks of chills and fever in human malaria 

 are thus accounted for. Ultimately the agametes develop into 

 gamonts which are usually easy to distinguish from the agamonts 

 and which are frequently differentiated into macrogametocytes and 

 microgainetocytes. The gametocytes are taken with the blood 

 into the digestive tract of an inverte])rate host (mosquitoes) where 

 the microgametes are formed and where union of gametes occurs. 

 The zygote, like that of some hsemogregarines, is motile and makes 



