UNICELLULAR FORMS 



75 



become single megagametocytes, or eggs. A single sperm unites 

 with an egg and this fertilized tgg enters the wall of the stomach 

 of the mosquito. Here it divides rapidly into a number of indi- 

 viduals, known as sporozoites. Then they burst free into the body 

 cavity of the mosquito and the parasites finally enter its salivary 

 glands. When this mosquito bites another human some of these 



f}£D BLOOD CORPUSCLES 



SPOROZOITE 



TROPHOZOITE 



SPOROZO/Te 



SPOROZOITE 



GAMETOCYTES 



MICROCAMETE 



MACROGAMETE 



SPERM 



FERTILIZATION OF EGG 



STOMACH WAU 

 PLASMODIUM zygote' 



Fig. 23. — Scheme representing the life history of the sporozoon, Plasmodium 

 vivax, which is the cause of one type of human malaria. Note that the divisions 

 occurring in the human red blood corpuscles are asexual fissions, while the 

 divisions in the wall of the stomach of the mosquito follow the fertilization of 

 the egg and are therefore sexual. (Re-drawn from Kiihn: Grundriss der allgemeinen 

 Zoologie. Georg Theime, Leipzig.) 



sporozoites are injected into the wound with the poison that the 

 mosquito injects to keep the blood from clotting. In the human 

 body they penetrate into the red blood cells and there form tropho- 

 zoites. Thus there are two types of reproduction in the life cycle of 

 the parasite. In the body of the mosquito reproduction is preceded 

 by the union of an egg and a sperm cell; this is known as sexual 



