IX] LIFE-CYCLE 157 



but the most favourable temperature is much higher (24- 



30 c.). 



The microgametocyte gives rise to 4 to 8 microgametes, 

 and the macrogametocyte undergoes division of the nucleus 

 and extrusion of a polar body before becoming a macro- 

 gamete ready for fertilization. After this process has 

 taken place, the male and female pro-nuclei are said to remain 

 apart for some time before uniting to form the single nucleus 

 of the ookinete. At a temperature of 28 C. the latter has 

 penetrated the gut -wall and become an oocyst by the end of 

 about 40 hours after the mosquito's feed. At this stage the 

 oocyst is an almost transparent spherical body containing 

 thin brown strands of melanin, and is surrounded by a well- 

 defined wall. The nucleus has already commenced to divide 

 and several daughter-nuclei are scattered through the cyto- 

 plasm. 



The following day the oocyst increases in size and the sporo- 

 blasts begin to separate off from each other. The melanin is 

 aggregated together into clumps lying between the sporoblasts. 

 On the fourth day the oocyst is now seen to contain well-defined 

 sporoblasts, each of which is covered by the growing sporo- 

 zoites. During the fourth and fifth days the oocyst still 

 continues to grow, attaining a diameter of about 50^ and 

 projects outwards into the coelom of the insect. The sporo- 

 zoites become completely formed and thus the oocyst is trans- 

 formed into a hollow sphere containing innumerable sporozoites 

 each about 14 ^ in diameter. About the seventh or eighth 

 day the cyst ruptures and the numerous sporozoites are liberated 

 into the ccelomic fluid. From here large numbers find their 

 way to the salivary glands, but some of them bore into the 

 developing eggs in the ovary. The fate of these latter is un- 

 known, for up to the present, beyond this fact, there is nothing 

 to support the view that the infection is transmitted to the 

 offspring of an infected mosquito. 



The above cycle of sporogony is thus complete in about 

 eight days, under these favourable conditions of temperature, 

 and after the sporozoites have entered the salivary glands 

 the mosquito becomes infective. At lower temperatures the 



