426 ZOOLOGY. 



protoplasm forms a long slender motile body, the micro- 

 gamete, analogous to a spermatozoon. These microgametes 

 are themselves filamentous and perform rapid undulatory 

 movements ; they are not provided with distinct flagella as 

 in Coccidiuni. 



In the megagametocyte (fig. 207, IX.6) the nucleus 

 remains undivided, and ejects a portion of its substance. 

 Conjugation then takes place ; a cone of attraction is 

 formed to meet the apex of one of the microgametes which 

 enters the megagamete, and whose chroniatin forms a 

 male pronucleus within the latter. The two pronuclei now 

 unite and the zygote is formed. 



4. Sporogony. The zygote instead of secreting a cyst 

 loses its spherical shape and becomes elongated and actively 

 motile : it is called a vermicule or ookinete. It bores its 

 way through the epithelium of the mosquito's stomach and 

 becomes encysted in the tissues outside the epithelium. 

 Here it increases greatly in size, the cysts in an infected 

 mosquito forming small rounded projections on the external 

 surface of the insect's stomach. In this stage the zygote 

 may be called the oocyte ; its nucleus divides without 

 mitosis, and then the protoplasm divides into corresponding 

 portions. The products of this division evidently corre- 

 spond to the sporoblasts of the Coccidium, and in their 

 formation a small quantity of residual protoplasm is left 

 containing the pigment granules of the zygote. 



The sporoblasts now behave very differently from those 

 of Coccidium or Monocystis : they form no external cyst, 

 but continue to grow, the nucleus in each divides into a 

 large number of nuclei; these pass to the surface of 

 the sporoblasts which grows out into a corresponding 

 number of spindle-shaped processes. In this way each 

 sporoblast gives rise to a large number of slender sporo- 

 zoites (Fig. 207, XVIII.) and a few masses of residual 

 protoplasm. Each sporozoite is about 14 p. long and 1 /A. 

 broad, and a single oocyst may contain several thousands 

 of them. 



When the sporozoites are completely developed the 

 bursts and they are set free in the haemocoel of the 



