134 LIFE-CYCLE OF MALARIAL PARASITE [CH. 



Conjugation now takes place, one of the microgametes 

 boring its way into a macrogamete. This is followed by union 

 of the two nuclei, and the resulting structure is known as the 

 zygote. At first the zygote is spherical, but it soon elongates 

 into a small worm-like body which is actively motile, moving 

 about in much the same way as a gregarine". The zygote is 

 then known as the ookinete or, by some writers, the vermicule, 

 because of the character of its movements. 



The whole of this process, including the formation of the 

 gametes and their union to form the ookinete, takes place in 

 the stomach (= hinder part of the mid-gut) of the mosquito, 

 and is completed within a comparatively short time. All the 

 above stages may be observed by placing a drop of blood con- 

 taining the gametocytes on a slide and slightly breathing on 

 it. If the preparation be now covered with a cover-glass and 

 examined under an oil-immersion lens, all the stages in the 

 evolution of the micro- and macrogametes, and their subse- 

 quent union to form the ookinete, may be followed in the living 

 state. 



The subsequent development of the ookinete can take place 

 only in the mosquito, and we shall proceed with the description 

 of the changes which it undergoes in this site. 



After moving about for some time, the ookinete bores 

 through the epithelium of the gut-wall and comes to rest 

 between this layer and the tissues immediately surrounding 

 it. Here the parasite becomes rounded off and secretes a thin 

 cyst-wall. This form is known as the oocyst and is parasitic 

 upon the mosquito, for it grows considerably in size, absorbing 

 nourishment from the surrounding tissues. During its growth, 

 the gut-wall is bulged out towards the body-cavity, and conse- 

 quently at this stage the stomach of the infected mosquito 

 has a very characteristic appearance (Fig. 53). As many as 

 500 oocysts have been found in the stomach-wall of a single 

 infected Anopheles. 



As the oocyst grows, its nucleus divides into a number of 

 daughter-nuclei, each of which becomes surrounded by a mass 

 of cytoplasm and gradually separated off from its neighbours. 

 These are known as the sporoblasts ; they are irregular in form 



