184 COMPARATIVE ANATOMY 



vermiform shape, and moves actively through the blood con- 

 tained in the mosquito's stomach. It is because of its activity 

 that it is frequently referred to as the ookinete, but it should be 

 borne in mind that the latter name signifies nothing more than 

 that the zygote exhibits a special adaptation to the particular 

 conditions under which it is formed. It has to escape from 

 the digestive cavity of the mosquito and take up a position 

 favourable to its future development. It accordingly bores 

 its way through the epithelial wall of the stomach, comes to 

 rest in the sub-epithelial tissues, again assumes a sub- 

 spherical shape, a thin and delicate cyst wall is formed 

 around it, and it passes into the stage known as the 

 oocyst. In this condition it is truly parasitic on the mosquito 

 that is to say, it absorbs nutriment from the juices of its 

 host, and grows so considerably in size that the oocysts for they 

 may be numerous form a number of nodules projecting from 

 the wall of the stomach into the surrounding cavities of the 

 body. As an oocyst grows its nucleus divides again and again 

 and eventually the cytoplasm divides into as many masses as 

 there are nuclei, the masses not being quite separate, but con- 

 nected with one another by radiating strands of protoplasm 

 (fig. 39, J and JT). As is so commonly the case, the whole 

 of the protoplasm is not used up in this way, but a certain 

 amount is left over as an anucleate mass of residual protoplasm 

 containing all the melanin granules. Each nucleated proto- 

 plasmic mass is comparable with a sporoblast of Monocystis, 

 and may be called by the same name. 



The sporoblasts now proceed to form sporozoites. In each 

 the nucleus divides repeatedly to form a number of small 

 nuclei, which travel to the periphery of the sporoblast. The 

 surface of the cytoplasm grows out into a number of slender 

 processes (fig. 39, Z), each of which takes a nucleus with it, 

 and thus a large number of elongate, slender, fusiform 

 sporozoites are formed, radiating outwards from a central 

 mass of residual protoplasm, which is of course the remnant 

 of the body of the sporoblast (fig. 39, M). This mass of 

 residual protoplasm may contain one or more abortive 

 nuclei. 



The formation of the sporozoites occupies some ten to 

 twelve days, during which time the oocyst increases greatly 

 in size. Finally it bursts, and the sporozoites are set free 



