INVBETBBRATA. 419 



an alveolar structure of unstained linin, but there is no 

 central body similar to the structures called nucleolus or 

 karyosome in other cases. When in a free state the 

 sporozoite exhibits spontaneous movements, but it soon 

 penetrates into an epithelial cell of the Lithobius and then 

 becomes motionless and spherical. The nucleus now 

 undergoes important changes. The chromatin granules 

 collect gradually to the centre of the spherical cell and 

 become fused with another substance to form a " karyo- 

 some." At this stage the parasite undergoes division 

 without conjugation, and this phase in the life-history is 

 called schizogony (Greek schizo, to split), the cell being 

 called a schizont. 



10. Schizogony. In the process of schizogony the 

 nucleus divides directly into a number of nuclei, which 

 travel towards the surface of the schizont. The proto- 

 plasm then grows out into small projections, each taking a 

 nucleus with it, and then each projection separates, form- 

 ing what is called a merozoite. The central part of the 

 protoplasm remains as a residuum. Each merozoite 

 escapes from the cell where it was formed, passes into 

 another epithelial cell and there grows into a new schizont, 

 and several schizogonous generations succeed each other. 



11. Sporogony. After a period of schizogony lasting 

 about five days the process of sexual reproduction begins, 

 and this is termed sporogony, because it results in the 

 formation of spores. The merozoites now grow into cells, 

 which give rise to the sexual or conjugating elements, 

 and these elements being now termed gametes, the cells 

 which form them are called the gametocytes. The gametes 

 are of two kinds one small and actively motile, the micro- 

 gametes ; the other large and motionless, the megagametes. 

 Obviously the former resemble spermatozoa, the latter 

 are similar to ova, in the Metazoa, and they may be 

 regarded as male and female. In the male gametocyte 

 the chromatin separates from the karyosome and travels 

 to the surface, leaving the rest of the karyosome at the 

 centre. At the surface the chromatin forms a number of 



