PROTOZOA 167 



within a red blood cell the merozoite becomes rounded off as a small 

 mass of protoplasm with a single nucleus, and begins to grow at the ex- 

 pense of the blood corpuscle. The parasite absorbs haemoglobin and 

 this is transformed into a pigment consisting of haematin which appears 

 in the cytoplasm of the parasite as characteristic black or brown 

 granules. These granules are also found in infections of the allied genus 

 Haemoproteus but not Leucocytozoon. After a few days of growth the para- 

 site multiplies by schizogony, giving rise to merozoites, the number of 

 which varies in different species. These burst out of the blood corpuscle, 

 which is entirely destroyed, and escape into the plasma of the bird. 

 Here each merozoite attaches itself to a healthy blood corpuscle and 

 actively forces its way in. Growth follows and schizogony is repeated 

 all over again. The periodical attacks of fever, so characteristic of 

 malaria, occur when the corpuscles are ruptured by the escaping para- 

 sites and poisonous substances are liberated in the blood stream. 



After several generations of merozoites have been produced a 

 striking change occurs. The merozoites develop into gametocytes 

 instead of schizonts which remain within the red blood corpuscles until 

 they are ingested by a mosquito feeding upon the blood of the bird. 

 Even at this stage it is possible to distinguish between the male and 

 female gametocytes. In the former the protoplasm stains faintly and 

 the nucleus is large and diffuse, while in the latter the cytoplasm stains 

 deeply and the nucleus is small. 



On entering the stomach of the mosquito (Fig. 3) the gametocytes, 

 apparently affected by the change of temperature, burst out of the 

 restraining membrane of the blood corpuscles. Long thin processes are 

 then formed from the surface of the male cell (microgametocyte) which 

 lash about continuously. These are the microgametes, which break 

 loose at intervals and swim about among the corpuscles in the stomach 

 of the mosquito. Meanwhile the liberated female cell (macrogameto- 

 cyte) remains as a more or less motionless sphere with the nucleus dis- 

 placed somewhat towards the surface of the cell. When a microgamete 

 comes near, it quickly penetrates the macrogamete and its nucleus 

 unites with that of the female cell. The spherical zygote resulting from 

 fertilisation rests for a while and then begins to elongate until it assumes 

 a wormlike form. It then makes its way through the contents of the 

 stomach by a gliding and bending motion until it reaches the epithelial 

 lining of the gut. Here it penetrates between the cells and finally comes 

 to rest under the elastic membrane which covers the outer surface of the 



