ENVIRONMENT AND SEX 295 



ensues in the salivary glands of the tsetse, preceding 

 the resumption of the stumpy, infective trypanosome 

 stage. 



The development of the malarial parasites affords an 

 example of the necessary transference of parasites to 

 a totally different environment, in order to complete 

 their own development. While sex forms commence 

 to develop in the human or bird victim, so far as is 

 known, they never reach maturity. That only occurs 

 when they pass into the gut of an Anopheline or 

 Culicine, as the case may be. There are -many 

 factors involved. Whether it is merely the decrease 

 in temperature that is directly responsible for 

 the production of sex forms and ultimately cysts, 

 or whether chemical stimulation is the cause, re- 

 mains to be proved. Probably both factors help, as 

 does also the degree of concentration of the medium. 



The development of sex forms is frequently 

 independent of change of host. Coccidia produce 

 their sex forms within the same host as the asexual 

 forms, and it seems that the diminution in nourish- 

 ment, together with chemical alterations in the food 

 available the indirect result of the action of the 

 parasite are largely responsible for the production 

 of the sexual forms of the Eimeria. The nutrition 

 seems of paramount importance in these cases. 

 When there is an abundance of food, schizonts only 

 are produced. When the host is overrun with para- 

 sites, and the food-supply is failing, then the sexual 

 phases of the parasite are formed in order to produce 

 stages capable of resisting extracorporeal conditions, 

 and subsequently becoming transferred to a new host. 



