53 



the first febrile attack. If this view be true, then it 

 follows that the gametes develop from forms already 

 present in the system, viz., the asexual forms, and 

 possibly the divergence into sexual forms takes place 

 from the youngest form of the parasite, i.e., the spore. 

 But it is possible that the divergence takes place at a 

 stage previous to the youngest form of parasite, i.e., 

 at a stage immediately succeeding the entry of sporo- 

 zoits into the blood, so that we have from the first 

 indifferent and sexual forms present, involving, indeed, 

 the existence of three kinds of sporozoits. Sexual 

 development has been supposed to proceed mainly in 

 the internal organs, e.g., bone marrow ; but it is being 

 gradually recognized that young forms of gametes are 

 also found in the circulation. Let us suppose that 

 we are now dealing with fully developed gametes in the 

 blood. We shall proceed to describe the further 

 changes undergone in the mosquito. The male cell 

 is, as we have seen, called the microgametocyte ; the 

 female cell, the macrogamete. These we can dis- 

 tinguish in the blood. Further flagellation can be 

 observed, i.e., the protrusion of so-called ' flagella,' 

 i.e., microgametes or spermatozoa. These ' flagella ' 

 break off and fertilize the female cell, the macrogamete, 

 a process which has been seen in Halteridium of birds, 

 but only once in man. 



This fertilized female cell or egg is known as a 

 Zygote. At a slightly later stage it is called the 

 V ermiculus or Ookinet (Figs. 10 and 14). Both these 

 terms are suitable ones, for the first describes the fact 

 that the zygote becomes worm-like in shape, and the 

 second, that the zygote moves. The Vermiculus stage 

 can be seen on the slide in the case of Halteridium, 

 but in the case of malaria parasites, only by taking the 

 blood from the stomach of the mosquito after a suitable 



