I MONOCYSTIS 55 



are formed each of which takes on a spindle shape and surrounds itself 

 with a characteristic boat-shaped cyst (" Pseudo-navicella " of micro- 

 scopists). Within this cyst (Fig. 21^/6) the zygote divides with mitosis 

 three times, becoming resolved into eight sausage-shaped bodies, each 

 with a nucleus near its centre, the sporozoites (Fig. 21, /7). So far as is 

 known nothing more happens during the life of the worm but if the worm 

 dies and its body disintegrates or if it is digested by a bird the cysts are 

 set free, their walls break down, and the " pseudo-navicellae " become 

 distributed through the soil. Presumably when swallowed by an Earth- 

 worm the boat-shaped cyst is dissolved, the eight sporozoites are set 

 free (Fig. 21, I) and make their way through the tissues of the worm to 

 the seminal vesicle where they bore into sperm-morulae and start the 

 life-cycle afresh. 



Plasmodium 



There exist probably a number of different species of malarial parasite 

 and of these three have had their life-histories fully worked out. The 

 relatively small differences in detail which mark off the species from 

 one another will be indicated after a general sketch of the life-history 

 has been given (Fig. 23). 



As is well known malarial fevers are characterized by the recurrence 

 of febrile attacks at definite intervals such as 48 hours or 72 hours. If 

 a drop of blood taken from a patient at the end of one of his febrile 

 attacks be examined microscopically the parasite will be found in the 

 amoebula stage — a minute amoeba-like creature which creeps about 

 slowly in the substance of a red blood -corpuscle (Fig. 23, A). The 

 amoebula nourishes itself at the expense of the corpuscle and increases 

 gradually in size (Fig. 23, B). As it does so a characteristic feature is 

 the appearance within its cytoplasm of minute particles of a dark brown, 

 almost black, pigment — one of the iron-containing pigments known to 

 ,the chemist as melanins, a product of the digestion by the parasite of 

 the red iron-containing pigment of the blood (Haemoglobin — see p. 141). 

 It is also very usual for fluid to accumulate within the amoebula as a 

 conspicuous vacuole which gradually attains to such a size that the 

 parasite assumes the appearance of a signet-ring— the nucleus being 

 pushed to one side (Fig. 23, C). With further growth the vacuole dis- 

 appears and the parasite occupies the whole of the interior of the corpuscle. 

 The portion of the life-history so far described is the trophozoite 

 phase. The full-grown trophozoite now becomes a schizont, i.e. a stage 

 which reproduces by schizogony. Its nucleus divides several times 

 (Fig. 23, D) and the cytoplasm segments into a number of fragments 



