16 THE PROTOZOA-THE DAWN OF LIFE 



The life-cycle typical of a Sporozoon may be divided into 

 three main periods : the first chiefly the period of growth, during 

 which the minute sporozoite, by the absorption of food from the 

 host, is growing into what is termed the sporont ; the second is 

 the period of reproduction or multiplication, which is accom- 

 panied by conjugation, and the resulting formation of a large 

 number of minute, often sickle-shaped germs, destined ultimately 

 to spread the species ; while the third is the period of rest, dur- 

 ing which the parasitic germs pass out from the old host to effect, 

 if circumstances are favourable, the infection of a new host. 



Up to the present three modes of infection have been observed. 

 The commonest, which may be called casual infection, takes 

 place by the host accidentally swallowing the spores along with 

 its food. The second, or inoculative, method is typical of the 

 malarial parasites, and is effected through the agency of an inter- 

 mediate host, such, for example, as the spot-winged anopheline 

 mosquito. The third is a very rare type, that of hereditary 

 infection, in which, as has been demonstrated in the silk-worm 

 disease, caused by a sporozoon parasite called Glugea bombycis, 

 the parasites penetrate the ovum of the host and produce spores 

 there, which germinate and infect the new generation of the host. 



It would be impossible here to describe all the numerous 

 species of Sporozoa, their appearance, habits, and life-history ; 

 but on account of the far- reaching economic importance of their 

 discovery, special mention must be made of those Sporozoa which 

 are the cause of malarial fevers in man. To Laveran, a French 

 medical man, we owe the discovery of the minute parasite in the 

 red blood-corpuscles of man which is the cause of malaria, while 

 Golgi demonstrated the coincidence of the stages of the inter- 

 mittent fever with those of the life-cycle of the parasite in the 

 human host which is the febrile cycle producing auto-infection 

 (regularly recurring attacks of fever every one, two, of three 

 days) 1 of the patient. 



In the blood of the affected person the parasite passes through 

 an asexual cycle, entering the red blood-corpuscle as an amoeboid 

 organism, and therein attaining its full size and becoming filled 

 with granules of black pigment, which are probably a decomposi- 

 tion product of the red colouring matter (haemoglobin) of the 



1 Termed, according to their periods, quotidian, tertian, and quartan fevers. 





