40 TAXONOMY 



their protoplasmic substance to which the name pseudopodia has been given. 

 The amoebae multiply rapidly by simple fission and, like many other 

 unicellular organisms, undergo from time to time a process of encystment. 

 The cysts are enclosed in a thick membrane which possesses great 

 powers of resistance, and from them, under suitable conditions, the new 

 amoebae emerge. Inasmuch as the amoebae are common in ponds, and 

 ditches, and in damp earth, it is evident that they can obtain easy access 

 to the human body. The best known pathological species, Amoeba coli, 

 has been frequently described in cases of dysentery. But it is not present 

 in all cases, and it seems to occur sometimes in the healthy intestine, 

 so that its causal connexion with dysentery has yet to be proved. Pure 

 cultures, free from bacteria, have not yet been obtained, nor has it been 

 possible to cause infection by inoculation in animals. An amoeba-like 

 organism of very doubtful legitimacy has been described associated with 

 cow-pox {Cytoryctes variolae), but it is certainly not the cause of the 

 disease. The great prizes for the discovery of the vaccine microbe have 

 yet to be won. 



Closely related to the amoebae is, no doubt, the Plasmodium malariae 

 {Haemamoeba, Laveranid), which occurs in the blood in cases of malarial 

 fever. It is a very minute amoeboid organism which lives in the plasma, 

 and (in greater numbers) inside the red blood corpuscles. At first colour- 

 less, it becomes pigmented later on from the accumulation of granules of 

 a black substance {Melanin) which arises from the decomposition of the 

 haemoglobin. The plasmodia are seen to be most numerous during the 

 periodical attacks of fever, one, three, or four days as the case may be, which 

 characterize the disease. They then either become disintegrated or break 

 up into a number of small spheres which have been called spores. Then 

 the number of amoebae increases again till the next paroxysm. Whether 

 these are true spores or not, is, like much else written about the malaria 

 parasites, uncertain. They have not been seen to germinate, nor has it been 

 possible to obtain pure cultures of the organisms. It seems, however, 

 certain that they are really the cause of the disease, for its symptoms 

 can be set up in a healthy body by the injection of blood containing the 

 plasmodia. How the parasites enter the body, whether by the respiratory 

 tract or the intestine, or, what is more probable, through minute wounds, 

 such as insect-stings, is not yet known. Their habitat and mode of life 

 outside the body are also unknown, but there can be no doubt that they 

 are saprophytic in malarial districts *. 



In the blood of birds, reptiles and amphibia, there are sometimes found 



* Researches now in progress on the west coast of Africa and elsewhere seem to show that certain 

 species of mosquitoes are the bearers of the malarial parasites which are introduced into the human 

 body by the insects' probosces. 



