64 AGRICULTURAL EXPERIMENT STATION. 



restricted. So far as known, not one of the whole great group 

 of the Sporozoa (Hterally, spore-producing animals), to which 

 the malarial organism belongs, can lead an active existence out- 

 side its appropriate host. Without exception they are obliga- 

 tory parasites," depending for their actual life upon the presence 

 of the host. "Probably all of them pass at least a portion of 

 their cycle within the very cells of their host. It is not surprising, 

 therefore, that not one of the Sporozoa has been found to lead 

 a saprophytic existence; that not one has been cultivated outside 

 of its host. With the bacteria, on the contrary, it is the exception 

 that they cannot be reared on nutrient media." 



"The host species of any Sporozoan are always few and closely 

 restricted ; frequently there is only one. Again, within the body 

 of the host these fastidious parasites elect only cells of a certain 

 tissue, and often even that tissue as it occurs in some particular 

 organ. Thus, the entire order Hsematozoa, to which the malarial 

 organism belongs, live in the red blood corpuscles; but in no other 

 part of the vertebrate body. Here they are completely shut in 

 from the outer world to^ which they have no direct access. The 

 blood-sucking habit of the mosquito affords them an exit : first, 

 to- the mosquito's body, where they pass through a cycle different 

 from that in the blood; and then, by the agency of the mosquito, 

 back tO' the circulation of man, beast or bird, according to the 

 special form oi malaria." 



The Hsematozoa differ from the rest of the Sporozoa, inas- 

 much as they live in two hosts, vertebrate and mosquito or some 

 other blood-sucking- parasite. In all forms of human malaria 

 yet investigated the carrier host is a mosquito of the genus 

 Anopheles. Tliis reveals the highly specialized nature of their 

 parasitism. Culex, so nearly related to Anopheles, so like it in 

 all its structure, physiology and habits, cannot become infected, 

 so far as we know, with any form of human malaria. On the 

 other hand, the malaria of birds, which differs slig^htly from 

 human malaria, cannot be transmitted by the agency of Anophe- 

 les, but only by Cnlex." 



"Can any species of Sporozoon pass from host to host without 

 an animal carrier? The answer to this question concerns us much. 

 As soon as it is demonstrated that even one form of the disease- 

 producing Sporozoon can pass from one vertebrate to another, 

 we naturally inquire whether this may not be the case sometimes 

 with the malarial parasite also. The answer is, emphatically, no ! 

 Notwithstanding there are numerous Sporozoa w^hich pass from 

 one host tO' enter another of the same species, with an interme- 

 diate sojourn in the outer world, this passage is made in the 

 form of encysted spores. Encystment is absolutely necessary to 



