198 POPULAR SCIENCE MONTHLY. 



salivary glands of the insect are located. They work their way through 

 the cell wall of this gland, into the gland cells, from which they are 

 drawn with the secretion and are finally poured into the lumen of the 

 gland (Fig. 3, C). When the proboscis of the mosquito is inserted into a 

 human host, and the salivary secretion is poured out, the sporozoites 

 pass with it into the blood, and thus effect infection of a new host. 

 Provided with their new potential of vitality resulting from conju- 

 gation, the young sporozoites grow and multi])ly in the blood cor- 

 puscles until they are numerous enough to cause the well-known 

 symptoms of malaria. Coming from the same brood, so to speak, they 

 have a similar rate of growth and multiplication, and so liberate their 

 melanin granules throughout the blood system of their human host, at 

 approximately the same time. 



The question is frequently asked: Is the mosquito the only agent 

 in the transmission of malaria? and when this is answered by the 

 somewhat modified affirmative, 'Yes, so far as we know,' it is usually 

 followed by the query: 'Why does malaria follow bad drainage, the 

 digging of sewers, laying of gas pipes, etc.?' This question may be 

 answered in two ways: First, it must be shown that these so-called 

 malarial fevers, which accompany such conditions, are in reality true 

 malaria; it is quite possible that hasty diagnosis in many cases gives 

 a wrong impression of the pretalence of this disease. Second, it is 

 conceivable that sporozoites may be carried in the blood, as typhoid 

 is said to be frequently carried in the digestive tract, without causing 

 symptoms of the disease until the natural resistance of the host is 

 weakened by decreased vitality, which may be brought about by bad 

 air, or by other means. 



It is quite possible that some other means of transmission than the 

 mosquito exists. The flea, for example, and other insects that prey on 

 man must be examined with this end in view. There is no reason to 

 believe that the sporozoites can be liberated in water, or suspended 

 in the dust of the air, and live, for, of all Sporozoa, the blood-infesting 

 forms are not protected against an external life. Thus we have seen 

 that the sporozoites of the gregarine or of Coccidium are incased in a 

 firm, calcareous shell, which protects them from drying and from other 

 dangers that might be encountered. With the malaria-organism there 

 is no such coating; the sporozoites are at all times naked bits of pro- 

 toplasm, which soon dry up and die, when exposed to the air or placed 

 in water. This fact also refutes the argument made by Bignami and 

 others that the })arasites are transferred directly from one individual 

 to another by sticking to the proboscis. It is probable that the mos- 

 quito is the original, or primary, host of the malaria-organism, and that 

 man and birds are secondary hosts, from whicli the parasites return 

 to the primary one for the vital function of conjugation. 



