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 conjugation, the young sporozoites grow and multiply in the blood corpuscles 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 prevalence 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 protoplasm, 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 parasites are transferred directly from one individual to another by sticking to the proboscis. It is probable that the mosquito is the original, or primary, host of the malaria-organism, and that man and birds are secondary hosts, from which the parasites return to the primary one for the vital function of conjugation.
