626 Insects and Disease 



which produce the cluster of cells constituting a multicellular animal, so, 

 in this case, the fertilized ovum, or zygote, gives rise to blasts, each of 

 which, however, becomes a separate animal. Prof. Ray Lankester suggests 

 for the blasts of the Haemamcebidae the simple term 'filiform young.' 



"At this point the investigations took a turn of extreme interest and 

 importance, scarcely second even to that attached to the first study of the 

 zygotes. Since the blasts are evidently the progeny of the zygotes, they 

 must carry on the life-history of the parasites to a further stage. How do they 

 do so? What is their function? Do they escape from the mosquito, and 

 in some manner, direct or indirect, set up infection in healthy men or birds? 

 Or, if not, what other purpose do they subserve? It was evident that our 

 knowledge of the mode of infection in malarial fever and perhaps even 

 the prevention of the disease depended on a reply to these questions. 



"As I have said, the zygotes become ripe and rupture about a week 

 after the insect was first infected scattering the blasts into the body-cavity 

 of the host. What happens next? It was next seen that by some process, 

 apparently owing to the circulation of the insect's body-fluids (for the blasts 

 themselves appear to be almost without movement), these little bodies find 

 their way into every part of the mosquito into the juices of its head, thorax, 

 and even legs. Beyond this it was difficult to go. All theory at least all 

 theory which I felt I could depend upon had been long left behind, and 

 I could rely only on direct observation. Gnat after gnat was sacrificed in 

 the attempt to follow these bodies. At last, while examining the head and 

 thorax of one insect, I found a large gland consisting of a central duct sur- 

 rounded by large grape-like cells. My astonishment was great when I 

 found that many of these cells were closely packed with the blasts (which 

 I may add are not in the least like any normal structures in the mosquito). 

 Now I did not know at that time what this gland was. It was speedily 

 found, however, to be a large racemose gland consisting of six lobes, three 

 lying in each side of the insect's neck. The ducts of the lobes finally unite 

 in a common channel which runs along the under surface of the head and 

 enters the middle stylet, or lancet, of the insect's proboscis. 



"It was impossible to avoid the obvious conclusion. Observation after 

 observation always showed that the blasts invariably collect within the cells 

 of this gland. It is the salivary or poison gland of the insect, similar to the 

 salivary gland found in many insects, the function of which, in the gnat, 

 had already been discovered although I was not aware of the fact. The 

 function is to secrete the fluid which is injected by the insect when it punc- 

 tures the skin the fluid which causes the well-known irritation of the punc- 

 ture, and which is probably meant either to prevent the contraction of the 

 torn capillaries or the coagulation of the ingested blood. The position of 

 the blasts in the cells of this gland could have only one interpretation 



