Insects and Disease 625 



since these are so extremely like each other. I elected to work with the 

 avian species, chiefly because the plague-scare in Bengal still rendered obser- 

 vations with the human species almost impossible. By feeding Culex 

 fatigans on birds with H. relicta and then examining the insects one, two, 

 three or more days afterwards, it was easy to trace the gradual growth of 

 the zygotes. Their development briefly is as follows: After the fertilization 

 of the macrogamete has taken place in the stomach-cavity of the gnat, the 

 fertilized parasite or zygote has the power of working its way through the 

 mass of blood contained in the stomach, of penetrating the wall of the organ, 

 and of affixing itself on, or just under, its outer coat. Here it first appears 

 about thirty-six hours after the insect was fed, and is found as a 'pigmented 

 cell' that is, a little oval body, about the size of a large red corpuscle, and 

 containing the granules of melanin possessed by the parent gametocyte 

 from which the macrogamete originally proceeded. In this position it shows 

 no sign of movement, but begins to grow rapidly, to acquire a thickened 

 capsule, and to project from the outer wall of the stomach, to which it is 

 attached, into the body-cavity of the insect-host. At the end of six days, if 

 the temperature of the air be sufficiently high (about 80 F.), the diameter 

 of the zygote has increased to about eight times what it was at first ; that is, 

 to about 60 microns. If the stomach of an infected insect be extracted at 

 this stage, it can be seen, by a low power of the microscope, to be studded 

 with a number of attached spheres, which have -something of the appear- 

 ance of warts on a finger. These are the large zygotes, which have now 

 reached maturity and which project prominently into the mosquito's body- 

 cavity. 



"All this could be ascertained with facility by the method I have men- 

 tioned: and it should be understood that gnats can be kept alive for weeks 

 or even months by feeding them every few days on blood, or, as Bancroft 

 does, on bananas. But a most important point still required study. What 

 happens after the zygotes reach maturity? I found that each zygote as 

 it increases in size divides into meres, each of which next becomes a blastophore 

 carrying a number of blasts attached to its surface. Finally, the blastophore 

 vanishes, leaving the thick capsule of the zygote packed with thousands 

 of the blasts. The capsule now ruptures, and allows the blasts to escape 

 into the body-fluids of the insect. 



"These blasts, when mature, are seen to be minute filamentous bodies, 

 about 1 2-6/1 in length, of extreme delicacy, and somewhat spindle-shaped 

 that is, tapering at each extremity. Prof. Herdman and I have adopted 

 this word 'blast' for these bodies after careful consideration, but others 

 prefer other names. They are, of course, spores; but spores which have 

 been produced by a previous sexual process, and are in fact the result of 

 a kind of polembryony. Just as a fertilized ovum gives rise to blasts, 



