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be attracted to the spot, and a struggle will follow in which, according to 

 the quantity and intensity of the virus, the leucocytes will either perish 

 or survive. In the first case, the germs will continue to develop, and will 

 finally poison the system with their toxins; if, on the contrary, the 

 leucocytes survive, they will have acquired the property of secreting 

 antitoxins, which will either completely or only partially protect the 

 system against the toxins which are being elaborated. Now, as these parti- 

 cular leucocytes multiply by division, they may transmit the newly-ac- 

 quired immunizing faculties to several generations of their descendants. 

 During the long residence of the same persons in an endemic focus, the 

 yellow fever germ will in all likelihood be introduced again and again, 

 producing only mild morbid phenomena or none at all, but each time new 

 sets of leucocytes will acquire immunizing properties, until finally, after 

 repeated exposures, all the leucocytes will have become similarly differen- 

 tiated, and permanent immunity follow. During a severe attack of the 

 disease, on the other hand, the whole system being invaded by the specific 

 toxins, the leococytes must either all perish or all that survive will remain 

 thereafter endowed with the immunizing faculty that has been surmised, 

 and permanant immunity will be there and then established. Absence from 

 yellow fever centres might, after some years, extinguish the special pro- 

 perties thus acquired by the leucocytes, and the immunity in such cases 

 would be lost. 



In Natural History there are instances in which, although no microbial 

 products are concerned, some explanation like the one just given seems 

 called for in order to understand the effects of the venom which some 

 insects pour into the wounds which they inflict. Taking as an example the 

 Havana mosquitoes (with which we are particularly concerned in this 

 article), a marked difference is observed in the effects produced by their 

 stings in new comers and in older residents. In the florid foreigner from 

 the North (specially in those whose skin is fair and delicate), during the 

 first weeks after arrival the mosquito sting is not more acutely felt, but 

 its effects upon the skin are much more visible. Soon after the insect 

 withdraws its lance a weal is apt to form, of a whitish colour, contrasting 

 with the red hyperaemic zone which at the same time, or soon after, to 

 developed around it. After longer residence the same persons will scarcely 

 feel the sting of the diurnal species, and no welt wild be produced by it, 

 but only a small pimple, scarcely raised above the surface, which may not 

 disappear, however, for several days. Here the doctrine of chemotaxis 

 seems to apply. The white welt sometimes formed indicates the attraction 

 of the leucocytes by the mosquito venom, and the non-appearance of such 

 w T elts in the same persons, after longer residence, would signify that the 

 venom is now neutralized by the leucocytes nearly as fast as it is instil- 

 led into the wound. 



Now, if both the chemotactic properties of the mosquito venom and 



