(2) Two healthy pups, each a month old, and born in the laboratory, were placed in a disinfected, flea-proof cage. A few days after, an infected dog was placed in the cage, so that fleas from the infected dog could pass on to the puppies. A month later the two pups became infected, parasites being found in them after liver puncture. A number of control puppies from the same litter remained uninfected and in good health. (3) Basile next used other laboratory-born puppies, a month old. Four of the litter were placed in a disinfected, flea-proof gauze cage in Rome. The cage was isolated from other dogs. Fleas obtained from an infected area in Sicily were placed in the cage. The puppies were examined by hepatic puncture, but were found to be negative for two months. Then two of the puppies showed infection, and six days later the remaining two puppies were found to be infected, and all four died. They showed irregular temperatures, and were getting thin. Control puppies remained healthy. From these experiments Basile concludes that fleas transmit leishmaniasis. However, Basile did not exclude the possible occurrence of natural herpetomonads in the gut of the fleas.[137] _Herpetomonas ctenocephali_ is known to occur in the gut of _Ctenocephalus canis_. A natural _Herpetomonas_ is also known in the gut of _Pulex irritans_, as well as a _Crithidia_ (_C. pulicis_, Porter). These natural flagellates of the fleas pass through non-flagellate stages, like the Leishman-Donovan body. In consequence Wenyon and Patton, among others, have criticized Basile’s results. Further, other investigators, such as Wenyon and Da Silva (1913), have repeated Basile’s flea experiments and been unable to confirm them. [137] See Fantham, _Brit. Med. Journ._, 1912, ii, p. 1196. In feeding and inoculation experiments the incubation period of the parasite may be long, and so it is necessary to wait a long time to see whether the parasite will develop. _Immunity._--Nicolle has tried some experiments with _L. infantum_ and _L. tropica_. He finds that in animals recovery from an attack of the former confers immunity against infection by the latter and vice-versâ. Laveran[138] records that a monkey having an immunity against _L. infantum_ was also immune to _L. donovani_. [138] _Annales Inst. Pasteur_ (1914–15), xxviii, pp. 823, 885; xxix, pp. 1, 71.