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168 tioned are derived from the crithidial types. The development in the salivary gland takes from two to five days before the forms are infective. "9. The fly is never infective until the glands are invaded. Trypanosomes from the proventriculus when injected into a monkey never produce infection. Trypanosomes may be found in the salivary glands as early as the sixteenth day of the cycle. An early infection of the salivary glands is always preceded by a very virulent and rapid gut infection.

" 10. The trypanosomes are never attached to the wall of the alimentary canal, and there is no intracellular multiplication in the gut cycle. A crithidial stage does not occur in the gut cycle. The trypanosomes are never found in the body cavity nor are they ever established in the rectum.

"11. Conjugation has not been observed, nevertheless the fly cycle as a whole has the biological significance of conjugation."

Considering the bionomics of glossina, it seems not improbable that some of the trypanosomes, after undergoing certain unknown changes, may enter the larva, and be thus transmitted by heredity to the vertebrate host. This has been suggested by Sambon. So far, however, such hereditary transmission has not been established. Yet it may be pointed out that it would conform in a remarkable manner to the habits of the babesise in relation to their invertebrate transmitters. To settle these and other questions of great practical importance in connection with the role of the tse-tse fly in trypanosomiasis, experiments with laboratory- reared insects are indispensable; and, further, observations should be extended to all the species of the genus. We have no assurance that Glossina palpalis and G. morsitans are the only efficient transmitters. Direct infection through breaches of surface or through mucous membranes (e.g. during coitus, as has been suggested by Koch) cannot be entirely excluded as a possibility. Predisposing causes.—— Neither age, sex, occupation, nor race per se has any influence on the