When the tsetse fly, _Glossina palpalis_, feeds on a man in whose peripheral circulation there are normal type trypanosomes we have an accumulation of such forms in the middle and posterior portions of the gut. From the eighth to the eighteenth day long, slender forms develop and pass forward into the proventriculus. None of the intestinal forms can cause infection when injected into animals. These proventricular types work their way into the salivary ducts and thence into the salivary glands, where further development takes place. Here we have shorter forms developing, which are similar in morphology to the normal blood type. It is at this stage that the fly becomes infective by the passing of these trypanosomes down the salivary ducts and through the channel in the hypopharynx to the subcutaneous tissues of the person bitten. High temperatures, 75 to 85°F., are favorable to development, while low temperatures, 60 to 70°F., are inimical to development, but do not kill the ingested trypanosomes. This explains the long period which at times elapses before a fly becomes infective. Under favorable conditions a fly becomes infective in twenty to thirty-four days and remains infective the rest of its life, up to 185 days. The infection is not transmitted to the pupa. This is an inoculative, cyclical or indirect type of infection. It is usually considered that a tsetse fly whose proboscis has just been contaminated with trypanosome blood is capable of transferring the infection for a few hours. This would be a mechanical or direct method of infection and such power for infection only lasts for a few hours. There are other groups of trypanosomes, not important for man, in which the cyclical development does not include the salivary glands. In the _T. pecorum_ group of small monomorphic trypanosomes development takes place only in intestines and proboscis, while in the _T. vivax_ group this occurs in the proboscis alone. [Illustration: FIG. 28.—_Glossina morsitans_ before and after feeding. Lateral view. (From Doflein after Austin.) MacNeal.] When tsetse flies feed on animals infected with trypanosomes only from 2 to 6% become infective. Again, it has been shown that where the wild animals on which tsetse flies feed may show an infection of from 16 to 50% yet not more than 2 out of every 1000 tsetse flies, caught and tried out on susceptible animals, show themselves infective. Both of the human trypanosomes of Africa have been cultured by using the N.N.N. medium in which rat’s blood was substituted for that of the rabbit. Human blood will also serve as a substitute. Growth however is not constant.