Chagas considers this second mode of multiplication to be strictly asexual. By this means the number of parasites in the vertebrate host is increased, and symptoms are produced. On the other hand the first mode of multiplication, seen in the lung capillaries, is considered by Chagas to be a process of gametogony, in which sexual forms are differentiated. He finds that (1) the adult trypanosomes exhibit a dimorphism in human blood rarely seen in artificially infected guinea-pigs. In these guinea-pigs (infected from guinea-pigs) the so-called gametogony in the lungs is seldom seen. (2) The intermediate host, _Triatoma_ (_Conorhinus_), becomes infective if fed directly on infected human blood, but very rarely so if fed on guinea-pigs. Chagas is led to believe that the occurrence of sexual forms constantly in the blood of man implies a greater resistance to infection on the part of man than on the part of guinea-pigs or other animals, assuming the general hypothesis that the formation of gametes represents a reaction of the Protozoön to unfavourable conditions. In human infection the number of parasites is always less than in laboratory animals, and their presence in the blood is transitory, lasting from fifteen to thirty days in acute cases. In many cases examination of the tissues at death has shown the presence of parasites in patients who did not exhibit them in the general circulation. [Illustration: FIG. 35.--_Trypanosoma cruzi_. Development in _Triatoma megista_. _1_-_6_, forms found in the mid gut of _Triatoma_; _7_ flagellate forms found in the posterior part of the gut of _Triatoma_. (After Chagas.)] _Life History in the Invertebrate Host._--About six hours after the ingestion of infected blood by the bug (_Triatoma megista_), the kinetic nucleus of the trypanosome moves towards the nucleus, and the flagellum is usually lost (fig. 35, _1_-_5_). The parasite becomes rounded and _Leishmania_-like (fig. 35, _3_-_5_), and multiplies rapidly by division. After a time, multiplication having ceased, the rounded forms become pear-shaped and develop a flagellum at the more pointed end. Crithidial forms (fig. 35, 7) are thus produced and pass into the intestine, where they multiply and may be seen in about twenty-five hours after the ingestion of blood. The crithidial forms may also be found in the rectum and fæces. The last stage in the invertebrate is a small, trypanosome-like type, long and thin with a band-like nucleus and conspicuous kinetic nucleus. These parasites are found in the hind gut and in the body cavity. They find their way into the salivary glands, and are the forms (fig. 36) which are transmissible to a new vertebrate host. The development in the bug takes about eight days altogether, after which time the bugs are infective.