Carini found cysts in the lungs of rats infected with _T. lewisi_. He thought the cysts were schizogonic stages of the trypanosome, comparable with those found in the lungs of animals sub-inoculated with _T. cruzi_. Delanoë (1912)[92] has found, however, that such cysts, containing eight vermicules, occurred in rats uninfected with _T. lewisi_. Delanoë concludes that the pneumocysts are independent of _T. lewisi_, and represent a new parasite, _Pneumocystis carinii_. The pneumocysts may be allied to the Coccidia, and must be considered when investigating the life-cycle of a trypanosome in a vertebrate host. Some of the stages of _T. cruzi_ may possibly be of this nature. [92] _C. R. Acad. Sci._, clv, p. 658. _Life-cycle in the Invertebrate Host._--This occurs in fleas, and has been investigated in considerable detail by Minchin and Thomson in _Ceratophyllus fasciatus_, and by Nöller in _Ctenocephalus canis_ and _Ctenopsylla musculi_. When infected rat’s blood is taken up by the flea, the parasites pass with the ingested blood direct to the mid-gut of the Siphonapteran. In the flea’s stomach they multiply in a somewhat remarkable manner, namely, by penetration of the cells of the lining epithelium, and division inside the epithelial cells. Inside these lining cells the trypanosomes first grow to a large size and then form large spherical bodies, within which nuclear multiplication occurs (fig. 38, A-F). Any one of these large spherical bodies contains at first a number of nuclei, blepharoplasts and developing flagella, the original flagellum still remaining attached for a time. The cytoplasm then divides into daughter trypanosomes which are contained within an envelope, formed by the periplast of the parent parasite. Inside the periplast envelope are a number of daughter trypanosomes “wriggling very actively; the envelope becomes more and more tense, and finally bursts with explosive suddenness, setting free the flagellates, usually about eight in number, within the host-cell” (fig. 38, F). The daughter forms escaping from the host cell into the stomach lumen of the flea are fully formed, long trypanosomes. [Illustration: FIG. 38.--_Trypanosoma lewisi_. Developmental stages from stomach of rat flea. O, ordinary blood type; A-F, stages occurring in gut-epithelium of flea, when the trypanosome becomes rounded and undergoes multiplication, forming in F eight daughter trypanosomes; G, type of trypanosome resulting from such division which passes back to the rectum. × 2,000. (After Minchin.)]