The trypanosomes (fig. 38, G) pass into the flea’s rectum. The next phase is a crithidial one. The parasites become pear-shaped, in which the blepharoplast (kinetic nucleus) has travelled anteriorly past the nucleus towards the flagellum (fig. 39). The crithidial forms attach themselves to the wall of the rectum, and multiply by binary fission (fig. 39, D). A stock of parasites is thus formed which, according to Minchin and Thomson, “persist for a long time in the flea--probably under favourable conditions, for the whole life of the insect” (fig. 39, A-I). From the crithidial forms of the rectum, according to Minchin, small infective trypanosomes arise by modification morphologically (fig. 39, J--M). The flagellum grows longer and draws out more the anterior part of the body, the blepharoplast migrates posteriorly, behind the nucleus, and carries with it the flagellar origin. These trypanosomes are small, but broad and stumpy (fig. 39, N), and can infect a rat. Minchin and Thomson formerly considered that the small, stumpy, infective trypanosomes pass forwards from the rectum into the stomach, and “appear to be regurgitated into the rat’s blood when the flea feeds.” However, the small infective trypanosomes were previously described by Swellengrebel and Strickland.[93] They may be found in the flea’s fæces. Nöller (1912)[94] has found that the development of _T. lewisi_ proceeds quite well in the dog flea (_Ctenocephalus canis_) in Germany. Wenyon confirms this, and states that the human flea, _Pulex irritans_, and the Indian rat-flea, _Xenopsylla cheopis_, are also able to serve as true hosts for _T. lewisi_. [93] _Parasitology_, iii, p. 360. [94] _Arch. f. Protistenkunde_, xxv, p. 386. [Illustration: FIG. 39.--_Trypanosoma lewisi_. Developmental stages from rectum of rat-flea. A, early rectal form; C, D, division of crithidial form; E, group of crithidial forms; F--I, crithidial forms without free flagella, some becoming rounded; J--M, transitional forms to trypanosome type seen in N, which represents the final form in the flea. × 2,000. (After Minchin.)]