THE GENUS TRYPANOSOMA 267 



the nucleus and cytoplasm (see above p. 256). The males and females 

 appear to lose the power of division, but, like indifferent forms, have 

 the power of penetrating epithelial cells of the gut and making their 

 way to various parts of the insect's body, including even the ovaries. 

 Under conditions of extreme cold and starvation of the insect, all 

 stages of the trypanosome die save these females, which appear to have 

 a remarkable power of resistance, and Schaudinn suggests that they 

 may be retained in the ovaries of the hibernating mosquitoes until 

 spring, when they may develop and infect the new generation. It is 

 in these forms that parthenogenesis occurs (see p. 168). The power 

 of changing, as crithidia does, from a free, flagellated, into a quiescent 

 parasite, not only in the gut of the mosquito, but also in the blood of 

 the bird, is a feature known to occur in no other trypanosome. Accord- 

 ing to Schaudinn and Sergent the intracellular parasite is the typical 

 form of the organism during the day, while it leaves the blood cell, 

 changes into a typical trypanosome, and grows during the night, 

 the change being induced, as Schaudinn believed, by the lowered 

 temperature of the bird at night. 



Although "latent bodies," encysted forms, and other non-flagellated 

 stages of trypanosomes have been observed by Moore and Breinl, 

 Minchin, Robertson, Laveran and Mesnil, and others, this is the only 

 case of trypanosomes known where, as in Herpetonionas donovani, 

 the flagellated organism becomes an intracellular parasite. The 

 phenomenon must be interpreted zoologically, as an indication of the 

 more evolved phylogenetic state of Tryp. noctuce and leading to the 

 group hemosporidia of permanently intracellular blood parasites. 

 In our opinion these facts do not justify the use of a different generic 

 name for Trypanosoma noctucB as Liihe proposes, biit are only further 

 evidence of the tendency to polymorphism exhibited by the group as a 

 whole. 



The Effects of Trypanosomes on Vertebrate Hosts. — The 

 great majority of trypanosomes, especially the parasites of cold- 

 blooded forms, have no evident effect upon their hosts. But among 

 warm-blooded animals they rank with the most deadly parasites 

 known. The horse, mule, and dog always succumb to infections of 

 Try p. evansi, the cause of surra, while cattle, camels, etc., are less 

 affected (Liihe). The organism of nagana. Try p. brucei, is fatal to 

 horses, dogs, and cattle, and that of mal de caderas (hip sickness) is 

 fatal to horses, rats, and mice. On the other hand, the rat try- 

 panosome, Tryp. lewisi, and the cause of galziekte (gall-sickness) in 

 cattle, Tryp. theileri, are relatively harmless. Immunity, in some 

 cases, is set up by one invasion of the parasites, wild animals, as 

 Bruce has shown, being immune to Tryp. brucei, which quickly kills 

 imported animals. Laveran and Mesnil ('02) showed that immunity 

 was conferred on rats by one infection with Tryp. lewisi. 



Human trypanosomiasis is particularly malignant, having a fatality 



