IMMUNOLOGY 863 



out by Massaglia (1907) and is illustrated in Figure 191 by T. rho- 

 desiense in the mouse. 



Whether there is a natural immunity against the trypanosomes in 

 the mouse cannot be answered at present, because of insufficient data. 

 On the one hand, W. H. Taliaferro, Johnson and Cannon (unpublished 

 work, see W. H. Taliaferro, 1929) reported no effect of splenectomy 

 on mice infected with T. equiniim, and on the other hand Schwetz 

 (1934) and Russeff (1935) found a slight effect of splenectomy in 

 mice infected with T. congolense and T. equiperdum respectively. 



With regard to acquired immunity, most data are in accord in show- 

 ing that the mouse does not develop any appreciable amount. As may 

 be seen in Figure 191, the rate of reproduction of the parasites remains 

 constant and fairly high (the C.V. varies between 8.9 and 10.5 per- 

 cent) and the parasites progressively increase in number until the death 

 of the host (see also control mouse, infected with T. equ'mum, in 

 Figure 193, which is drawn on a semilogarithmic scale). 



In 1933 Krijgsman showed that the rate of increase of T. evansi 

 in the mouse and rat is not uniform during the acute rise, but that two 

 periods of high rates of increase, approximately between eighteen and 

 thirty-two hours and sixty and sixty-six hours, alternate with three pe- 

 riods of lower rates of increase at the beginning, middle, and end of 

 the infection. He believed that the terminal low rate of increase is due 

 to a destruction of parasites, as evidenced by the occurrence of more 

 degenerating forms, but that the earlier low rates are due to a partial 

 inhibition of reproduction. He reached this conclusion because he found 

 no increase in degenerating trypanosomes in the blood and in spite of 

 the fact that he found no diminution in the percentage of division 

 forms. He visualized the mechanism of this partial inhibition of re- 

 production as a uniform retardation of all stages of the cycle of 

 growth and cell division. There is no doubt that theoretically such 

 a uniform lengthening of all stages would give a partial inhibition of 

 reproduction, without affecting either the percentage of division forms 

 or the coefficient of variation. Nevertheless, the existence of such a 

 mechanism is doubtful, in view of the fact that in T. leivhi, as well as 

 in cells in general, a retardation of cell division is characterized by an 

 increased length of the resting stage (the so-called adult stage of T. 

 leivisi), and not by a gradual slowing down of the whole process with 



