1024] Kessfl: Amwhtu of Culture Rats and Mire r)27 



First, the incidence of infection among old rats is much lower than 

 among young rats or rats of middle age. Secondly, in 27 per cent of 

 the cases in this investigation rats and mice which had a previously 

 determined amoebic infection have been known to clear themselves 

 of the infection without the administration of curative measures. 

 Thirdly, among rats and mice subjected to precisely the same feeding 

 experiments, some animals became infected while others remained free 

 from amoebic infection. No data were kept for the rats, but among 

 the mice those that did not become infected as a result of the feeding 

 experiments were old mice that had been diagnosed as amoeba-free at 

 the beginning of the experiment. 



These results have been obtained from experiments bearing on 

 subjects other than immunity, but they present striking suggestions 

 and open the way for further investigation along this line. 



Reproduction. — In so far as is known, all reproduction in amoebae 

 is by the asexual method. There has been no recent evidence to sup- 

 port the earlier idea of autogamy and no sexual cycle or syngamy has 

 been determined. It is generally accepted that asexual reproduction 

 may occur either in the motile amoeba or in the encysted stages. 



Although Schaudinn (1903) and Mathis and Mercier (1917) 

 described the proces,s of multiple schizogony as occurring in Enda- 

 moeha coH, conclusive evidence has not been produced for this method 

 of reproduction. It is the opinion of the writer that irregularly 

 shaped cysts (pi. 47, tig. 49), which may simulate motile amoebae in 

 shape, may have been interpreted as motile amoebae and the theory 

 of schizogony based on this evidence. 



Binary fission seems to be the usual method of reproduction in 

 motile amoebae; at any rate, motile amoebae with more than two nuclei 

 have never been seen in this investigation. Motile amoebae with two 

 nuclei have been observed and, while they are not common, the fact 

 that they have been found indicates that the process of binary fission 

 is quite normal. 



Probably the more usual method of reproduction of parasitic 

 amoebae is by multiple nuclear formation within the cyst. A mono- 

 nucleate amoeba encysts and by nuclear divisions produces a cyst with 

 successively two, four, eight, or sixteen nuclei. In the three amoebae 

 here described from the rat and mouse, eight nuclei is the common 

 number for the mature cyst. 



The question of periodicity of encystment has been discussed in an 

 earlier paper (Kessel, 1923a), but the growing impression among 

 investigators is that there is no regular periodicity of similar intervals 



