25s 



THE AMEBIC AX NATURALIST 



[Vol. L 



individuals of a polymorphic coelenterate are present in 

 potential in the fertilized egg of the coelenterate, but we 

 are less accustomed to the idea that polymorphic indi- 

 viduals are present in potential, in the fertilized cell of a 

 protozoon. Research in recent years has shown that suc- 

 cessive generations of Protozoa may be more or less pro- 

 gressively differentiated, so that a cell picked out at one 

 phase of the life cycle is quite a different type of indi- 

 vidual from one picked out at another phase. Which, for 

 example, would be the ' 1 type ' ' individual of the dimorphic 

 Foraminifera? Which would be the type in the repro- 

 ducing flagellated and ameboid stages of Nagleria punc- 

 tata? of different phases in the life history of C entropy xis, 

 Arcella, or Difflugia? or of intestinal and blood-dwelling 

 stages of Plasmodium? The morphological differences 

 here indicate that the protozoan life history involves 

 differentiation analogous to that of a polymorphic meta- 

 zoon, and justify the comparison of the whole life cycle 

 with the development and differentiation of a metazoon, 

 especially that of a metagenetic type such as coelenterate 

 or trematode. 



The importance of the whole life cycle, first demon- 

 strated by Maupas, was fully recognized by Schaudinn 

 and applied by him to the study of parasitic forms. The 

 monographs resulting from this study, especially those 

 on Coccidium schubergi, Plasmodium vivax and on rhizo- 

 pods, are classics in the literature of Protozoa, and models 

 which later students have followed. 



Through Schaudinn 's work, and by later researches, 

 the sequence of events in different parasitic types has 

 been made out with painstaking care until to-day, we 

 know the general history of the majority of injurious 

 human protozoan parasites, the modes of transmission 

 from host to host, the types of intermediate hosts and 

 what happens in them. In short, we know enough to 

 furnish an adequate basis for public and private prophy- 

 laxis which, in the hands of sanitary commissioners and 

 public health officers, has put an end to epidemics of yel- 



