Cell and Protoplasm. 109 



living* emulsion. Finally a number flow together to 

 form a relatively large amoeboid mass or plasmodium. 

 A somewhat similar life-history is known in many cases, 

 and the point is that we have here a " cell-cycle" in the 

 life-history of an individual, i.e. a passage from phase 

 to phase amoeboid, encysted, flagellate, amoeboid . . . 

 and so on. These phases are regarded as primitive 

 reactions of the protoplasm in relation to the variations 

 in the environment (food and other forms of energy). 



(2) Among the unicellular animals, or Protozoa, there 

 are three chief types : the amoeboid Rhizopods, the 

 encysted Sporozoa or Gregarines, and the ciliated or 

 flagellate Infusorians. It may be said that each of 

 these accents one chapter in the life-cycle of the simple 

 Protomyxa, and there are many cases in which, although 

 one phase is dominant, another may occur temporarily. 

 Thus a young Sporozoon may be amoeboid, or an Amoeba 

 may become encysted in unpropitious environment. 



(3) But this general classification of the Protozoa into 

 three main sets, which becomes more intelligible in the 

 light of Protomyxa's cell-cycle, is also harmonious with 

 that of the cells in the higher animals. Thus the Rhizo- 

 pods, with their changeful outflowing processes of living 

 matter, are comparable to the white blood corpuscles, 

 to phagocytes, to many young ova, and to other amoe- 

 boid cells of Metazoa. The parasitic Gregarines or 

 Sporozoa, which have a rind and no motile processes, 

 may be compared to degenerate muscle-cells, to mature 

 ova, or to other passive encysted cells in Metazoa. And 

 the Infusorians, with their lashes, may be compared to 

 the cells of ciliated epithelium, or to the active sperma- 

 tozoa of most Metazoa. And further evidence of the 

 cell-cycle is readily procurable, as when a ciliated cell in 

 the trachea sinks down into amoeboid form, or when an 

 amoeboid young ovum becomes encysted in becoming 

 mature 



(4) The suggestion for it has remained little more 

 acquires further significance when the author points out 

 that the three chapters plainly represent the three main 

 functional possibilities: (a) the amoeboid units, neither 

 very active nor very passive, form a median compro- 



