xvm PAST HISTORY 353 



The Differentiation of Unicellular Animals. When we 

 take a survey of the kingdom of the unicellular or non- 

 cellular animals, and try to avoid losing sight of the 

 wood in our study of the trees, we recognise what Geddes 

 has expounded in his conception of the cell-cycle, that 

 there are three great pathways. On the one side, there 

 are 'the highly active Infusorians, most of which move 

 quickly by means of cilia or flagella ; on the other side, 

 there are sluggish and parasitic Sporozoa, which tend to 

 sink into a passive encysted stage ; and between the two 

 are the amoeboid Rhizopods, neither very active nor 

 very passive, but representing a median compromise. 

 These three lines correspond to three physiological 

 regimes of lavish expenditure and "living dangerously," 

 of preponderant saving and a life of ease, and of balance 

 between these extremes, which are obviously extremes of 

 relatively high katabolism and relatively preponderant 

 anabolism. The most primitive organisms, like some of 

 the Proteomyxa and Myxomycetes, pass through a cycle of 

 phases from flagellate to amoeboid, from amoeboid to en- 

 cysted, often with the interpolation of another plasmodial 



-chapter, in which a number of minute amoeboid units 

 coalesce into one composite mass. The Infusorians, 

 Sporozoa, and Rhizopods are, as it were, accentuations 

 of the phases that occur in the life-history of the simplest, 

 and often show traces of other phases besides the one 

 that they mainly represent. The idea of the cell-cycle is 

 corroborated by reference to multiccllular animals, where 

 we find illustrations of (1) very active cells, as in ciliated 

 epithelium, (2) very passive cells, as in fatty tissue or in 

 cartilage, and (3) amoeboid cells like the white blood- 

 corpuscles. Moreover, a spermatozoon is usually very 

 active, like a flagellate Int'usorian ; a mature ovum is 

 often encysted and quiescent, like a Sporozoon ; while an 

 immature ovum is often amoeboid, as is well seen in the 

 freshwater Hydra. When a young ovum accumulates 

 reserves and becomes passive, or when ciliated or flagellate 

 cells lapse into amoeboid phase, we hear as it were echoes 



of the primitive cell-cycle. 



Other Great Steps. In the course of time there must 



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