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not be formed and heat will not be evolved. The cell-wall which appears 
on the assumption of the quiescent state is therefore the equivalent of the 
carbonic acid and water, which were before being excreted — an incom- 
pletely oxidised waste product, which by reason of its insolubility and 
coherence acquires at once its permanent morphological importance 
and its protective use. The plasmodial stage which terminates the 
cycle seems an almost mechanical union of exhausted cells, and such 
cases as those of the union of Actinosphaeria and Gregarines formerly 
alluded to, furnish grounds for regarding the process of conjugation as 
specialised from that of plasmodial union. Division may take place at 
any stage, most frequently, however, after cell-union. 
That this cell-cycle is not beyond the reach of experimental phy- 
siology, and that such a form as the amoeboid does really underlie that 
of apparently far more highly differentiated organisms are evidently 
shown by a very simple experiment. When a living Actinosphaerium 
is treated with dilute solution of ammonium carbonate, its pseudo- 
podia are retracted or disappear, its stroma vanishes, and it sinks into 
a simple granular amoeboid mass with blunt pseudopodia and finally 
bursts and dies. 
If division takes place continuously in the encysted stage the re- 
sultant multicellular aggregate is a vegetable, if in the amoeboid or the 
ciliated a more or less distinctly animal organism arises. In plants the 
cell cycle is represented almost solely by the resting-stage, though the 
ciliated phase lingers on here and there in the antherozoid, and the 
amoeboid in the oospore. In an undifferentiated aggregate like Mago- 
sphaera the cell-cycle is evident, in a Sponge-Gastrula equally so. 
The development of the tissues of all the higher animals from similar 
embryonic amoeboid cells ought to be interpreted in the same way. 
Some become quiescent and these develop an intercellular substance 
truly analogous to cellulose, but probably differing only in the non- 
economisation of nitrogenous matter, others are differentiated into 
muscular tissue or remain merely amoeboid, others become ciliated. 
And thus a classification of the normal tissues from the same point of 
view as that adopted for the Protista becomes possible. We recognise 
the utility and physiological import of the present hypothesis when we 
consider the cell theory in its functional as well as in its structural 
aspect. Since the activities of the body are the aggregate activities of 
its component cells, such phenomena as the variations of muscular and 
nervous tonus, of ciliary activity, and even those of rest and sleep, be- 
come intelligible as phases of the same rhythm of increasing and de- 
creasing cellular activity. 
Variation and disease are much akin, for pathological changes 
