50 



CELL 



language respectively termed anabolism and kata- 

 bolism (see PROTOPLASM). 



But only a few cells, comparatively speaking, 

 live a free and independent life. The majority are 

 component elements in higher unities. In these 

 the original many-sidedness of function is more or 

 less lost, or at anyrate in abeyance, and that 

 exactly in proportion to their degree of subordina- 

 tion. Even in individual cells there is a tendency, 

 obviously within narrow limits, towards differen- 

 tiation that is, to the restriction and specialisa- 

 tion of certain parts for certain functions. But 

 when the cells form elements of a larger whole, 

 the division of labour finds full effect. From 

 position and other conditions the cells cease to be 

 uniform or metaphorically many-sided. Certain 

 sets predominate in contractility, others in irrita- 

 bility, others in secretion, others again in storage, 

 and so on. In such cases one function predominates 

 over the others, which are subordinate or only 

 dormant possibilities. Thus arise muscle-cells, 

 nerve-cells, glandular cells, fat-cells, and the like. 

 Compared with Amoebae, those cells must have a 

 simpler physiology ; they may have gained in com- 

 plexity of structure, but have lost in manifoldness 

 of function. The aggregation of similar cells, 

 usually with one predominant habit or function, 

 results in the formation of tissues (see BIOLOGY, 

 EMBRYOLOGY, FUNCTION, PHYSIOLOGY, REPRO- 

 DUCTION, and cognate articles in this work). 



One general physiological fact may, however, 

 be referred to which will greatly assist in under- 

 standing the life both of independent cells and 

 of those which form the elements of tissues. A 

 survey of the unit-organisms, both among plants 

 and animals, reveals the existence of three well- 

 marked phases. Some cells are emphatically 

 active, equipped with motile lashes (cilia or 

 flagella), and obviously liberal in their expendi- 

 ture of energy. Others are just the reverse of 

 this, emphatically passive, wrapped up in them- 

 selves and without motile processes, obviously 

 economical in their expenditure, conservative of 

 their income. A third set form a mean between 

 these two extremes, are neither encysted like the 

 latter nor lashed like the former, but furnished 

 with the relatively slow-moving processes charac- 

 teristic of Amcebfe, and living in a via media be- 

 tween activity and passivity. These three types 

 may be termed respectively ciliated, encysted, and 

 amoeboid, or active, passive, and moderate. That 

 these types generally correspond to the three 

 great divisions of the Protozoa shows that they 

 represent the three main possibilities of cellular 

 life. Now in the very simplest forms all the three 

 phases occur in one life-history ; no step has, as it 

 were, been taken in any one of the three direc- 

 tions ; the primitive cells are in a state of physio- 

 logical indifference. What has happened in the 



Fig. 4. Phases of Cell-life. (After Geddes.) 



Development of passive or resting, intermediate (amoeboid), and 



active (motile) states. 



higher classes of Protozoa Infusorians, Gre- 

 garinids, Rhizopods is that one phase has been 



accentuated to the more or less marked sub- 

 ordination of the others. Not that the emphatic 

 adoption of one line of cell-life excludes the others ;. 

 they may in fact occur as temporary stages, or as 

 pathological deviations. 



But while simple observation is sufficient to 

 establish the existence of a cycle of phases in 

 the life of primitive cellular organisms, such as 

 Protomyxa, and the existence of three main lines 

 of specialisation among the Protozoa, the import- 

 ance of this conception of a 'cell-cycle' becomes 

 increased and justified when the facts are con- 

 sidered physiologically. If we start from a simple 

 cell, such as an Amoeba, it is evident enough, from 

 what has been already said as to the twofold 

 nature of all vital processes, that the principal 

 physiological possibilities are the three phases 

 above indicated. On the one hand, with pre- 

 ponderance of income over expenditure, of con- 

 structive over destructive changes, of anabolism 

 over katabolism, the cell must tend to become- 

 larger in size, more weighted with stored material, 

 more sluggish or passive in habit, and more 

 rounded in form. But if the reverse take place, 

 the cell will tend to become less bulky, more 

 active or locomotor in habit, and more elongated 



Fig. 5. Protomyxa : 



1, encysted; 2, dividing; 3, spores escaping as ciliated bodies,, 

 passing into 4, amoeboid state ; 5, ' plasmodium ' forming from; 

 lusion of amoaboid cells. 



in form. The sweated-oft' cyst of the former, 

 the motile processes of the latter, are expres- 

 sions of exactly opposite constitutions and condi- 

 tions. A third physiological possibility remains, 

 that namely of continuing in a position of average 

 equilibrium between income and expenditure, be- 

 tween anabolism and katabolism, in a middle way 

 between the fitful fever of extreme ciliated activity 

 and the sluggish sleep of encysted passivity. 



Now if we take these two facts the existence 

 of a primitive cycle through which cells tend to- 



Fig. 6. The Cycle of Cell-life : 

 a, encysted ; b, ciliated ; c, amoeboid ; d, plasmodial. 



pass, and the existence of three main physiological 

 possibilities which lie behind the cycle we are in 

 a better position to understand both the changes 

 exhibited in normal and pathological conditions 

 by individual cells, and the various forms of cells 

 as they occur in the tissues of the higher organisms. 

 Thus lashed cells such as those of the windpipe of 

 mammals, the skin of many lower worms, the 

 inside of a Hydra, the male elements of most 

 animals and many lower plants, emphasise one 

 phase in the cycle, and it is not surprising to 

 find that in certain conditions they may sink 

 down into the amoeboid type. Or again, the 

 amoeboid character of young ova, preceding the 

 more passive and encysted condition of the mature 

 cells, is in view of the ' cell-cycle ' a most natural 

 procedure. In many cases artificial stimulus of 

 various kinds has been shown to make cells 

 pass from one phase to another of the primitive 

 life-cycle theoretically possible to all. In the- 

 same way the preponderance of cellulose in cells 



