THE MECHANISM OF LIFE 531 



cell, for example of the size of a man. At the same time it is 

 comprehensible that under certain circumstances cells whose surface 

 is considerably increased in proportion to their mass, such as the 

 flat leaves of Caulerpa, or those cells whose protoplasm is in 

 continual circulation between the surface and the interior, such as 

 the plasmodia of Myxomycetes, are capable of reaching a very 

 considerable size, especially when by multiplication the nuclear 

 substance presents a considerable increase. In these cases the 

 difference between the outer and the inner layers of the cell-body 

 cannot develop in the same degree as in compact cells. But where 

 the cell-body is a compact mass, where there is no active streaming 

 of the contents toward the surface, and where only one nucleus is 

 present in the protoplasm, the cell cannot surpass a certain size. 

 If, therefore, the living substance of such a cell is not to perish 

 by growth, at some period in its growth a correction of this dispro- 

 portion between mass and surface and of the disturbance of 

 metabolism conditioned by it must come in ; such a connection is 

 realised in the reproduction of the cell by division. 



The reproduction of the cell by division is, accordingly, to be 

 considered merely as a result of growth, and the morphologists for 

 a long time have rightly termed reproduction a continuation of 

 growth, " a growth beyond the measure of the individual." 

 Unfortunately our knowledge of the special mechanics of the 

 process of cell-division is thus far limited ; but it is to be expected 

 that a comparative physiological investigation of the well-known 

 morphological facts, when especially directed to the mechanical 

 conditions, as they are realised in various ways in different forms 

 of cells, will yield gratifying results. It is especially important 

 always to keep in mind and to select as the starting-point of the 

 investigations the metabolic relations of the individual parts of the 

 cell. The mechanical results of the metabolic relations between 

 the individual parts of the cell and the medium, are fitted to throw 

 some light upon the processes of cell -division, many of which 

 appear wonderfully complex. The most important factor in the 

 explanation of the mechanics of the characteristic figures of cell- 

 and nuclear division is probably to be found in the mechanical 

 movements caused by the chemical relations between the individual 

 parts of the cell ; among these, diffusion-processes and changes of 

 the cohesion and surface-tension of different cell-elements play a pro- 

 minent role. Some time ago Biitschli (' 76) expressed the view that 

 the radiating figure that is formed in the protoplasm about the cent- 

 rosome in nuclear divison is an expression of diffusion-processes 

 arising between the centrosome and the protoplasm; and later 

 (' 92, 3) he showed that, when warm gelatine-foams poured upon a 

 glass plate dry and coagulate, radiation-phenomena, exactly like 

 those of the karyokinetic figure, are caused by the traction of 

 the contracting air about the air-bubbles (Fig. 262). It may, 



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