538 GENERAL PHYSIOLOGY 



of the cell, the correlation between it and the medium necessarily 

 changes, because the relation of the surface and the mass of the 

 living substance is gradually altered. As a result of this there is 

 also a change in the metabolism. In other words, there is in the 

 growing cell a continual succession of different metabolic conditions 

 forming a very gradual transition, every succeeding condition 

 resulting necessarily from the preceding one. Since here, as 

 everywhere else in the physical world, form is among other things 

 a function of matter, it is thus made clear that with a change of 

 metabolism the form of the cell under certain circumstances will 

 also change, and thus a continual succession of different form-con- 

 ditions goes hand in hand with the succession of different metabolic 

 conditions, in other words, there is a development. It follows 

 from this that the development of the cell is a real epigenesis 

 in the sense of Caspar Friedrich Wolff, i.e., a succession, of con- 

 stantly new forms, and not a more distinct appearance of already 

 preformed, but hitherto imperceptible, structural differentiations 

 of the living substance. According to the extent of the changes 

 in the correlation between medium and cell the change of form 

 will be expressed in one case less, in another case more strongly ; 

 it will be most pronounced where, as in the development of the 

 cell-community of plants aitd animals from the egg, the cells 

 dividing as a result of growth remain in connection with one 

 another and act upon one another, in other words, where the 

 relations between cell and medium change rapidly and continually. 



c. Structure and Liquid 



Special importance has been attached elsewhere to the fact that 

 living substance possesses the essential properties of a liquid. In 

 the production of form another factor, namely, structure, plays an 

 important role. Since at first sight it might appear as if structure 

 and a liquid state are mutually exclusive, it will be advantageous 

 to examine briefly this question. 1 



If by structure there is understood a definite mutual relation of 

 the smallest particles of which a substance is composed, the funda- 

 mental requisites of the inauguration of structure are the mutual 

 attraction and grouping of definite particles. We can speak of 

 structure only where certain particles attract one another and 

 become grouped. This requirement is fulfilled not merely in solid 

 bodies, but in a certain measure in liquids, for in liquids also, as 

 cohesion shows, the individual parts attract one another. The 

 difference in the structure of liquids and solids is in reality a 

 gradual one, depending wholly upon the degree of consistency ; 

 imperceptible transitions exist between the two states. The 

 difference between them consists essentially in the fact that 



1 Cf. Verworn ('97). 



