THEORY OF THE DEVELOPMENT OF ORGANISMS 109 



Naegeli strikingly says, ' implies a constancy oi 

 perpetual change.' 



Thus, growing protoplasm can assume only such 

 shapes as allow it to remain in constant touch with 

 the outer world. A cubical or spherical mass of 

 cells could not grow by the formation of new layers 

 of cells on the outside, for these layers would 

 deprive the centrally placed masses of cells of their 

 conditions of existence. Similarly, an extended 

 membrane of cells or an epithelial layer cannot add 

 indefinitely to its thickness, else would the ceils 

 furthest removed from the outside be injured in 

 their relations to surrounding things. To satisfy 

 its essential conditions, protoplasm can grow only 

 with a proportionate extension of its external 

 surfaces. This is secured by the cells becoming 

 arranged in threads and membranes, and its result 

 is that the threads by branching, and the mem- 

 branes by folding, produce structures whose com- 

 plexity increases with growth. 



This conception that the shape of growing 

 organisms is in many respects the necessary con- 

 sequence of the specific characters with which 

 protoplasm is endowed, explains the great contrast 

 between animals and plants in their general 

 organisation. The contrast is the result of the 

 difference between animal and plant metabolism, 

 and between the ways in which animals and plants 

 obtain their food. Plant cells elaborate protoplasm 

 from the carbonic acid of the air, water, and easily 

 diffusible solutions of salts, obtained from the sea or 

 from the soil. For the chemical work of combining 



