AX INTRODUCTION TO THE STUDY OF GROWTH. 2> 



simple division of the former volume by the latter, it 

 appears that the average volume of the cells under the 

 condition named, is 2,942 cu. /*, equivalent to a cube 14 /*, 

 on each edge, which is by no means small. 



The nutrition of a cell depends on the surrounding 

 medium from which it derives food, either directly by 

 the incorporation of small particles, or, as is more usual 

 in higher animals, by the diffusion of the nutrient sub- 

 stances in a fluid form. For this latter process the 

 surface of the cell must be exposed to the nutrient fluid, 

 a condition easily fulfilled in the case of single cells or 

 those arranged in thin layers. When, however, the 

 mass of cells becomes large, the nourishment of those 

 below the surface is indirectly accomplished by the 

 conveyance of the nutrient fluid to them through vessels 

 that penetrate the mass. Between the blood in these 

 vessels and the surrounding cells the process of diffusion 

 then takes place. This arrangement leaves to the cells 

 most directly in contact with the ingested material the 

 peculiar duty of preparing the food for distribution by 

 diffusion, and here arises a good instance of the division 

 of labour among the cells leading to that mutual de- 

 pendence of which the completed organism offers so 

 many examples, for it holds true through the entire 

 series of the activities of the animal body, that each 

 is carried on by elements specially modified for the 

 purpose. 



All these different forms of physiological activity ex- 

 hibited by the higher animals are found to be present in 

 many of the unicellular organisms, and are assumed to 

 be present in them all. For this reason, the development 

 of different capacities by different groups of cells is best 

 regarded, neither as the sudden acquisition of a new 

 power, nor as a peculiar capacity only inherent in a 

 particular group of elements, but as arising simply from 



