THE UNIT OF STRUCTURE 35 



Presumably it always absorbs as much as it can get, the simple 

 law of growth with cell division making it impossible for it 

 ever to get too much. The cells which in the liver are so for- 

 tunate as to be placed on the route along which food is carried 

 into the body retain the appetite of an amoeba, but lose its 

 capacity for growth and cell division. They return to the 

 blood-stream, when it is deficient in food, the stores which 

 they took up when food was in excess. 



The specialization of a gland-cell is opposite in kind to that 

 of a liver-cell. It takes up no more food than it requires, 

 but it has developed a great capacity of producing from the 

 food a substance which would no doubt be needed for its own 

 purposes were it an isolated cell, but which the gland- cell 

 places at the service of the body as a whole. An amoeba can 

 digest proteid substances. A cell of the pancreas produces the 

 ferment necessary for the digestion of proteins, and secretes 

 it into the alimentary canal. 



To take another instance of specialization. An amceba 

 responds to stimulation by changing its shape. It contracts 

 in one direction, expands in another. A muscle-fibre has 

 developed the capacity of contraction at the expense of all 

 other functions. During the course of its growth it changes 

 from a round cell into one that is elongated. The elongation 

 is in the direction in which it acts with greatest efficiency. Its 

 cell-substance is very highly specialized in order that it may 

 have the maximum capacity of contraction in this direction. 



Sensory cells develop to a maximum the capacity of respond- 

 ing to external force ; nerve- cells, the capacity of conducting 

 the impulses generated in sensory cells. The body is a republic 

 in which every citizen develops to the highest degree the 

 capacity of doing the thing which his situation makes it 

 desirable for him to do. 



The possibility of isolated cell life, and the necessity within 

 certain limits of cell division, have led biologists to dwell too 

 much upon the independence of the separate cells of which 

 the body is composed. Protoplasm organizes itself into 

 cells, but cells are not necessarily anatomically distinct. 

 They may be the partially separate elements of a syncytium, 

 or there may be but the faintest traces of cell separation. 

 The objection to looking upon cells as isolated, self-complete 



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