THE BIOLOGICAL UNIT 35 



condition has given us a different notion about 

 the relation of the life of the cells in the body as a 

 whole. Ever since Harrison showed how living 

 cells could be kept apart from the body of the frog 

 tadpole, and that they would develop normally 

 when immersed in coagulated lymph, we have been 

 wondering what is the distinction between the life 

 of cells and the life of the whole organism. 



Carrel isolated some fragments of connective 

 tissue in 19 12 which were still active in 1920, after 

 more than seven years of life outside of the body, 

 the rate of growth having remained the same after 

 more than a thousand transplantations. Such ex- 

 periments throw new light on the problem of sen- 

 ility and death. For it is conceivable that the 

 length of life of the biological unit outside of the 

 body greatly exceeds its normal duration when 



Figure 13. A drawing of an ameba seen from the side. 

 There is no definite cell wall and the pseudopodia can be 

 formed in any direction. There is no mouth but food can be 

 taken into the body at any place. 



