144 



HISTOLOGY 



series of imaginary cellular bodies in which the surface is supposed to be 

 everywhere equal in its power to transfer a given amount of material 

 in a given length of time, and in which this material is likewise passed by 

 physiological processes from cell to cell inside the body. Each cell, in- 

 cluding the outer cells, is supposed to use what food it needs upon 

 receiving it, and to pass the rest along equally to all of its neighbors. 



Let us suppose, for discussion, that a body is the simplest kind of a 

 mass, a cube, or better still a sphere (Fig. 130, A). It can then be seen 

 that a cell situated in the center of this sphere can only receive food 



FIG. 1 30, A , B, and C. A , body of cells small enough to secure necessary exchanges through sur- 

 face cells. B, a body of cells too large to do the same. C, same sized mass of cells as in 

 B, but with surface increased by invagination until sufficient to work necessary exchanges. 



materials from the surface of the body after this food has passed through 

 every cell that lies between it and the body surface. Suppose further 

 that, in this spherical body, only enough material to supply three rows 

 of cells can be passed through the outer row or layer of cells (in A) 

 besides what it requires for its own use, and that in this case the organism 

 is just able to live. 



If now we suppose that the organism is larger (Fig. 130, B), to have, 

 for instance, six rows of cells from center to surface, and, remembering 

 that the surface cells are capable of supplying only three other rows of 

 cells with food, it follows that in this case they have more than they can 

 do, and the inner cells must perish both from lack of food and from an 

 inability to get rid fast enough of the waste substances that are poisonous 

 to them. The inner cells would die first, as is exemplified in the case of the 

 cancerous growth that breaks down in its interior when it has reached a 

 certain size and has not developed a circulation. 



Two changes in the structure of the body are possible that would 



