28 PRINCIPLES OF ANIMAL BIOLOGY 



make up the complex process of embryonic development. All these changes 

 are brought about in or by the cells. 



Through this series of multiple changes there is built up an organism 

 which, in most cases, closely resembles its parents. Here again it is 

 found that the cells are responsible for each step and for the final result. 

 The complete heritage of the eventual adult animal must of course be 

 represented in some manner in the egg. The proper distribution of the 

 material of heredity, whatever it is, through all of the divisions and re- 

 arrangements which the embryo undergoes, is at every point cared for 

 by the struUure and functioning of cells. What these structures and activi- 

 ties are cannot be stated here, even by name, but are dealt with in sub- 

 sequent chapters. 



An adequate statement of the cell doctrine, as must be apparent from 

 the foregoing considerations, must do more than generalize upon the 

 structure of living beings. It must include the fact that the cell is also 

 the unit in physiological processes, including those of embryonic develop- 

 ment and inheritance. 



Influence of the Cell Theory. — The theory of the universality of 

 cells was a great generalization. It would have been a most important 

 milestone in biological progress, even if it had been in itself less funda- 

 / mental. For zoology had had no great generalizations before the cell 

 theory was brought forth. There was nothing in the field of biology to 

 compare with Newton's laws of motion in physics. Biologists would 

 have been pardoned for despairing of ever bringing the whole living world 

 under one point of view in some specific regard. The cell theory, there- 

 Y fore, was a great unifying influence. Contemplation of it must have 

 prepared biologists for other great generalizations. Less than half a 

 century later, another fundamental proposition, the theory of evolution, 

 had been accepted by practically the whole biological world; and it is 

 not improbable that the idea of evolution w'as adopted the more readily 

 because the cell theory had shown that basic unity was as much to be 

 expected among living things as in physics and chemistry. 



The knowledge that animals and plants are universally constructed 

 of cells led to new problems. Comparative morphology could now extend 

 its principles to an undreamed-of host of minute structures. All the com- 

 ponents of cells, including those of the nuclei and specialized organs, 

 which are mentioned in the description of a typical cell below, have been 

 discovered since Schwann's time; and the occurrence of these in cells of 

 widely different kinds courted comparisons. Physiology now became the 

 physiology of cells, or was capable of such extension. General physi- 

 ology, as distinct from the physiology of the organs of higher animals, 

 has grown out of this discover^'. A knowledge of the permeability of 

 cell membranes, of the functions of electrolytes in living substance, of the 

 release of energy by chemical reactions within the cells, are some of the 



