252 BIOLOGY AND ITS MAKERS 



theory, in which the original generalization became consoli- 

 dated with the protoplasm doctrine. 



Further Modifications of the Cell-Theory. — The reformed 

 cell-theory was, however, destined to undergo further modifi- 

 cation, and to become greatly extended in its application. 

 At first the cell was regarded merely as an element of struc- 

 ture; then, as a supplement to this restricted view, came the 

 recognition that it is also a unit of physiology, viz., that all 

 physiological activities take place within the cell. Matters 

 did not come to a rest, however, with the recognition of these 

 two fundamental aspects of the cell. The importance of the 

 cell in development also took firmer hold upon the minds of 

 anatomists after it was made clear that both the egg and its 

 fertilizing agents are modified cells of the parent's body. It 

 was necessary to comprehend this fact in order to get a clear 

 idea of the origin of cells within the body of a multicellular 

 organism, and of the relation between the primordial element 

 and the fully developed tissues. Finally, when observers 

 found within the nucleus the bearers of hereditary qualities, 

 they began to realize that a careful study of the behavior of 

 the cell elements during development is necessary for the 

 investigation of hereditary transmissions. 



A statement of the cell-theory at the present time, then, 

 must include these four conceptions: the cell as a unit of 

 structure, the cell as a unit of physiological activity, the cell 

 as embracing all hereditary qualities within its substance 

 and the cell in the historical development of the organism. 



Some of these relations may now be more fully illustrated. 



Origin of Tissues. — The egg in which all organisms above 

 the very lowest begin, is a single cell having, under the micro- 

 scope, the appearance shown in Fig. 76. After fertilization, 

 this divides repeatedly, and many cohering cells result. The 

 cells are at first similar, but as they increase in number, and 

 as development proceeds, they grow different, and certain 



