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, vzz., 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 numbcr, and 
as development proceeds, they grow different, and certain 
