THE CELL-DOCTRINE 121 



seventeenth century. Hooke, Leeuwenhoek, Grew, and Mal- 

 pighi recorded the presence of cells without recognizing 

 them as universal. The simplest organisms, which consist of 

 but a single cell, were studied for more than a century and a 

 half before their cellular nature was recognized. Cells and 

 their nuclei were described in many plants and in a few ani- 

 mals, during the first third of the nineteenth century. But 

 it was not until 1838 that a cell-theory was promulgated for 

 the animal body by Theodor Schwann, and, in the following 

 year, for the plant body by Matthias Schleiden. The value 

 of this generalization was at once apparent. It unified and 

 explained observations of the most diverse sort throughout 

 organic nature, giving an explanation, first, of microscopic 

 structure and, second, of embryological development. 



Since its universal acceptance toward the middle of the 

 nineteenth century, the theory has found confirmation in 

 whatever direction the microscope has been turned. The 

 cell is now recognized as the unit of structure, and so of 

 function, throughout the organic world. The problems of 

 embryology, of physiology, of pathology, and even of 

 heredity are, in the last analysis, cell problems. The domain 

 of cellular biology has steadily expanded, until there exists an 

 apparently inexhaustible field of investigation in cell chem- 

 istry and physics, as well as the structural features, which 

 remain unexplored. 



Cells were originally observed in the form of walled com- 

 partments, to which the term cell was fittingly applied. It 

 was soon realized, however, that the walls, which had seemed 

 the important feature, were not universal. The earlier con- 

 cept of a series of minute cavities was displaced before the 

 middle of the nineteenth century by the discovery that the 

 protoplasm, or semi-fluid material enclosed within the wall, 

 and not the wall itself, constituted the living stuff. More- 

 over, it was found that the walls, particularly in animal cells, 

 were frequently represented by membranes so delicate as 

 to seem non-existent. A "mass of protoplasm containing a 



