The progressive development of the cell theory of organic structure, and the several steps which have led to a recognition of protoplasm as a factor in evolution and in the processes of vegetable and animal nutrition, may be profitably reviewed to illustrate the erroneous inferences made from superficial and defective observation. In 1755 Rosenhoff described the Proteus animalcule, now familiarly known as the Amæba, without being aware of the importance of the discovery in furnishing a type of the form or conditions of matter required for the manifestations of life.
Bichat laid the foundation for the study of the minute structure of animals in his work on anatomy, published in 1801, in which the different organs of the body were described as made up of tissues, to each of which was assigned a special function, and the attention of anatomists was then given to the distribution and arrangement of these structural elements, while their intimate relations, arising from a common origin, were not detected.
The next step of real progress was made in 1838 by Schleiden, who traced all vegetable tissues to the common form of nucleated cells from which they had their origin, and in 1839 Schwann perfected the cell theory of organization by extending the same conception to animal tissues. Cells were then recognized as the ultimate units of organic structure, which were variously modified to adapt them to diverse special purposes. This cell theory of organized structure was generally adopted, and cells were defined as closed membranes or sacs, containing a more or less fluid substance which served to nourish them. The cells were looked upon as independent units, which multiplied by a process of budding or by self-division, and a new factor was introduced in the discussion of the mooted question as to what constitutes the individual in plants. Schwann "regarded the plant as a cell community in which the separate elements were like the bees in a swarm," and this appeared to be a logical inference from the accepted cell theory of organization.
This view was, however, based on an erroneous assumption as to the essential constituents of the cell, and the progress of discovery gradually led to the demonstration of a material and physiological bond of union in the various tissues of plants. In 1835 Dujardin made the discovery that the bodies of Foraminifera, a group of animals of simple organization, including the Amæba, were composed of a glairy contractile substance, which he called sarcode (rudimentary flesh), and in 1840 Von Mohl called attention to the importance of the inner lining of the cell wall of plants, which he designated the primordial utricle, with its inclosed contents, to which he gave the name of protoplasm (primitive plastic or organizable matter), and these, he claimed, repre-