504 THE POPULAR SCIENCE MONTHLY. 



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 vege- 

 table and animal nutrition, may be profitably reviewed to illus- 

 trate the erroneous inferences made from superficial and defective 

 observation. In 1755 Rosenhoff described the Proteus animal- 

 cule, now familiarly known as the Amoeba, 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 struc- 

 ture 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 distribu- 

 tion and arrangement of these structural elements, while their in- 

 timate 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 per- 

 fected the cell theory of organization by extending the same con- 

 ception to animal tissues. Cells were then recognized as the ulti- 

 mate units of organic structure, which were variously modified to 

 adapt them to diverse special purposes. This cell theory of or- 

 ganized structure was generally adopted, and cells were defined 

 as closed membranes or sacs, containing a more or less fluid sub- 

 stance 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 discus- 

 sion 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 dis- 

 covery 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 Amoeba, 

 were composed of a glairy contractile substance, which he called 

 sarcode (rudimentary flesh), and in 1846 Yon Mohl called atten- 

 tion to the importance of the inner lining of the cell wall of 

 plants, which he designated the primordial utricle, with its in- 

 closed contents, to which he gave the name of protoplasm (primi- 

 tive plastic or organizable matter), and these, he claimed, repre- 



