7O GENERAL PRINCIPLES OF ZOOLOGY. 



History of the Cell Theory. The conception of the 

 cell, as adopted in the morphology of animals and plants, 

 has in the course of time undergone many changes, which 

 must be known to some extent in order to understand 

 completely the name and the conception. When Hooker, 

 Marcello Malpighi, and Nehemia Grew, in the seventeenth 

 century, introduced the term into vegetable anatomy, they 

 understood thereby small chambers surrounded by firm 

 walls and filled with air or fluid contents. When, also, at 

 the beginning of the present century, it was correctly recog- 

 nized that the cell is the anatomical and physiological 

 vegetable unit by changes of which all the other parts of 

 the plant are formed, and when the English botanist 

 Brown discovered in the interior of the cell that small 

 body previously overlooked, the kernel of the cell, or nu- 

 cleus, the old conception remained, and as such was ac- 

 cepted by Schleiden in his cell theory. Schleiden added 

 ,as new a completely erroneous view of the origin of 

 cells; that in a sort of matrix, the "cytoblast," first a 

 granule, the nuclear body, was formed, then around this 

 granule a membrane, the nuclear membrane, arose by pre- 

 cipitation, and around the thus completed nucleus a larger 

 membrane (the cell membrane) was precipitated. Thus 

 for the formation of the cell the nucleus would be of most 

 importance. 



The Schleiden-Schwann Cell Theory. Since it is the 

 nuclei of cells which are most easily demonstrated in the 

 .animal body, and even now are particularly useful for de- 

 ciding questions concerning the presence of cells, it is 

 readily understood how the Schleiden theory, which placed 

 the nucleus so much in the foreground, should have led 

 Schwann to apply the cell theory to the animal kingdom, 

 and thereby raise it to a principle of general application. 

 We usually therefore, speak of the Schleiden-Schwann cell 

 theory. 



As a result of this theory the walls, the cell membrane, 

 were regarded as most important for the function of the 

 cell ; through the cell membrane diffusion currents must 



o 



