GENERAL ANA10M7. 59 



in deciding questions concerning the presence of cells, it is readily under- 

 stood how Schleiden's theory, which placed the nucleus so much in the 

 foreground, should have led Schwann to apply the cell theory to the 

 animal kingdom, and thus 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 mem- 

 brane diffusion-currents must pass between the surrounding medium 

 and the contents of the cell ; the character of the membrane and of the 

 cell-sap must determine the condition of the diffusion-currents, and 

 hence the functional character of the cell ; the different appearance of 

 tissues depends chiefly upon the fact that the cells, spherical in the 

 beginning, change their form ; in the case of fibrillar connective tis- 

 sue, for example, they increase enormously in length and become fine 

 fibrillaB. Since the life of an organism is nothing else than the co-operative 

 work of all its cells, they flattered themselves that through the cell theory 

 .and the discovery, brought about by it, of the physical unity of the animal 

 -and vegetable body they had made an important advance in the great 

 problem of the physical explanation of the phenomena of life. Cell gene- 

 sis also seemed, according to the theory, to be just as satisfactorily 

 explained on a mechanical basis as the formation of a crystal. In the 

 ' cytoblast ' the nuclear bodies, nuclear membrane, and cell membrane 

 must be formed by deposition just as in the process of crystallization. 



Reform Movements. Since that time our conception of the nature of 

 cells has completely changed. The cell does not, after the manner of a 

 crystal, arise as a new formation in a matrix, but it presupposes the 

 existence of a living mother-cell, from which it arises by division or bud- 

 ding. Just so also the cell is not a physical unit, but is itself an organism 

 which shows to us all the enigmas of life, the physical basis of which our 

 investigations must ever keep in view as a goal, though it be still indiscern- 

 ibly distant. The membrane and cell-sap are of quite subordinate impor- 

 tance for the existence of the cell; rather the most important thing in it is 

 the previously disregarded substance, for which von Mohl introduced the 

 name protoplasm. According to the newer conception the cell is practically 

 a small mass of protoplasm, usually, probably always, provided with one 

 or more nuclei. This newer conception of the cell has developed so gradu- 

 ally, and has so slowly supplanted the Schleiden-Schwann view, that the 

 old name has been retained, although it no longer at all fits the new con- 

 ception. We have indeed become so thoroughly accustomed to the name 

 that we no longer notice the contradiction of terms when we call a solid 

 lump without a membrane a ' cell.' 



Discovery of Protoplasm. The reformation of the cell theory was begun 

 by discoveries which were made in very different regions and only lately 

 have been brought to a focus. 



1. At about the beginning of the nineteenth century, Bonaventura 

 Oorti and Treviranus had seen that the chlorophyl bodies, which cause the 

 green color of plants, in many species stream around in a lively manner in 

 the interior of the cell, but Mohl was the first to find out that this motion 



