THEODOR SCHWANN 255 



which, at an earlier period, nucleus and cell were precipitated as layers 

 around the nucleolus. It must, however, be remarked that the identity 

 of these two processes cannot be so clearly proved as that of the 

 processes by which nucleus and cell are formed ; more especially as 

 there is a variety in the phenomena, for the secondary deposits in 

 plants occur in spiral forms, while this has at least not yet been demon- 

 strated in the formation of the cell-membrane and the nucleus, al- 

 though by some botanical writers the cell-membrane itself is supposed 

 to consist of spirals. 



The power of attraction may be uniform throughout the whole 

 cell, but it may also be confined to single spots ; the deposition of new 

 molecules is then more vigorous at these spots, and the consequence of 

 this uneven growth of the cell-membrane is a change in the form of 

 the cell. 



The attractive power of the cells manifest a certain form of elec- 

 tion in its operation. It does not take up all the substances contained 

 in the surrounding cytoblastema, but only particular ones, either those 

 which are analogous with the substance already present in the cell 

 (assimilation), or such as diflfer from it in chemical properties. The 

 several layers grow by assimilation, but when a new layer is being 

 formed, different material from that of the previously-formed layer is 

 attracted : for the nucleolus, the nucleus and cell-membrane are com- 

 posed of materials which differ in their chemical properties. 



Such are the peculiarities of the plastic power of the cells, so far 

 as they can as yet be drawn from observation. But the manifestations 

 of this power presuppose another faculty of the cells. The cytoblas- 

 tema, in which the cells are formed, contains the elements of the ma- 

 terials of which the cell is composed, but in other combinations ; it is 

 not a mere solution of cell-material, but it contains only certain organic 

 substances in solution. The cells, therefore, not only attract materials 

 from out of the cytoblastema, but they must have the faculty of pro- 

 ducing chemical changes in its constituent particles. Besides which, 

 all the parts of the cell itself may be chemically altered during the 

 process of its vegetation. The unknown cause of all these phenomena, 

 which we comprise under the term metabolic phenomena of the cells, 

 we will denominate the metabolic power. 



The next point which can be proved is, that this power is an attri- 

 bute of the cells themselves, and that the cytoblastema is passive under 



