196 THEORY OF THE CELLS. 



considerably in superficial extent than in thickness ; and thus 

 an intervening space is formed between each layer and the 

 one preceding it, by which cells and nuclei are formed into 

 actual hollow vesicles. From this it may be inferred that 

 the deposition of new molecules is more active between those 

 which lie side by side along the surface of the membrane, than 

 between those which lie one upon the other in its thickness. 

 Were it otherwise, each layer would increase in thickness, but 

 there would be no intervening cavity between it and the pre- 

 vious one, there would be no vesicles, but a solid body com- 

 posed of layers. 



Attractive powder is exerted in all the solid parts of the cell. 

 This follows, not only from the fact that new molecules may 

 be deposited everywhere between those already present, but 

 also from the formation of secondary deposits. When the 

 cavity of a cell is once formed, material may be also attracted 

 from its contents and deposited in layers ; and as this depo- 

 sition takes place upon the inner surface of the membrane 

 of the cell, it is probably that which exerts the attractive in- 

 fluence. This formation of layers on the inner surface of the 

 cell-membrane is, perhaps, merely a repetition of the same 

 process by which, at an earlier period, nucleus and cell were 

 precipitated as layers around the nucleolus. It must, how- 

 ever, 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 de- 

 monstrated in the formation of the cell-membrane and the 

 nucleus, although 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 manifests a certain form of 

 election 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 





