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 power 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 imner 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 
