THEORY OF THE CELLS. 193 
it cannot continue long in existence after being separated 
from its swarm. The manifestation of the power which resides 
in the cell depends upon conditions to which it is subject only 
when in connexion with the whole (organism). 
The question, then, as to the fundamental power of orga- 
nized bodies resolves itself into that of the fundamental powers 
of the individual cells. We must now consider the general 
phenomena attending the formation of cells, in order to dis- 
cover what powers may be presumed to exist in the cells to 
explain them. ‘These phenomena may be arranged in two 
natural groups: first, those which relate to the combination of 
the molecules to form a cell, and which may be denominated the 
plastic phenomena of the cells; secondly, those which result 
from chemical changes either in the component particles of the 
cell itself, or in the surrounding cytoblastema, and which may 
be called metabolic phenomena (70 perafoAtkov, implying that 
which is liable to occasion or to suffer change). 
The general plastic appearances in the cells are, as we have 
seen, the following: at first a minute corpuscle is formed, 
(the nucleolus) ; a layer of substance (the nucleus) is then pre- 
cipitated around it, which becomes more thickened and ex- 
panded by the continual deposition of fresh molecules between 
those already present. Deposition goes on more vigorously at 
the outer part of this layer than at the inner. Frequently the 
entire layer, or in other imstances the outer part of it only, 
becomes condensed to a membrane, which may continue to take 
up new molecules in such a manner that it increases more 
rapidly in superficial extent than in thickness, and thus an 
intervening cavity is necessarily formed between it and the 
nucleolus. A second layer (cell) is next precipitated around 
this first, im which precisely the same phenomena are repeated, 
with merely the difference that in this case the processes, espe- 
cially the growth of the layer, and the formation of the space 
intervening between it and the first layer (the cell-cavity), go 
on more rapidly and more completely. Such were the pheno- 
mena in the formation of most cells; m some, however, there 
appeared to be only a single layer formed, while in others (those 
especially in which the nucleolus was hollow) there were three. 
The other varieties in the development of the elementary parts 
were (as we saw) reduced to these—that if two neighbouring 
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