THEORY OF THE CELLS. 195 
when no part of the cell is yet formed. Therefore, the greater 
the attractive force of the cell is, the less concentration of the 
fluid is required; while, at the commencement of the forma- 
tion of a cell, the fluid must be more than concentrated. But 
the conclusion which may be thus directly drawn, as to the 
attractive power of the cell, may also be verified by observation. 
Wherever the nutrient fluid is not equally distributed in a 
tissue, the new cells are formed in that part into which the 
fluid penetrates first, and where, consequently, it is most con- 
centrated. Upon this fact, as we have seen, depended the 
difference between the growth of organized and unorganized 
tissues (see page 169). And this confirmation of the foregoing 
conclusion by experience speaks also for the correctness of the 
reasoning itself. 
The attractive power of the cells operates so as to effect the 
addition of new molecules in two ways,—first, in layers, and 
secondly, in such a manner in each layer that the new mole- 
cules are deposited between those already present. This is 
only an expression of the fact ; the more simple law, by which 
several layers are formed and the molecules are not all de- 
posited between those already present, cannot yet be explained. 
The formation of layers may be repeated once, twice, or thrice. 
The growth of the separate layers is regulated by a law, that 
the deposition of new molecules should be greatest at the part 
where the nutrient fluid is most concentrated. Hence the 
outer part particularly becomes condensed into a membrane 
both in the layer corresponding to the nucleus and in that 
answering to the cell, because the nutrient fluid penetrates 
from without, and consequently is more concentrated at the 
outer than at the imner part of each layer. For the same 
reason the nucleus grows rapidly, so long as the layer of the 
cell is not formed around it, but it either stops growing 
altogether, or at least grows much more slowly so soon as 
the cell-layer has surrounded it; because then the latter 
receives the nutrient matter first, and, therefore, in a more 
concentrated form. And hence the cell becomes, in a general 
sense, much more completely developed, while the nucleus- 
layer usually remains at a stage of development, in which 
the cell-layer had been in its earlier period. The addition of 
new molecules is so arranged that the layers imcrease more 
