SURVEY OF CELL-LIFE. 183 
growth does not necessarily proceed equably on all sides, but 
that the new molecules may be deposited in greater abundance 
in certain situations. Let us take the instance of a round 
_ cell, the cell-membrane of which is already developed, and 
suppose the deposition of new molecules to be confined to one 
particular part of the cell-membrane, that part would become 
expanded, and so a hollow fibre would grow forth from the cell, 
the cavity of which would communicate with the cell-cavity. 
The same result would take place, but more easily, if the new 
molecules were disposed unequally previous to the period when 
the external stratum of the precipitate, which is formed around 
the nucleus, had become condensed into a distinctly perceptible 
cell-membrane. The hollowing out of the fibre would then be 
less perfect, and the growth of the fibre must advance, particu- 
larly as regarded its thickness, before any manifest distinction 
between wall and cavity could be perceived. 
The cause of this irregular disposition of the new molecules 
may, in some instances, be due to circumstances altogether ex- 
ternal to the cell. If, for instance, a cell lay in such a position 
that one side of it was in contact with a concentrated nutri- 
tive material, one could conceive that side of the cell growing 
more vigorously, even though the force, which produces the 
growth of the cell, should operate equably throughout the entire 
cell. Such an explanation cannot, however, be received at all 
in most instances, but we must admit modifications in the 
principle of development of the cells, of such a nature, as that 
the force, which affects the general growth of the cells, is 
enabled to occasion an equable disposition of new molecules in 
one cell, and an unequal one in another. 
Amongst the changes which more or less completely deprive 
the original cells of their individuality, are to be classed, in the 
first place, the coalescence of the cell-walls with one another, 
or with the intercellular substance; secondly, the division of 
one cell into several; and, thirdly, the coalescence of several 
primary cells to form a secondary one. 
A coalescence of the cell-membrane with the intercellular sub- 
stance, or with a neighbouring cell-wall, appears to take place 
in some cartilages for example. At first the cell-membrane has | 
a sharply-defined external contour, by degrees the boundary 
line becomes paler, and at last is no longer perceptible with 
