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, particularly 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 external to the cell. If, for instance, a cell lay in such a position that one side of it was in contact with a concentrated nutritive 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 substance, 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
