SURVEY OF CELL-LIFE. 183 



growth docs 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 cf 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 w r ith the intercellular substance : secondlv, the division of 

 one cell into several ; and, thirdly, the coalescence of several 

 primary cells to form a secondary one. 



A coalescence of the ccll-mcmbranc 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 



