74 FORMATION OF SHELLS OF ANIMALS, ETC., 
avow my opinion upon this point; and, therefore, I 
may briefly notice, that although I partially admit many 
of the facts connected with the general structure of 
cells, such as an exterior investment, or, as it is called, 
a cell-membrane, cell-contents, and very frequently 
a central part, differmg from the rest, called the 
cell-nucleus, I am not of opinion that these parts are 
endowed with a distinct and separate vital power, by 
which one cell can act independently of another and of 
the surrounding parts, and grow and multiply by the 
exercise of its own individual vital endowments. I wish 
particularly to notice also, that my object in bringing 
these researches before the public is not to attack the 
cell theory, but that it is to explain certain facts which 
have come under my notice, and that I wish to deal only 
with facts, and such deductions from them as the facts 
themselves will be shown to justify. Notwithstanding, as 
it must sometimes occur, that the decision of a doubtful 
question will depend upon a comparison of evidence, it 
will be impossible to avoid making allusions to the 
opinions and views of others. But to return to the 
subject of the formation of these urinary calculi, I may 
notice that, if the cell-development theory be tenable in 
any case of the formation of adventitious structure, it is 
so in this. It is true that, in this instance, the connexion 
between the living structures and the adventitious pro- 
ducts is not so clear as in many others, but still it may be 
assumed that it did once exist, but that now it has ceased; 
however, that there still exists sufficient connexion be- 
tween them, through the medium of the urime acting as a 
blastema or pabulum, to furnish the cells with the requi- 
site supply of material for their individual growth and 
multiplication. Such, in substance, is the explanation 
