182 SURVEY OF CELL-LIFE. 
cover the internal surface of the cell-membrane, or not be in 
contact with each other at all. I have not observed any such 
secondary stratified depositions in animals. 
The variations which may occur in the growth of the cell- 
membrane in simple cells, depend upon the circumstance as to 
whether or not the addition of new molecules takes place 
equably at all parts of the cell-membrane. In the first case 
the form of the cell remains unchanged, and the only other 
distinction possible would be grounded upon the fact as to 
whether the greater part of the new molecules were deposited 
between the particles which lay side by side upon the super- 
ficies of the cell-membrane, or between those which lay one 
behind another in its thickness. The first mode of growth 
produces an expansion of the cell-membrane, the effect of the 
second is more especially to thicken it. Both modes are gene- 
rally combined, but in such a manner that the expansion of the 
cell-membrane prevails in most instances. 
A great variety of modifications in the form of the cells may 
be produced by the irregular distribution of the new mole- 
cules. The globular, which is their fundamental form, may 
be converted into a polyhedral figure, or the cells may become 
flattened into a round or oval or angular tablet, or the expan- 
sion of the cells may take place on one or on two opposite sides, 
so as to form a fibre, and these fibres again may either be flat, 
being at the same time in some instances serrated, or lastly, 
the expansion of the cells into fibres may take place on dif- 
ferent sides so as to give them the stellate form. Some of 
these changes of form are, no doubt, due to mechanical causes. 
Thus, for example, the polyhedral form is produced by the 
close crowding of the spherical cells, and these, when separated 
from one another, sometimes assume their round figure again ; 
such is the case with the yelk-cells. Some of the other 
changes would seem to be capable of explanation by exosmosis. 
If, for example, the contents of a round cell be so changed, 
that a fluid is generated in it which is less dense than the 
surrounding fluid, the cell will lose some of its contents by 
exosmosis, and must, therefore, collapse, and may become 
flattened into a table as in the blood-corpuscles. Such expla- 
nations, however, are unsatisfactory in by far the greatest num- 
ber of instances, and we are compelled to assume, that the 
