38 ON CELLS AS THE BASIS 
globules under examination, in which it lies, and those cells, 
consequently the existence of a spherical cell-membrane in the 
globules, extremely probable. More than nine tenths of the 
globules in question present such a nucleus; in many the 
special cell-membrane is indubitable, in most it is more or 
less distinct. Under such circumstances, we may be permitted 
to conclude that all those globules which present a nucleus of 
the characteristic form and position, have also a cell-membrane, 
although, from the causes before specified, it may not be per- 
ceptible. The different tissues will also afford us many instances 
of other circumstances which tend to prove the existence of 
an actual cell-membrane. An example of what is referred to 
would be afforded by an instance, in which a certain corpuscle 
(furnished with a nucleus), about the cellular nature of which 
a doubt existed, could be proved to be only a stage of deve- 
lopment, or modification in form, of an indubitable cell. The 
cell-nuclei and their distance from each other when scattered 
in a tissue, also serve as indications, when the outlines of the 
cells have to be sought for. They likewise serve to guide 
conjecture as to the earlier existence of separate cells, mm 
instances where they have coalesced in the progress of develop- 
ment. When a globule does not exhibit a nucleus during 
any one of the stages of its development, it is either not a cell, 
or may at least be preliminarily rejected, if there be no other 
circumstances to prove it such.’ Fortunately, these cells devoid 
of nuclei are rare. 
In addition, however, to the cellular nature of the elementary 
structures of animal tissues, there are yet other points of 
accordance between them and the cells of plants, which may 
generally be shown in the progress of their development, and 
which give increased weight to the evidence tending to prove 
that these elementary structures are cells. The exceedingly 
frequent, if not absolutely universal presence of the nucleus, 
even in the latest formed cells, proves its great importance for 
their existence. We cannot, it is true, at present assert 
that, with regard to all cells furnished with a nucleus, the 
latter is universally the primary and the cell the secondary 
formation, that is to say, that in every instance the cell is 
formed around the previously existing nucleus. It is probable, 
however, that such is the case generally, for we not only meet 
