ON CELLS, ETC. oT 
form presented by the cells, for they may be flattened even to 
the total disappearance of the cavity, or elongated into cylinders 
and fibres. From these circumstances, many of the cells which 
now come before us for consideration, have been described as 
mere globules, or granules, terms which do not express their 
true signification, and even when they were spoken of as cells, 
or cells furnished with a nucleus, the description rested only 
upon a slight analogy, since but very few of them (for example, 
the pigment-cells), were proved to be actually hollow cells. 
But—as the precise signification of the nucleus is unknown, and 
as the celi-membrane is not proved to be anything essential to 
those cells (and this follows from their accordance with vege- 
table cells), upon the analogy with which the proof of the 
cellular nature of the rest of the globules provided with a 
nucleus will be based,—there is no contradiction involved in the 
supposition that a nucleus may be contained in a solid globule 
as well as in a cell. 
From the difficulties of this investigation above detailed, it 
will be seen that a given object may really be a cell, when even 
the common characteristics of that structure, namely, the per- 
ceptibility of the cell-membrane, and the flowing out of the cell- 
contents, cannot be brought under observation. The possibility 
that an object may be a cell, does not, however, advance us 
much; the presence of positive characteristics 1s necessary in 
order to enable us to regard it as such. In many instances 
these difficulties do not present themselves, and the cellular 
nature of the object is immediately recognized ; in others, the 
impediments are not so great but that the distinction between 
cell-membrane and cell-contents is at least indicated, and in 
such cases other circumstances may advance that supposition 
to a certainty. The most important and abundant proof as to 
the existence of a cell is the presence or absence of the nucleus. 
Its sharp outline and dark colour render it in most instances 
easily perceptible ; its characteristic figure, especially when it 
encloses nucleoli, and remarkable position in the globule under 
examination, (being within it, but eccentrical, and separated 
from the surface only by the thickness of the assumed cell-wail,) 
all combine to prove it the cell-nucleus, and render its analogy 
with the nucleus of the young cells contained in cartilage, and 
with those of vegetables, as also the analogy between- the 
