SECTION II. 
ON CELLS AS THE BASIS OF ALL TISSUES OF THE ANIMAL BODY. 
Tue young cells contained within the cartilage-cells (see 
plate I, fig. 8, ff) may be regarded as the elementary form 
of the tissues previously considered, and may be described as 
round cells having a characteristic nucleus, firmly attached to 
the internal surface of the wall. As the above were proved to 
correspond with the vegetable cells, it follows, that it is only 
necessary to trace back the elementary structure of the rest 
of the tissues to the same formation, in order to show their 
analogy also with the cells of plants. In some tissues this 
proof is easy, and immediately afforded ; in others, however, it 
is obtained with much difficulty, and it would frequently be 
altogether impossible to demonstrate the cellular nature of 
some, if the connexion between the different steps in this 
investigation were lost sight of. The difficulty arises from the 
following circumstances: Ist. The minuteness of the cells; in 
consequence of which it is not only necessary to use a power 
magnifying from 400 to 500 diameters, but it is also frequently, 
indeed generally found impossible to press out their contents. 
2dly. The delicate nature of the cell-membrane. When this has 
a certain density, its external as well as internal outlmé may 
be recognized, and the distinction between it and the cell-con- 
tents may thus be placed beyond a doubt. Butif the cell-mem- 
brane be very delicate, the two outlines meet together im one line, 
and this may readily be regarded as the boundary line of a 
globule, not enclosed by a special enveloping membrane. 3dly. 
The similar power of refraction possessed by the cell-wall and 
cell-contents, in consequence of which the internal outline 
of the former cannot be observed. 4thly. The granulous nature of 
the cell-membrane, which when the contents are also granulous, 
cannot be distinguished from them. Lastly, the variety of 
