SECTION II
ON CELLS AS THE BASIS OF ALL TISSUES OF THE ANIMAL BODY.
The 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 outline may be recognized, and the distinction between it and the cell-contents may thus be placed beyond a doubt. But if the cell-membrane be very delicate, the two outlines meet together in one line, and this may readily be regarded as the boundary line of a globule, not enclosed by a special envelopmg 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
