THE ANIMAL BODY.
to a greater or less extent, of their individuality by the cells, should serve as the scale for their degree of development. We give the name of independent cells to those in which the wall remains distinguishable from the neighbouring structures throughout the whole progress of its expansion. We apply the term coalesced cells to those in which the wall blends, either partially or entirely, with the neighbouring cells, or intercellular substance, so as to form an homogeneous substance. The cell-cavities, in such instances, are separated from one another only by a single wall, as we have already observed in cartilage. This is the first degree of coalescence; the cacti present an example of it in vegetables. The second is that in which the walls of several cells lying lengthwise together, coalesce with one another at their points of contact, and the partition walls of the cell-cavities become absorbed. In this way not only the walls but the cavities of the cells also become united, as in the spiral and lactiferous vessels in plants.
Upon these more or less important modifications of the Cell-life the following classification of the tissues is based: 1st. Isolated, independent cells, which either exist in fluids, or merely lie unconnected and moveable, beside each other. 2d. Independent cells applied firmly together, so as to form a coherent tissue. 3d. Tissues, in which the cell-walls (but not the cell-cavities) have coalesced together, or with the intercellular substance. Lastly, tissues in which both the walls and cavities of many cells blend together. In addition to these, however, there is yet another very natural section of the tissues, namely, the fibre-cells, in which independent cells are extended out on one or more sides into bundles of fibres. The naturalness of this group will form my excuse for sacrificing logical classification to it, and inserting it as the fourth class (4th), consequently, that last mentioned, consisting of tissues, in which the cell-walls and cell-cavities coalesce, becomes the fifth (5th).
All tissues of the animal body may be comprised under these five classes; the classification, however, gives rise to some difficulties. For instance, the fibres of cellular tissue and fat must be placed in very different classes, so also the enamel of the teeth and the proper dental substance. A second difficulty arises from the fact, that transitions take place, the5
