THE ANIMAL BODY. 65 
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 : 
Ist. 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 intercel- 
lular 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 mdependent 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 diffi- 
culty arises from the fact, that transitions take place, the 
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