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 : 

 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 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 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 

 (lth), 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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