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CAPILLARY VESSELS. 159 blood constitutes the contents of the primary cells, as well as of the secondary ones—the vessels produced by their coalescence; and the blood-corpuscles are young cells which are developed in their cavities.

Thus this last class, comprising tissues, which, in their functions, are the most characteristic of the animal kingdom, exhibits the same principle of development that we have met with in all the others; namely, that cells originate in the first place, and that. these become transformed into the elementary parts of the tissues. The elementary cells in this class, however, undergo more essential changes during their transformation than those of any previous one. They not only do not remain, as in the first two classes, independent, that is provided with a special cavity and particular wall; not only does a coalescence of the walls of neighbouring cells take place, as in the third class, but the cavities of the different cells also unite together in consequence of the absorption of the coalesced partition-walls of the several cells, so that the primary cells cease to exist as distinct objects. It is to a certain extent the opposite process to that which occurred in the fourth class, where, in addition to the prolongation of the cells, a splitting of them into several, probably hollow, fibres, a sort of division of the cells took place. The type of the transformation of the primary cells, as presented by nerve, muscle, and capillary vessels, is not, however, altogether limited to this class, but has been already exhibited by previous classes, and even in plants. Some of the pigment-cells have been cited before as examples, and the generation of the cells of the liber observed by Meyen was brought forward as an instance of perfect analogy in vegetables.

The independent existence of each separate primary cell is, no doubt, lost as a consequence of this perfect coalescence of several cells; not so, however, its character as Cell in general. On the contrary, several primary cells contribute to form one secondary cell, having the full signification of one independent cell. Each secondary cell in muscle and nerve forms a closed Whole, and the distinction between cell-membrane and cell-contents or secondary deposit seems to continue throughout life. In this way the nerves bring every part of the body into con-