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130 the cell-walls then become blended, in such manner that merely simple septa remain, dividing each succeeding cell-cavity from its neighbour. These septa, however, become absorbed, so that the cavities of the different cells communicate. Instead of a number of primary cells, we then have one single long one, which we shall call a secondary cell. The cavity of such a one, therefore, consists of the united cavities of the original cells, and its cell-membrane of all their blended cell-membranes, except that the parts with which they were in contact are absorbed. The growth of the secondary cell proceeds hike that of any simple independent cell. This appears to be the process of formation in muscle and nerve, so far, at least, as the observations, which will presently be communicated, extend. When the primary cells have a stellate figure, their bodies are not applied in rows, as in nerve and muscle, but are generated in larger interspaces filled with cytoblastema or with cells of another kind. Their prolongations, however, come in contact, the walls coalesce at the points of junction, and the blended septa then become absorbed, so that the cell-cavities, which were at first separated, now communicate. In this manner, when several prolongations of one cell come into contact with those of another, or of several others, we obtain, in the place of isolated, hollow, stellate cells, a network of canals, which are, in the first instance, somewhat thicker at the parts corresponding to the bodies of the cells, but become of pretty equal dimensions, in consequence of more vigorous expansion of the communicating prolongations. This appears to be the mode in which the capillary vessels are formed. The following detailed statement of observations upon the relation which muscles, nerves, and capillary vessels bear to elementary cells, will show how far the description just given, as the probable mode of formation, is to be regarded as proved by these, as yet, very incomplete researches, and will also indicate what deficiencies have yet to be supplied.

1. Muscle. To ascertain the relation which this tissue bears to the elementary cells, we must have recourse to the history of its development. I was, unfortunately, prevented from investigating the earliest formation of muscular fibre, in consequence of not being able to obtain any very young