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184 the microscope. We cannot, at present, lay down any general law respecting the circumstances under which such a coalescence occurs; it presupposes that the cell-membrane and intercellular substance are homogeneous structures, and may perhaps always take place when such a state exists.

As regards the subdivision of the cells, we have already seen how a jutting out of the cell-membrane may be produced by its more vigorous growth in certain situations. But a jutting inwards into the cavity of the cell may also result from the very same process. Now, if we imagine this jutting inwards to take place in a circular form around a cell, as the consequence of a partial increase in the force of its growth, it may proceed to such an extent, that one cell may be separated into two, connected together only by a short peduncle, which may afterwards be absorbed. This would illustrate the most simple form of subdivision in a cell. In the animal cells, however, which undergo subdivision, that is, the fibre-cells, the process is more complicated; firstly, because when an elongated cell subdivides, it splits into many fibres; and, secondly, because the cells are so very minute. The process, therefore, cannot for these reasons be accurately traced, and the following is all that we can detect: a cell becomes elongated on two opposite sides into several fibres; from the angle, which the fibres on either side form with each other, a striated appearance gradually extends over the body of the cell; this formation of striæ becomes more and more distinct, until the body of the cell splits entirely into fibres.

The coalescence of several primary cells to form a secondary cell is, to a certain extent, the opposite process to the last. Several primary cells, of muscle for instance, are arranged close together in rows, and coalesce into a cylinder, in the thickness of which lie the nuclei of the primary cells. This cylinder is hollow and not interrupted by septa, and the nuclei lie upon the internal surface of its wall. These are the facts of the process, so far as they have as yet been observed. One can form a conception of so much as is yet required to render them complete. If two perfectly-developed cells coalesce together, their walls must first unite at the point of contact, and then the partition-wall between the cavities must be absorbed. Nature, however, does not by any means require that these acts should occur at precisely defined periods. The coalescence may take